Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.


 Library of Excerpts

Lateralization and Language III

"The predominance of the right hand over the left was also reported by Dennis even among Egyptian art forms 3,500 to 4,500 years old, where the ratio of left- to right-handers was 9:111 and 5:100, respectively. However, going further back, Parello, using the "Draw-a-Man Test," found that ancient paleolithic man, from 1,750,000 to 8,000 years B.C., was probably either more ambidextrous or that there was a greater proportion of left-handers than there are now. This is an extremely interesting finding, which fits in very well with evidence from Hole, that the main paleolithic substence economy to 8,000 B.C. was hunting, while agriculture began shortly thereafter. Thus evidence will be presented to show that among the present traditional hunting-fishing population such as the Eskimo, Barry also found a lower degree of conformity on the Asch Conformity Test and more independent values. Hence a higher number of left-handers was predicted for the Eskimo, while observed incidence is 11.3 percent. Conversely, the majority of the agriculturalists of the world since 8,000 B.C., such as the Hong Kong Chinese Hakka, have harsher socialization with very few left-handers (an observed incidence of 1.5 percent). (Dawson, John L. (1977) An anthropological perspective on the evolution and lateralization of the brain. Annals of the New York Academy of Science 299: pp. 426)

Chart describes left-handedness percentages in different cultures including: Eskimo male 12.5, female 10.3; Australian Aborigines 10.7 male, 10.3 female. See book wall for additional interesting stats. (Dawson, John L. (1977) An anthropological perspective on the evolution and lateralization of the brain. Annals of the New York Academy of Science 299: pp. 430)

"In a study of the effects of the kwashiorkor protein-deficiency disease by Dawson, certain males were found to manifest gynecomastia as a result of an inability of the kwashiorkor-damadged liver to inactivate the normal amount ofestrogen found in the male. The increased estrogen levels are thought to stimulate both the growth of the male mammary gland so as to resemble that of the female and cause the accompanying atrophy of the testicles. These feminized males with gynecomastia had the predicted significant reversal of normal male cognitive style, with lower spatial and higher verbal skills as contrasted with controls. The normal male usually has higher spatial but lower verbal abilities whereas females normally have lower spatial, but higher verbal values." (Dawson, John L. (1977) An anthropological perspective on the evolution and lateralization of the brain. Annals of the New York Academy of Science 299: pp. 432)

"The findings of significantly lower incidence of left-handedness among agricultural peoples, as contrasted with the hunting and fishing societies, provide considerable support for the hypothesis that argued that due to higher cultural pressures towards conformity in agricultural societies the incidence of left-handedness would be much lower than that among the more independent hunting peoples." (Dawson, John L. (1977) An anthropological perspective on the evolution and lateralization of the brain. Annals of the New York Academy of Science 299: pp. 440)

"This significantly higher spatial skills are found in the hunting peoples such as the Eskimo and Australian semi-desert Arunta, which stems in part from the "permissive socialization" needed to develop greater independence and superior spatial skills, and in part from "natural selection processes" since good spatial orientation is required in hunting activities for survival.... To summarize these arguments, as higher male spatial ability has had biological survival advantages in evolution, this explains the development of a "sex hormone/selective genetic inheritance/socialization" relationship in developing higher male spatial skills, which has been paralleled by the development of higher female perceptual-motor / linguistic skills and reproductive activities, similarly controlled by a "sex-hormone/selective genetic inheritance/socialization relationship." Dawson, John L. (1977) An anthropological perspective on the evolution and lateralization of the brain. Annals of the New York Academy of Science 299: pp. 443)

"A plethora of examples attest to variability increased above normal in many morphometric traits in Down syndrome ... This classic paper [Levinson et al., 1955] in an attempt to counter the popular misconception of unusual phenotypic uniformity of Down Syndrome. This classic paper made and still makes many valuable points, namely, greater than normal variability in birth weight, psychomotor development, pattern and timing of tooth eruption, closure of anterior fontanel, speech development and handedness (normal right-handedness 95%, Down syndrome 48%, ambidextrous 18%, undetermined 20%)." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 48)

"Shepard (1975, 1981, 1982) has suggested that the evolution of human language may have been facilitated by the evolution of hemispheric specialization, such that the capacities for grammatical and conceptual transformation underlying language in one hemisphere may represent modifications of more ancient capacities for the mental transformation of spatial representations that were originally carred out in both hemispheres. Similar ideas have been advanced by Bradshaw (1988, 1993) and Corballis (1983, 1991a,b). (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 22)

"This suggests that even if female choice alone were driving encephalization in our ancestors (which I don't believe was the case) modern males and females would still be expected to have very similar brain sizes and mental abilities. This is because demorphism in brain size evolves very slowly whereas encephalization evolved very rapidly. The lack of sexual dimorphism in modern human brain size is therefore not a strong argument against the hypothesis that human mental evolution was driven by selective mate choice." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 116)

"According to Professors W.S. Condon and W. Ogston of Boston University, as early as twenty minutes after birth the human neonate moves in precise and sustained synchronous organizations of change of movement with the articulated structure of its mother's speech. " (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 98)

"The essential point being made here is that it may be insufficient to look for one cause of handedness. There may be several interacting causes, genetic and environmental, some coded in genes and others depending on developmental pathways. These developmental pathways may begin before of after birth. The question of cause becomes a question of whether the "recipe" can be deciphered, both in the sense of what ingrediants go into the making of handedness and in the sense of their timing. Every cook knows that it makes a lot of difference whether the eggs are added before of after beating the cake mixture. Similarly, we need to discover the sequence in which the several "causes" of handedness influence the eventual outcome." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 17)

in the brain"Notice that the mouth and hand areas are in fairly close proximity. Any special influence on the corticle growth of either area might also affect the other." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 25)

"Sensations from the skin and joints include information about touch, temperature, pain, and pressure, which are referred to collectively as the somatosensory system." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 29)

"The parietal association areas are especially important for coordinating information from the several senses, as would be expected from their position between the somatosensory, visual, and auditory areas. The parietal cortex immediately posterior to the somatosensory strip is concerned with keeping track of where the limbs are located in space, as essential basis for the planning of new movements. Next to this area is a region that is especially responsive to visual stimuli that are within reach for grasping. These findings by Mountcastle and his colleagues (Mountcastle, Lynch, Georgopolous, Sakata, & Acuna, 1975; outlined in Kolb & Winshaw, 1980) confirm that parietal cortex is involved in the coordination of behaviors of hand and eye. The enlargement of parietal areas in the human brain was presumably a prerequisite for the development of human technological skills. The angular gyrus seems to be necessary for connections to be made between visual and auditory inputs (Geschwind, 1979). This areas is especially large in man and is one of the last to be myelinated during individual growth. In learning to read and write, the coordination of information about how words look with how they sound is presumed to depend on this organ." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 30) [note connections to dance; late myelination etc.]

"Although other analysts did not always consur (Harris, 1980a), recent microscopic examination of the worked surfaces of stone tools led Keeley (1977) to conclude that workers at the Clacton site some 200,000 years ago were consistently right-handed. " (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 40)

"There is no clear evidence available as to hand preferences in fossil hominids. To the extent structural asymmetries of the brain in modern man are associated with handedness, the evidence for similar structural asymmetries in hominids suggests that these precursors of modern man included more right- than left-handers. Evidence from early artefacts, works of art, and biblical references to sinistrality give no indications of time when the distribution of human handedness differed from the modern pattern." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 61)

"Less than 1% of Japanese school-children were reported by Komai and Fukuoka (1934) to use the left and for writing, but incidences of left-handed throwing, cutting with scissors, striking a match, and kicking a football (5-8.5%) were comparable to those found for English children today (Chapter 10). Chinese children were found to use the left hand rarely for eating (1.5% or writing (.7%) in Taiwan but more often (6.5%) in the United States (Teng, Lee, Yang, & Chang, 1976). A population sample of parents and children in Hawaii is especially interesting because differences in incidences of left-handed writing were found between parents of different ethnic origin (Japanese versus European) but not between their children, presumably because of an easing of social sanctions against left-handedness in Japanese families living in Hawaii (Ashton, 1982). A difference between generations, with more left-handers among children than parents, has been found in all studies of the families of university students (Tables 3.6 and 16.7). It was also found for both ethnic groups in Ashton's (1982) sample, which was not university-based. A questionnaire given to Londoners attending a dental hospital for treatment found about 10% left-handed writers in younger respondents (15-44 years) but only about 3% in older respondents (55-64 years) (Fleminger, Dalton, & Standage, 1977). Burt (1937) reported incidences of 3-4% in his studies in his schools of 1913 and 1923, at about the time the older Londoners were at school. Studies of trends in adult populations in the United States (Porac, Coren & Duncan, 1980) and in Australia (Brackenbridge, 1981) have documented falling incidences of left-handedness with increasing age." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 68-9)

"Bingley (1958) reviewed several studies that reported incidences of left-handedness of 14 to 18% in epileptics, in contrast to 8 to 9% in controls. Not all studies found an excess of left-handers in epileptics. In Bingley's (1958) series of 90 cases of temporal lobe epilepsy, the incidence was the same (6.7%) as for other cases operated for temporal lobe tumour (6.5%). In an unpublished study of adult epileptics, I found no evidence of an excess of left-handed or mixed-handers." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 72)

"Positive support claimed by Bakan (1977) includes a study of university students who gave information by questionnaire on their own birth histories and handedness: 40% of left-handers and 41% of ambilaterals reported stressful births, in contrast to 22% of right-handers (Bakan, Dibb, & Reed, 1973). A report by Leviton and Kilty (1976) supported Bakan's hypothesis gave insufficient details ofthe sample, and since some 60 to 70 left-handers were divided between four birth order categories, numbers within groups wre almost certainly inadequate for statistical test." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 74)

"The other finding in the Smart sample concerned children of 21 mothers who had their first-born child at 39 years or older; 9 (43%) of the children were non-right-handed. In the NCDS survey there were no significant effects for maternal or paternal age (McMannus, 1981)." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 75)

"The observation of Barnes (1975) that left-handers were a little slower to establish regular breathing immediately after birth may be due, as she suggested, to some characteristic of the nervous system of the potential left-hander rather than that left-handedness is caused by damage consequent on cerebral hypoxia. Respiratory distress was not associated with increased incidence of sinistrality in the NDCS sample (McManus, 1981). Finally, left-handers are often found amoung those with the highest levels of achievement in the art, music, sport, and theatre (Barsley, 1966). The proposition that all these people suffered early brain damage, however minimal, seems most implausible." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 76)

"In her survey of 11- to 12-year-old Glascow school-children, Clark (1957) noticed a curious sex difference in the association between left-handedness in children and parents. In her sample of 330 children there were approximately equal numbers having a left-handed father and a left-handed mother; but of 32 strongly left-handed children, 5 had a left-handed mother and none had a left-handed father. Falek (1959) noticed that in his small sample of personally visited families there was a higher proportion of left-handed children in families with a left-handed mother (37.5%) than in families where the left-handed parent was the father (12.8%). Table 4.5 summarises the findings for this comparison for five studies with large N's in each group. The trend is consistent though not always statistically signigicant. This trend was not found by Carter-Saltzman (1980) in her biological families, but N's were smaller. Sex differences in families are more complicated if the sex of children is taken in to account. The stonger effect of maternal left-handedness on children's handedness was more noticeable for daughters than sons in several sets of data. (Annett, 1973a; Chamberlain, 1928; Rife, 1940) but not in that of McGee and Cozad (1980). (Leiber and Axelrod, 1981a, did not differentiate sex of children.)" (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 77-8)

"Chapanis and Gropper (1968) compared 32 right-handers and 32 left-handers... Over-all there was not much difference between right- and left-handers in using their preferred hand. However, left-handers were considerably better than right-handers in using their nonpreferred hands.....Peters and Durding (1979) compared right- and left-handers for speed of tapping by each hand. The handedness groups were virtually identical for the preferred hand, but left-handers were significantly faster than right-handers with their nonpreferred hand." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 88-9)

"There are a number of suggestions that musical abilities might be associated with non-right-handedness. Oldfield (1969) found that almost 20% of staff and students of university schools of music answered yes to the question "have you ever had any tendency to left handedness?" --but found a similar response rate in his psychology student controls. Byrne (1974) reported a higher incidence of mixed-handedness in instrumentalists. Deutsch (1978) found that mixed-handers (mildly rather than strongly left-handed as assessed by questionnaire) made fewer errors than strongly right- or left-handed subjects on a test of pitch identification. The advantage was confirmed in a second experiment (Deutsch, 1980). Craig (1980) found left-handers superior to right-handers for identifying rhythms when beats were presented in both ears simultaneously." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 95-6)

"First, one of the recurring themes of the literature is that the right hemisphere is specialised for "holistic" or "gestalt" processing, while the left hemisphere is specialised for "analytic" of "sequential" processing. LEA and LVA are often found when stimuli must be responded to quickly, in a direct global fashion, whereas REA and RVA are often found (even for the same stimulus material) when more complex judgements that take time have to be made. This contrast has been found for judgements of nonverbal stimuli as well as verbal ones, leading to the inference that the essential difference between the hemispheres does not relate to the verbal or nonverbal nature of the stimulus material but rather to the holistic or analytic nature of the processing required. The point here is that sequential and analytic processing must involve mechanisms for holding information over time, which depend, in turn, on the symbolic representation of the material to be analysed. The symbols used are probably verbal for most types of material. Hence, the characterisation of the left hemisphere as sequential or analytic does not add signigicantly to the original inference that it is specially adapted for carrying information in verbal symbols." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 112)

"The idea that only a very small difference between the two hemispheres is involved, a capacity to make finer time discriminations on the left side, supplies a hypothesis as to just the sort of developmental nudge that might, in normal circumstances, be sufficient to induce the left hemisphere to serve speech." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 113)

"There is an important point to be made about this feedback process with regard to brain organization. The brain of the infant is analogous, in some respects, to that of a split-brain adult because the cerebral commissures are largely unmyelinated (Yakovlev & Lecours, 1967). In discovering how the mouth must be shaped to produce the fine defferences between /b/ and /p/, or between /l/ and /r/, it would be advantageous to have a close link between the somatosensory feedback from the mouth and the auditory feedback from the ear. If the mouth were being controlled from one side and the sound analysed on the other, this intimate link would be absent. Subcortical pathways might be available, but they would involve some delay, in comparison with the virtually instantaneous link if motor production and auditory analysis were controlled from the same cerebral hemisphere. The conclusion seems obvious that there would be great advantages for the acquisition of speech if the control of the speech apparatus and auditory analysis were on the same side. The side could be the right or the left, but the evidence from hemiphegic children, dichotic listening, and split-brain patients suggests that there is something special about the left hemisphere of most people, which makes it more fitted than the right hemisphere of most people, which makes it more fitted than the right to serve speech acquition. Summarising the argument, it may be said that the advantages of having the motor and sensory aspects of speech control located on the same side of the brain are sufficient to account for the evolution of some mechanism that attempts to ensure this concordance, but it happens to have evolved for the more efficient control of speech form the left side." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 119)

"Piaget (1951) noticed that several behaviors that depend on representative capacities are acquired at about the same time: the imitation of absent models, pretend play, using words symbolically, and recognising pictures as representations of objects." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 120)

"With regard to the issue of bilaterality of cerebral speech, Hacaen and Sauguet (1971) concluded that "cerebral ambilaterality is not a characteristic of all left handers, but only those who belong to the familial type." Hacaen, De Agostini, and Monzon-Montes (1981) suggested that their new analysis confirmed the greater cerebral ambilaterality of FS+ left handers." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 131-3)

[FS + is familial sinistrals] "On theoretical grounds, if FS+ reduces the chances of the presence of the typical pattern of cerebral specialization, groups should be ordered as follows: The strongest asymmetries should be found in FS- right-handers, followed by FS+ right handers, then FS- left-handers, and FS+ left-handers. Bryden (1982) calculated verbal-non-verbal differences from scores for two verbal and two nonverbal tasks in data of Piazza (1980). The predicted order of size of laterality effect held except for small and almost certainly trivial reversals for FS- and FS+ left-handers. However, this study is but one of many where effects are less clear. It is not certain that FS+ has any detectable effect." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 146)

"The possibility that patterns of lateral specialisation might be related to musical aptitude rather than musical experience was suggested by findings of Gaede, Parsons, and Bertera (1978). Subjects classified for low and high musical experience and low and high musical aptitude (assessed through tests of musical memory and aptitude) were compared for the perception of musical chords and musical sequences presented monaurally to either ear. Over all subjects, errors were fewer for chords presented to the left ear and for melody sequences presented to the right ear. The most interesting apsect of these findings was that low-aptitude subjects (even if they had had 5 or more years of music lessons) showed larger differences between ears than high aptitude subjects. They conclude by suggesting, "that low aptitude for music may be the result of adherence to a rather rigid hemispheric strategy whether analytic or sequential (left hemisphere) or 'holistic and synthetic' (right hemisphere). Conversely, high aptitude for music may be based on the brain's ability to use both strategies flexibly" (Gaede, Parsons, & Bertera, 1978). If they are correct in these inferences, Gaede and colleagues are suggesting a way in which differing patterns of cerebral specialisation, in the sense of a capacity for the hemispheres to cooperate rather than to compete, may have implications for ability. Other studies have found musicians to have no over-all bias to either side but to be more variable than nonmusicians (Gordon, 1980; Morais, Peretz, Gudanski, & Guiard, 1982). (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 149)

"The weaker lateral differences observed in females could result form the use of more consistent verbal stategies for the processing of material presented to both hemispheres in females than in males. The weight of evidence points, in my view, to the conclusion that females are more dependent on the left hemisphere than males, not less. This conclusion is consistent with the evidence of female advantage in the early acquisition of language skills. It is also possible that female weaknesses in spatial reasoning result from attempts to solve all problems verbally, as suggested by Sherman (1978). This analysis of sex differences suggests that patterns of cerebral specialization may, indeed, have implications for the development of psychological functions." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 159)

"Schaller (1963) detected a bias to use the right hand first in the chest-beating displays of gorillas. This is a highly automatic ritualised behavior that need have no relevance to hand skill." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 171)

"The only vertebrate for which an unequal distribution of limb preferences seems to have been reported is not a mammal but a bird. Parrots often prefer the left claw in holding food (Friedman & Davis, 1938, cited in Walker, 1980). These data are of great importance in considering the origins of human hand preference. It is clear that mammals can show strong side preferences. The development of handedness in individuals is part of the mammalian (including primate) pattern. There is a major difference form the human distribution of handedness, however, in that no species bias has been discovered to the right side ( or to the left side) in any mammal other than man." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 173)

"These studies imply that handedness does not seem to be a noteworthy characteristic for most right-handers and that under-reporting of left- and mixed-handedness in relatives is more likely to occur in right-handed than left-handed informants. Biases such as these must be considered when analysing data for handedness in famileis (Chapter 6)" (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 186)

"Only 3 to 4 per 1,000 claim to write with either hand. That is, true ambidexterity is very infrequent and much less common than changes of preference between tasks." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 186)

"An symmetry that should be noted, although not studied in my student samples, concerns fingerprints. Rife (1955,1978) studied large samples of students and found slight but consistent differences between right- and left-handers. Both right- and left-handers tend to have certain patterns more often on the left than on the right palm, but this asymmetry was smaller in groups of left=handers than right-handers. Jantz, Fohl, and Zahler (1979) counted the numbers of finger ridges on each side (radial and ulnar) of all 10 fingers in right- and left-handed males and females. There were very few significant differences between handedness groups, but there were consistent trends for right-handers to have a higher and a more variable ridge count than left-handers in both sexes. Correlations between counts were higher for right-handers than left-handers, both male and female. It is not easy to interpret these findings, but Jantz and colleagues point out that they imply that handedness is influenced by prenatal developmental factors, and these could be genetic or environmental." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp.

"There is no reason to believe, on present evidence, that the distribution of L-R skill differs between Oriental and Western samples (Ashton, 1982), or that there are differences in extent of right shift." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 258)

"the hypothesis that must immediately leap to mind is that it is something to do with the human tendency to develop speech in the left cerebral hemisphere. No other primate is capable of speech in the full sense of human vocalisation of meaningful sounds. Females have a slight advantage over males in the rate of speech acquisition, and they are much less likely to suffer delays and handicaps in the acquisition of speech and reading. Some boost to the left hemisphere's growth or organisation, which made it readier to serve speech, might incidentally increase the skill of the right side of the body in motor functions. The fact that some people are right-brained or bilateral for speech could arise in just the way suggested for handedness---accidental variation in the absence of the factor inducing left-brainedness. There must be some genes that man does not share with the rest of the animal kingdom, and what could be more probably than that they have a role in human speech?" (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 259)

"If right-brainedness arises, in most cases, as a natural varient in those who lack the right-shift factor, and if in such cases each hemisphere is equally likely to serve speech, the size to the RS- proportion of the population can be inferred to be twice the proportion of right-hemisphere speakers. An estimate of this proportion is offered by the data in Table 14.4 for incidences of right-hemisphere lesion. Omitting the data of Penfield and Roberts for the reasons explained, there is a total of 647 dysphasics, of whom 9.27% suffered unilateral right-hemisphere lesions. This suggests that 18.54% of the population are RS-, and this estimate is used to infer the frequency of the RS- gene in Chapter 15. It is important to ask how much confidence can be placed in this estimate of right-braindedness." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 271)

"Figure 15.1 shows the distribution of L-R times in the 115 children. The hatched distribution shows the times of 20 children with one or more parent whose left-handedness was judged possibly pathological (based on a clear history of personal birth complications or abnormally slow right-hand time on the peg-moving task, and the absence of a close left-handed relative). Table 15.4 gives the L-R mean and also the percentage of children with a bias to the left hand for peg moving, in children with and without a pathologically sinistral parent. The former were biased to the right hand as the controls in Table 15.3. The 95 children whose parents did not fulfil any of the criteria of pathological sinistrality above were not significantly biased to the right hand, and 42% showed a left-hand advantage for peg moving. Another analysis for evidence of genetic influence was possible in the combined L X L samples. This was an examination of the effect of presence of left-handers among the parents and siblings of the left-handed parents. Parents with a close left-handed relative would be more likely to be RS- than parents without such a relative. Table 15.4 gives the L-R means of children classified for presence or absence of left-handed relatives of the parents. The 33 children of families where neither parent had a close sinistral relative was significantly biased to the right hand, and only 30% showed a peronal bias to the left hand. None of the other groups were significantly biased to the right hand, and 39 to 50% were personally biased to the left hand. This analysis demonstrates that the bias to the right hand in the children differs according to the handedness of the parents' relatives, although all the parents were phenotypically left-handed and all the children had the experience of being reared in a L X L family." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 288-290)

"If the RS factor is responsible for the bias to the left hemisphere for speech, as argued in Chapter 14, it clearly does not cause human speech but merely increases the chances that speech will depend on the left hemisphere. The shortness of the evolutionary time scale, the relative paucity of genetic differences between man and chimpanzee (Chapter 1), and the limited role of the RS factor all argue for some very simple agent for the RS. Parsimony alone should urge us to look for a simple mechanism. Very small changes in genotype can have major consequences for individuals. Changes that seem to be of great evolutionary significance from the human viewpoint could well depend on very small modifications of the genetic code. This section argues that the RS could depend on a single change at one gene locus (single allele). It is hypothesized that when this allele is present on one or both chromosomes, some advantage is likely to be conferred on the left cerebral hemisphere, which tends to induce speech on that side and incidentally increases the skill of the right hand in comparison with the left hand. When the right shift allele (rs+) is absent on both chromosomes, the alternate allete(s) at that locus (rs-) are indifferent or neutral for speech and handedness." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 291-292)

"One of the striking findings for children and parents in L X L families is an absence of sex differences in hand preference and L-R skill." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 299)

"All of these observations point to the hypothesis that sex differences for handedness occur in those likely to carry the rs+ gene but are absent in those of rs- - genotype. That is, sex differences are a function of the gene when it is present but absent when the gene is absent. The absence of sex difference is not just a function of left-handedness; the children of L X L parents included more right- than left-handeders but still showed a lack of sex differences. (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 302)

"If development was slower (in twins or in males), differences between the hemispheres might be reduced. Chi, Dooling, and Gilles (1977) found, in a study of fetal brain growth, that twins were delayed by two of three weeks in the appearance of cerebral convolutions in comparison with nontwins. No difference were observed in this study between the sexes. " (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 304)

"It may be concluded that twinning and sex are associated with differences in neonatal maturity, in rate of language acquisition, and in incidences of right-handedness. All of these differences are consistently in the direction expected if the expression of the rs+ gene is correlated with neonatal maturity; that is, greater in females than males and greater in the singleborn than in twins. The postulate that twins have a reduced right shift now seems an essential part of the RS theory and not at all "ad hoc". (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 305)

"Mismatches between observed and predicted numbers of females were largely due to greater numbers of left-handed females in the families of left-handed children of left-handed mothers, and whether there is a true excess of left-handed children of left-handed mothers, and whether this applies especially to daughters, was explored. There was no clear evidence for this excess when data depended on self-report. It is possible that the apparent excess in mixed-report data is due to under-reporting of the true incidence of maternal left-handedness. When incidences of parental handedness were "corrected" by small adjustments of percentages (up to 3%), substantial improvements of fit were obtained." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 335)

"What could the disadvantages of the rs - - and rs + + genotypes be? From first formulating the RS theory (Annett, 1972) it was expected that the right shift induced speech development in the left hemisphere, and that those who lacked the right shift might be at some disadvantage for speech and language development." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 338)

"The most unexpected and dramatic outcome of this analysis is the finding that left-handers have an absolute advantage in hand skill and that the mechanism of action of the rs + gene might be to impair the function of the right hemisphere. This is a very important contribution to the hypothesis of a balanced polymorphism and to an understanding of the findings for mathematicians described in Section 17.2." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 347-8)

"This analysis of relations between mathematical ability and laterality strongly support the idea that genotypes of the rs+ locus differ in potential for certain intellectual skills. The study was expected to demonstrate an advantage for the rs - - , but it led to a demonstration of a disadvantage for the rs ++. The evidence is in accord with the possibility that strong biases to the left hemisphere and right hand associated with the rs ++ genotype are detrimental to mathematical thinking. The stronger expression of the rs + gene in females than males is also consistent with the universal observation that females are less likely to show mathematical ability than males." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 354)

"The RS theory suggests that a substantial minority of the population lacks some boost to the development of language skills enjoyed by the majority. In the 18% of the population hypothesized to be rs - -, patterns of cerebral specialization are expected to depend on chance." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 358)

"This pattern of findings was not anticipated on the initial hypothesis that developmental language problems are associated with the rs - - genotype, but it is in accord with the more general hypothesis that the most advantageous genotype is the heterozygote and that both homozygotes are at risk. The main clues found in Chapter 17 as to the risks associated with the rs ++ genotype were that the strongly dextral have slow left-hand speeds and are likely to be absent among mathematicians. Do dyslexics show poor left-hand speeds, either as a group or only among the strongly dextral? Can dyslexics at the left and the right sides of the distribution be differentiated for hand skills, or for any other characteristic such as clumsiness or mathematical ability?" (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 366)

"This detailed comparison of dyslexics and birthday sample controls leads to the conclusion that dyslexics are not abnormal for peg-moving times but that they are more likely to include individuals at both extremes of the normal range, while those in the centre of the range are underrepresented." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 367)

"In the general population about 18% are expected to be rs - - and about 32 % rs + +. The dyslexic sample was not compatible with the sum of these two distributions in the same relative distributions if the proportions were about equal or in favour of the rs - -. For example, if the proportions of rs - - and rs + + were taken to be 60:40, the means as in Table 16.5, and the standard deviation of both distributions as 8, the D value obtained was .035. This close fit demonstrates that the dyslexic sample could consist of these two genotypes, but of course it does not prove the hypothesis correct." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 369)

"The small number of male dyslexics who described themselves as not enjoying making things showed extremely strong dextral biases. ... If strong dextrality is associated with a limitation of right-hemisphere function, and the right hemisphere has a special role in visuo-spatial skills, certain kinds of clumsiness would be associated with strong right-handedness, as suggested here." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 369-370)

"The rs + gene is hypothesised to facilitate left-hemisphere speech. If the mechanism of gene action gave direct advantages to language growth, it would seem to follow that the rs + + should have especially well-developed language skills. However, the findings for strongly right-handed dyslexics (Chapter 18) introduced the unexpected possibility that the rs + + might have problems in learning to read. This seemed very surprising while the gene was thought to facilitate language development, but no longer surprising if the mechanism of gene action is to impair right-hemisphere function. Many avenues for further research are opened up by these speculations. The hypothesis of a balanced polymorphism with heterozygote advantage suggests that within the school population, children of rs + - genotype should be especially well equiped for the acquisition of all language skills. These children probably learn in school relatively easily, while children of rs - - and rs + + genotype are more likely to have learning problems." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 379)

"The possibility that the rs + gene might be associated with risks to capacities for skilled performance was suggested when the single-allele hypothesis was first formulated (Annett, 1978b), and it was later discovered that left-handers have absolute advantages in hand speed over right-handers (Section 17.1.2). This was clear for the nonpreferred hand of both sexes in both groups of samples; male left-handers in the combined main samples also showed faster preferred hand times than mixed- and right-handers (Figs. 17.2 and 17.3). These observations, and the analyses for mathematicains and dylexics, led to the idea that the gene promotes left-hemisphere specialisation for language through right-hemisphere impairment. Skilled performance might be handicapped by right-hemisphere impairment in several ways. There could be defects in capacities for visuo-spatial thinking, for the fine control of both hands, or for fast reactions to stimuli on both sides." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 382)

"If it should be confirmed that in outstanding (star) tennis professionals, the incidence of left-handedness is about 30%, it would sugest that these players are more likely to be of rs - - genotype (see Table15.2). This would imply that any impairment of the right hemisphere, even in male heterozygotes, is associated with some loss of potential for skilled sensorimotor performance." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 384)

"Leiber and Axelrod (1981b) found that in parents of undergraduates there were signigicantly more sinistrals among those of higher rather than lower educational and occupational status. The higher proportion of left-handers, and also of mixed-handers, was observed for fathers and mothers, but statistically signigicant for the former only." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 387)

"Both of these analyses of L-R differences found the Maya considerably less biased to the right hand than the Ladinos, and in both cases the English bias in intermediate. In comparison with my English samples, the Ladinos show a stronger right shift, and the Maya a slightly lesser right shift. These differences concern the L-R differences between the hands in skill, which are presumed to underlie observed perferences. So far in this book, it has been assumed that differences between racial groups in hand preferences for writing and other skilled actions are due to cultural pressures that affect the threshold at which sinistraity is manifested. If the present observations for differing z scores on measures of skill are confirmed in further research, the possibility must be considered that there are racial differences in right shift. Racial differences could occur either because there were differences in the frequency of the rs + gene or because there were differences in the expressivity of the gene. Differences in expressivity have been postulated between the sexes and between twins and the singleborn, and these have been tentatively attributed to differences in relative growth and/or maturity at birth. Cross-cultural studies of neonatal maturity and development in infancy have reported several differences between racial groups, but the evidence is insufficient to reach any firm conclusions about the relative rates of maturation of Landinos and Mayan children (Super, 1981). Assuming a lesser right shift is associated with lesser maturity at birth (as postulated for males and twins), the Maya would be expected to be slightly less mature than Landinos. Studies of Maya Indians in Mexico (Brazelton, Robey, & Collier, 1969) and of Hopi Indians in the United States (Dennis, 1940) both concluded that development in infancy paralleled that of Caucasian children but was approximately 1 month delayed; this delay was found in the latter sample that could be relevant to the expression of the rs + gene. The newborn observed were very small (about 5 lbs.), though not premature. On tests of development, items concerning quality of vocalisation could not be scored because of the paucity of social babbling. The main aim of the mothers seemed to be to keep the infants quiet, but it seems remarkable that the voice play that is so evident in Caucasian children in the second half of the first year should be so reduced." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 394)

"A surprising left-ear dichotic listening superiority was reported for a group of native Navajo college students (Scott, Hynd, Hunt, & Weed. 1979)" (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 394)

"Faster times for left-handers were found in Maya and in Landinos children, as in English children. Thus the assumptions basic to the balanced polymorphism hypothesis, that the bias to the right hand gives advantages to language learning but at the cost of actual hand speed, is supported by this independent cross-cultural study." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 396)

"The important point with regard to speech sounds is that learning is likely to be more efficient if feedback from the vocal tract and from the ear are on the same side of the brain. Sensorimotor feedback from the mouth and from the ear presumably must be co-ordinated if speech production and perception are to be efficient. Studies of left-hemisphere language through electrical stimulation mapping have found that phoneme discrimination and the control of mouth movements tend to depend on the same cortical areas (Ojemann & Mateer, 1979); see Section 2.4). If sensory information from the mouth and from the ear were received on opposite sides of the brain, coordination would depend on connections via the corpus callosum, and the use of this longer route might hinder the infant's discovery of some of the subtleties of speech timing. There is considerable variation between infants in the quantity of voice play during the first year; some, but not necessarily all, of this variablility may be associated with effects of the rs+ gene. Ramsay (1980) found that infants who used left-hand reaching strategies were delayed in comparison with infants using right-hand stategies in producing combinations of phonemes in speech." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 400)

"Why the left side should be especially fitted, in most people, to serve the speech output-input loop is not certain, but it seems likely that it depends on the enlargement of the planum temporale in the last quarter of fetal life, in about two out of three brains. Evidence has been found for bias towards a larger motor speech area on the left also (see Section 6.3). Somatosensory mouth areas have not been compared on two sides, to my knowledge. The main point for the RS theory is that the close proximity of the mouth and hand areas in the sensorimotor cortical strip suggests that any advantage to the left-hemisphere mouth area would be likely to give an incidental advantage to the right hand. Alternatively, if the gene works by inhibiting the growth of right-hemisphere mouth cortex, there would be an incidental inhibition of the growth of the sensorimotor cortex for the left hand. Comparisons of the cerebral hemispheres of fetal brains were made by Cunningham (1902) for evidence of superior growth of the left motor cortex in the arm area; the finds were opposite of those expected in that there was a slight advantage to the right motor cortex, which Cunningham believed was detectable in ape as well as human brains, though less marked in former. Chi, Dooing, and Gilles (1977) also found that fetal brain development was slightly faster on the right than the left side. Cunningham noted, however, that in four out of five brains belonging to the eighth month of fetal development there was an excess of growth on the left side. This asymmetry of fetal development has not been remarked upon, to my knowledge, by any more recent observer. Whether the gene works by accelerating or decelerating growth on the right or the left, some in equality is introduced, which biases the random chances of superiority on either side in favour of left-hemisphere speech and preference for the right hand." (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 402-3)

"However, Moffat and Hampson (1993) have found that salivary testosterone levels are significantly lower in left-handers than in right-handers. While circulating testosterone levels in adults may not correlate well with fetal exposure to testosterone, these data provide suggestive evidence against the Geschwind hypothesis --- one would expect higher, rather than lower, levels of testosterone in left-handers." (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 151)

"The regulation of the growth processes through which these relative advantages and disadvantages are produced in those carrying the rs + gene varies with any factor that influences growth. The maximum asymmetry in favour of the left hemisphere is expected to occur in females who carry a double dose of the gene (rs + +) and who are early maturers. Lesser asymmetries are expected in those of rs + - genotype, in males, twins, and any whose developmental progress is delayed or distorted by extraneous factors. Thus, the incidence of right-handedness should be highest in early-maturing girls and lowest in late-maturing boys. It should be relatively easy to test this prediction. Evidence for an advantage in verbal compared with spacial test scores in early developers and the reverse pattern in late developers has been reported by Waber (1976). A review of the development of verbal and nonverbal abilities of children with sex chromosome anomalies (XO XXY, and XXX sex chromosome complements) has led to the view that the critical variable may be maturation rate. Netley and Rovet (1983) tentatively suggested that the pattern of findings would be consistent with the possibility that rapid development gives relative advantages to verbal skills and deficits of spatial skills, while late development gives the reverse pattern. This is almost, but not quite, the suggestion made by the RS theory. Early maturers are expected to show stronger expression of the rs + gene than late maturers; this implies a stronger bias to the left hemisphere for speech but at the cost of some weakening of the right hemisphere. Early maturers should show rapid speech acquisition but poorer visuo-spatial skills. Later maturers risk poorer development of the speech control mechanisms but avoid the risks of right-hemisphere impairment of early maturers. In normal development, these relative advantages and disadvantages are probably so slight as to be of little practicle significance. ... Outstanding achievement in any area that requires maximum efficiency on both sides of the brain, for skilled control of the hands (as in surgery or in playing musical instruments), for sports (as in tennis, football, and gymnastics) and for the application of spatial and mathematical concepts is unlikely in those who carry a double dose of the gene (rs + +). ... The sexes differ slightly for effects associated with the rs + gene because the gene is expressed more strongly in early maturers and because females tend to mature earlier than males. It would be as rediculous to suppose tht the rs+ gene is the only factor involved in sex differences in ability as it would be to suppose it is the only factor involved in musical or sports ability. " (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 404-6)

"Urion (1988) compared relatives of 100 language-disabled boys with those of 100 control boys. Data on handedness of the probands were not presented, but the two control groups of relatives did not differ with respect to either handedness or autoimmune diseases. Urion went on to divide the language-disabled sample into two groups, with a high, or low, proportion of non-dextral relatives. The first group had a much higher incidence of autoimmune diseases in their relatives, leading Urion to suggest that there may be one familial subtype of language disorder linked with non-dextrality and autoimmune disorder." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 155)

[abstract] Schizophrenic symptoms are conceived as arising from inter-individual variability in the distribution of those fibres that connect asymmetrical regions of the hemispheres related to language. Language (a bihemispheric phenomenon) arose as a result of a genetic change that allowed the two hemispheres to develop with a degree of independence. One component, the phonological output sequence, became localised to the dominant hemisphere, interacting through the corpus callosum with other component functions, including the associated meanings, in the non-dominant hemisphere. Nuclear symptoms are a consequence of failure of segregation of these two functions. This failure is associated with abnormal connectivity between the hemispheres and relates particularly to those regions that are late developing and differ between the sexes. (Crow TJ (1998) Schizophrenia as a transcallosal misconnection syndrome. Schizophr Res 1998 Mar 10;30(2):111-114)

"It is possible to distinguish at least three theories that predict an increase in the rate of non-right-handedness in children with developmental language disorders. We may start with the theory of atypical cerebral lateralization, first proposed by Orton, and discussed at length in Chapter 11 in relation to specific reading difficulties. This theory maintains that adequate language functioning depents on verbal operations being predominantly mediated by the left cerebral hemisphere. Non-right-handedness is seen as an indirect manifestation of atypical cerebral lateralization, and so should be unusually common in lanuage-impaired children. Annett's right shift theory, also reviewed in Chapter 11, may be regarded as a variant of this theory. According to Annett and Kilshaw (1984) we would expect children who lack the right shift factor (who do not have strong left hemisphere language superiority, and do not show large skill differences between the hands) to be at particular risk for developmental language disorders. The theory also predicts that extreme right-handedness (where weak left hand skills are indicative of poor right hemisphere functioning) might be common in this group. The second theory to consider is that of pathological left-handedness. At first glance, it seems plausible that specific language difficulties in children, like acquired aphasia in adults, might arise as a consequence of a focal lesion of the left cerebral hemisphere. Such a lesion might also impinge on brain areas concerned with motor function, so raising the probability that a naturally right-handed individual will shift to become left-handed. The third possibility is that children with specific language disorders may show increased randomness in their hand preferences. The notion of increased randomness was first introduced in Chapter 9 in the context of mental impairment. According to this theory, reliable and consistent hand preference for an activity is a function of motor skill, which will depend both on practice and on neurological status. Increased randomness might be predicted in children with specific developmental disorders as well as in those with more global mental impairment, in so far as these disorders are associated with neuromotor immaturity." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 131)

"The studies on lateralization of motor and language functions in this population caution against equating morphological brain symmetry with lack of language lateralization. Rather, just as with dyslexic children, it seems likely that language-impaired children have verbal functions lateralized to the left hemisphere, but this hemisphere is not sufficiently well-developed to process language adequately." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 136)

"A link between gender and handedness is widely though not universally accepted. Annett's (1985) review of the literature confirmed that, although some studies have failed to find sex differences, most large-scale investigations do report a slight preponderance of males among non-right-handers. Her own work using peg-moving to assess relative skill of the two sides found that, on average, females show a more pronounced superiority of the right hand over the left than do males." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 146)

"Annett's (1975, 1985) right shift theory (see Chapter 3) leads to much more precise predictions than other theories concerned with cerebral lateralization, because handedness is treated as a condition variable that can be measured in terms of relative proficiency of the two hands. According to this theory, handedness is determined by environmental factors interacting with genotype. Individuals who are homozygous for the right shift allele (rs + +) will be very likely to show both left hemisphere language representation and right-handedness, because this genotype strongly boosts early left hemisphere development." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 127)

"On closer inspection of their data, Annett and Kilshaw found that hand difference scores for right-handed dyslexics were more extreme than those of controls. This finding led them to reformulate the theory to propose that both types of homozygote, rs - - and rs + +, are at risk for learning disabililties. Annett's original account suggested that overcommitment to left hemisphere languge processing (in rs+ + individuals) might result in selective impairment of right hemisphere visuospatial skills. However, the dyslexia study of Annett and Kilshaw and a later study of intellectual development by Annett and Manning (1989) indicated that verbal as well as visuospatial development tends to be poor in those who are very strongly right-handed. Annett has argued that the disadvantages in intellecutual development associated with both types of homozygote can explain why the rs- gene is maintained in the population." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 128)

"What would give greater credence to Annett's genetic theory of dyslexia would be a demonstration that strongly right-handed and non-right-handed dyslexics had different types of reading problem. .... Although atypical cerebral dominance seems likely as a cause of dyslexia, the related idea of a neuromaturational lag is more attractive. On this view, lack of strong handedness is not a sign of bilateral language mediation, but of neurological immaturity. Unfortunately, reliance on handedness inventories in the majority of studies makes it difficult to test this notion against published data. As argued in Chapter 5, inventories confound cases where there is a delay in establishing hand preference and those where there is a stable mixed or left hand preference. It may be that by failing to distinguish these different subgroups of non-right-hander, links between handedness and developmental dyslexia have been obscured. If dyslexia reflects neurodevelopmental immaturity, then it is only unstable hand preference that is relavant. Although his sample size was small, Harris's (1957) finding of excess mixed-handedness among 7-year-old but not 9-year-old poor readers is compatible with the notion that a lag in establishing hand preference may be an indicator of neuromotor immaturity in dyslexics." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 129)

"If the results form these three studies are combined, the overall proportion of left-handers is 13.3 per cent for autistic children and 8.3 per cent for matched controls, not a significant difference. However, if left- and mixed-handers are summed, then the frequency of non-right-handedness among autistic children is considerably higher than that found in age-matched normally developing children although it is similar to that found in other children of the same intellectual ability." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 111)

"Evidence against a genetic predisposition to left-handedness in autism came from studies by Boucher (1977), Tsai (1982) and Fein et al. (1985), all of which found that the overall rate of left-handed relatives in autistic individuals was similar to expected population values, and non-right-handers and right-handers had similar numbers of left-handed relatives." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 111)

"A second, theoretical, point is that even if we accept that there is a significant excess of non-right-handedness in dylexia, the percentage is well below that which would be expected if weak cerebral lateralization were the major cause of dyslexia." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 125)

"The data reviewed so far point toward an explanation of increased non-right handedness in autism arising as a consequence of generally poor motor functioning which results in a failure to learn the types of motor skills for which hand preference is normally shown. Lack of hand preference rather than stable left-handedness is what differentiates autistic from normal children. Neither pathological left-handedness nor a genetic predisposition towards decreased cerebral lateralization seem able to account for available data." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 113)

"Another possibility is that there may be developmental changes in language processing in autism: in this study, evidence of left hemisphere language functions was strongest in the older subjects." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 115)

"Interest in handedness in Rett syndrome was fuelled by a report by Nomura et al. (1984) who noted that out of 11 girls with the syndrome five were left-handed, one was right-handed and five had never been observed to show a preference. Handedness ceased to be apparent as children grew older and hand function declined so that the child could no longer grasp objects. Olsson and Rett (1986) studied handedness is 33 children with Rett syndrome. A range of toys and foods was presented, with hand preference being coded if one hand was used for grasping at least three times more often than the other. Olsson and Rett reported a striking difference between children aged above and below 7 years. For the younger group, there was a confirmation of the findings of Nomura et al., with nine out of the 14 girls using the left hand more than the right, one using the right more than the left, and the remainder showing no hand preference. However, for those aged above 7 years, only one out of the 12 preferred the left hand, whereas nine out of 12 preferred the right. These older children had evidence of asymmetical pathology affecting the upper limb, with the left side more abnormal than the right." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 116)

"In a mata-analysis of published data, Searlman et al. (1989) found that certain indices of birth stress (e.g. low birthweight, rhesus incompatibility, caesarian delivery) were associated with decreased right-handedness, but effects were tiny and only reached significance with the huge samples generated by combining data from many sources. The clearest evidence for a link between left-handedness and perinatal condition comes form studies of infants of extremely low birthweight. O'Callaghan et al. (1987) reported that 21 out of 39 children with birthweight below 1000g were left-handed at 4 years of age, compared with 8 per cent of other infants admitted to intensive care. None of the very low birthweight group had cerebral palsy. Ross it al. (1987) found that while 80 per cent of a full-term control group were right-handed, only 63 per cent of perterm children with very low birthweight were, this being a statistically significant difference. Furthermore, the two groups did not differ in the distributions of parental handedness. Only six children in this sample had asymmetry of body tone, but four of these were left-handed. In addition, within the preterm group, the non-right-handed children had significantly lower IQ's impaired expressive language and a higher frequency of articulation defects relative to right-handed children. Although this study appears to provide strong support for the notion that very preterm children are vulnerable to brain damage affecting handedness, it should be noted that not only was left-handedness more frequent in this sample, but so too was mixed handedness." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 90-1)

"Furthermore, those left-handers with poor non-preferred hand scores had fewer left-handed relatives than other left-handers. In general, children with poor scores of the non-preferred hand were more likely to have had a history of neurological impairment, and obtained lower scores on tests of intellectual function, than did other children." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 98)

"By fitting the model shown in Figure 7.5 to data obtained with this large sample, it was estimated that it follows that around one in 20 of all left-handers are pathological left-handers, and just over one-third of left-handers with very poor non-preferred hand skill are pathological left-handers. This study indicated that it is not implausible to postulate that an increase in left-handedness due to pathological influences may be found even in populations where frank neuromotor abnormality is not evident. ... We need to develop better ways of distingishing pathological from non-patholgical left-handers. Familial sinistrality, the most widely used index, yields such a high rate of misclassification as to make it worthless. Hypotrophy on one limb, strength of hand preference and poor motor skill of one side are all promising indices." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 100)

"Furthermore, where a distinction has been made between mixed- and left-handedness, it is clear that in general it is lack of hand preference rather than left-handedness that is particularly common in mentally handicapped people (Dart 1938, Porac et al. 1980)." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 106)

"Teng et al. (1976) pointed out that cases of pathological left-handedness will constitute a higher proportion of the population of all left-handers in countries such as China where there is strong social pressure to be right-handed. They found that twinning was associated with both decreased right-handedness and reduced college entrance is a large Taiwanese population, which is consistent withthe view that there is an excess of left-handedness among twins due to the influence of pathological factors. This suggests that if we could find some independent way of discriminating between pathological and natural left-handers in Wesern cultures (see p.95), we might be able to obtain clearer evidence for different distributions of handedness in twins and singletons. However, against this view is Annett's (1985) finding of a significant excess of left-handers among twins as compared to singletons even when probable pathological left-handers (those scoring more than two standard deviations below the mean on a test of manual dexterity) were excluded from consideration." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 36)

"Cohen and Forget (submitted for publication) compared right-handed men, women, and genetic male transexuals on verbal and nonverbal dichotic tasks to investigate relations between hormone therapy and auditory cerebral specialization of speech and non-speech stimuli in adults. In verbal tasks, the usual REA obtained for all subject groups; in nonverbal tasks, results revealed a left ear advantage in the processing of nonverbal information for men only; women and transsexuals exhibited similar performances in nonverbal tasks. Cautious interpretation of the data is some possible hormonal involvement, in adults, in the modulation of right hemispheric cognitive processing. In light of the inherent difficulties of isolating the early influence of testosterone, the studies that effectively submit a test of the GBG hypothesis are those of Grimshaw, Bryden, and Finegan (1993), and Grimshaw, Niccols, and Finegan (1990) who attempting to determine whether dichotic listening performance and frequency of occurance of left-handedness were correlated with levels of testosterone measured prenatally, during amniocentesis, in populations of children. These two studies do not support GBG's claim that higher levels of testosterone are associated with reduced left hemisphere dominance. If anything, the reverse appeared to be true." (Forget, H. & Cohen, H. (1994) Life after Birth: The Influence of Steroid Hormones on Cerebral Structure and Function is Not Fixed Prenatally. Brain and Cognition 26: 247)

"Indeed, there is evidence that at 19 to 32 weeks gestation the development of various sulci and gyri is delayed by two to three weeks in twins compared to singletons (Gilles et al. 1983). Annett noted that language development is often delayed in twins and she argued that this too was an indication of delayed maturation. It would be interesting to see whether a raised frequency of left-handedness is particularly striking among twins who are late to talk... ...Boklage (1981) argued that non-right-handed parents are more likely than right-handed parents to produce twins. He asked parents of twins to specify the handedness of themselves, their twins and other family members. A singleton control group was not used, but the rate of left-handedness reported in twins (around 20 per cent for both MZ and DZ) was higher than in studies assessing singletons by comparable means. Furthermore, the rate of left-handedness in first-degree relatives was not significantly different from that in twins (18 per cent in brothers, sisters and fathers, and 13 per cent in mothers). Much lower rates of left-handedness were reported in second-degree relatives of twins." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 38)

"In our sample, LH or ambidextrous individuals tended to have lower, not higher, T concentrations. However, this does suggest that some relationship between T and hand preference may in fact be present, and it will be important to work out the mechanisms that account for the difference in free T, as these may eventually shed light on the ontogeny or correlates of hand preference." (Hampson, E. & Moffat, S.D. (1994) Is testosterone related to spatial cognition and hand preference in humans? Brain and Cognition 26: 262)

"In contrast, there were significant relationships between the handedness of children and that of their biological parents. Furthermore, the proportions of non-right-handers were similar for those with a non-right-handed mother and those with a non-right-handed father. This rules ot explanations in terms of very early non-genetic maternal influences, such as maternal carrying or feeding practices or prenatal environment of the fetus. The finding that biological relationships are more important than postnatal experiences points towards a genetic explanation." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 34)

"Studies of the parietal and temporal lobes have shown that areas of the left hemisphere known to be involved in language comprehension are typically larger than the corresponding regions on the right. An early observation was that the Sylvian fissure is of greater length on the left than on the right and usually ends at a higher position on the left. The region of the temporal lobe known as the planum temporale (Fig. 2.3), which is part of Wernicke's area, is significantly larger on the left side than the right in about 70 per cent of people, and if one averages across all individuals, the left planum temporale is 40 per cent larger than the right. Furthermore, the same type of asymmetry has been shown in radiographic examination of the skulls of primitive man. Asymmetry of the planum temporale has been found in fetal brains and this has been used as evidence that the left hemisphere specialization is present before birth." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 27)

"One hundred thirty-six alcoholic men and 48 alcoholic women admitted consecutively to an adult alcohol and substance abuse unit were studied in an attempt to replicate previous reports of an association between alcoholism and handedness. Each individual received a structured admission interview, and handedness was determined by a modification of the Edinburgh Inventory. Left-handedness was more frequent in men with alcoholic fathers and in first-born men. These data are discussed in the context of a recent theory relating left-handedness, immune disease, developmental learning disorders, and fetal testosterone." (London WP, Kibbee P, Holt L (1985) Handedness and alcoholism. J Nerv Ment Dis 173(9):570-572)

The relationship between cerebral lateralization and dichotic ear asymmetry, while significant, is far from perfect, and the proportion of right-handers who show a left ear advantage for verbal stimuli (suggestive of right hemisphere processing) is around 20 per cent, far greater than the estimates based on neurological data of the proportion of right-handers with right hemisphere speech (Satz 1977)." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 25)

"However, more recent reports have revived interest in this theory. Churchill it al. (1962) investigated hand preference in over 1000 2-year-olds whose birth position had been recorded as left or right occiput anterior. They found a small but significant association between handedness and position at birth (Table 1.II). Handedness was also related to parental hand preference, but the association between child handedness and birth position remained significant even after excluding those with left-handed parents. A weak trend for a link between head position at birth and handedness at 7 years was confirmedfor boys but not for girls in the Collaborative Perinatal Project (Ehrlichmann et al. 1982). Churchhill et al. considered a range of explanations for this association, noting that asymmetrical damage to the brain during delivery may be more important than asymmetrical position in utero." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 15)

"Dawson (1972, 1974) has pointed out that hunting and fishing cultures, such as Eskimo and Australian Aborigine peoples, tend to show higher rates of left-handedness than agricultural communities, such as the Temne and the Chinese Hakka, and he suggested that this may be accounted for by the emphasis on independent values and the relatively low degree of conformity found in nomadic groups." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 13)

"The highest proportion of left-handedness that I could discover from a reliable source was for the Kwakiutl Indians of British Columbia, 17 to 22 per cent of whom were left-handed or ambidextrous for writing (Marrion 1886). [fornote says 'Marrian reported that no fewer than 6 per cent of Kwakiutl Indians could write with either hand. However, there is no tradition of written languge in this culture, and many adults do not write after leaving school. Marrion (personal communication) noted that many treated writing their name as an activity akin to drawing.'" (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 12)

"It is undisputed that in the fetal environment, testosterone can have profound effects on neonatal brain development. In animal studies, when hormonal levels have been controlled experimentally, many developmental processes are affected by exposure to testosterone, and thus lead to anatomical differences between males and females (i.e., sexual dimorphism.). Most of these are mediated by estrogen, from which testosterone is converted by normal enzymatic operations involving aromatase, a catalytic enzyme found within the brain and expressed quite early in development (McEwen, Lieberburg, Chaptal, & Krey, 1977). Ironically, the role of the female hormone estrogen in this process occurs to a significant extent only in males and not females, since the estrogen secreted by the developing ovaries never reaches the brain. High levels of alpha fetoprotein in the neonatal serum bind these estrogens and prevent their access to the central nervous system. By contrast, estrogen has an effect on brain development in males, since conversion of testosterone to estrogen takes place within the developing brain itself." (Small S.L., Hoffman G.E. (1994) Neuroanatomical lateralization of language: sexual dimorphism and the ethology of neural computation. Brain and Cognition 26: 307-8)

"Although the spatial distribution of information in a visual scene lends itself naturally to parallel processing, the temporal distribution of information in language does not. In particular, language processing requires explicit sequential processing, from the levels of interpreting of producing auditory speech signals or visual graphemes to the levels of syntactic and semantic processing of full sentences. This is not to say that parallel processing cannot or does not come into play in the performance of such functions. In fact, in receptive language processing, many sounds and even words can be processed simulataneously. But there is inherent temporal order to the information, and realizing the interpretive significance of the temporal order or producing language with the appropriate temporal order requires sequential processing. Perhaps parts of the left hemisphere have some evolutionary advantage for sequential processing, even if not an absolute advantage. Certain information suggests that indeed this may be the case. Dichotic listening procedures constitute one of the most compelling lines of research supporting this idea. With similar phonemic characteristics of acoustic stimuli, but different rates of change of the formant transitions, the magnitude of the normal right ear advantage is dichotic listening tasks can be altered (Schwartz & Tallal, 1980). In particular, reducing the rate of change of the acoustic stimuli reduces the hemispheric asymmetry in processing language." (Small S.L., Hoffman G.E. (1994) Neuroanatomical lateralization of language: sexual dimorphism and the ethology of neural computation. Brain and Cognition 26: 306)

"Luria (1970) reported that aphasia was present in the acute state in nearly all soldiers with penetrating brain wounds in the primary speech areas in the left hemisphere. A year after their injuries the rate of recovery from these left-sided lesions was much higher among lefthanders and among righthanders with familial lefthandedness." (Geschwind & Galaburda 1987: 73, Cerebral Lateralization)

"Although it is not entirely understood how the gonadal hormones affect the development of the CNS, we know that the morphological and structural repercussions of perinatal exposure to these steroids are at the level of the cell body, axons, dendrites, and synapses. The consequences of this exposure on growth of neurites, as shown by the majority of studies, are largely facilitory or enhancing. A few studies have, however, reported inhibitory effects of testosterone. For example, in the male rat, the thickness of the cerebral cortex is apparently greater on the right side than on the left and removal of the testes at birth alters this cortical laterality. Furthermore, in the neonate male rat, concentration of estrogen receptors is greater in the left than in the right cortex (Diamond, 1991). Testosterone has also been implicated in influencing the size of specific areas of the corpus callosum (Fitch, Berrebi, Cowell, Schrott, & Denenberg, 1990). More recently, Witelson (1991) has shown that lower levels of androgens lead to less axon elimination in specific areas of the brain, apparently resulting in specific patterns of functional asymmetry. She found that left-handed men had a larger isthmus of the corpus callosum than right-handed men." (Forget, H. & Cohen, H. (1994) Life after Birth: The Influence of Steroid Hormones on Cerebral Structure and Function is Not Fixed Prenatally. Brain and Cognition 26: 244)

"In a study comparing pre- and postmenstrual dichotic performance in a large sample of women, the right ear advantage (REA) for auditory language-related stimuli was significantly more pronounced in the postmenstrual phase of the cycle (Altemus, Wexler, & Boulis, 1989). Changes over the menstrual cycle were also observed on visual cerebral asymmetries in lexical dicision and line orientation tasks (Chiarello, McMahon, & Schaefer, 1989). Moreover, shift of functional cerebral asymmetry in face perception was also associated with variations in the mentrual cycle (Heister, Landis, Regard, & Schroeder-Heister, 1989)." (Forget, H. & Cohen, H. (1994) Life after Birth: The Influence of Steroid Hormones on Cerebral Structure and Function is Not Fixed Prenatally. Brain and Cognition 26: 246) [T?]

"However, the story became very much more interesting when it is was discovered that dyslexics were overrepresented at both the left and right extremes of the laterality continuum (Annett & Kilshaw, 1984; replicated by Annett & Manning, 1990). The RS theory expects people who lack the rs+ gene for cerebral asymmetry (rs- genotypes) to be at risk for speech-based processes and thus children with weak phonological processing should be more frequently found at the left of the laterality distribution. This was demonstrated in two samples of normal children for handedness (Annett, 1992) and in undergraduates for dichotic listenig asymmetry (Annett, 1991). Thus the RS theory expects dyslexics to include an excess of mixed and left-handers only if they are dyslexics with weak phonology. But some dyslexics do not have weak phonology. They have other types of problem which arenot well specified but can be at risk because they carry two copies of the rs+ gene (rs+ + genotypes), because the mechanism of gene action is probably to handicap the right hemisphere (Kilshaw & Annett, 1983). Thus, dyslexics at the left and right extremes of the laterality distribution may have difficulties in learning to read for quite different reasons, one speech-based and one not speech based." (Annett, M. (1994) Geschwind’s Legacy. Brain and Cognition 26 (2): 239-40)

"Partially because of the inspiration provided by the GBG model, other cognitive neuroscientists have begun to consider how a host of prenatal factors may serve as precursors of functional hemispheric symmetry in humans. In the present section, I outline briefly some of the factors that have been considered and the scenarios to which they have led. ... Various scenarios have been proposed to suggest how certain functional asymmetries might arise from the interaction of these slight asymmetries in maturation rate and the changing nature of environmental stimulation that reaches the brain (for review and discussion, see Hellige, 1993)." (Hellige, J.B. (1994) Babies, bath water, and the chicken’s way out. Brain and Cognition 26 (2): 233)

"Using this approach [sampling journals], 394 effects sizes drawn from 266 studies were sampled. The analysis followed the procedure presented by Hedges and Becker (1986). An overall analysis was conducted. It revealed a nonsignificant mean weighted d of .21. However, the effect sizes were not homogenous. They were thus partitioned in terms of the testing modality (viusal, auditory, tactile) and the type of task (verbal, nonverbal). this analysis revealed mean weighted effect sizes ranging from .17 (auditory verbal) to .25 (tactile nonverbal), all nonsignificant at the .05 level. These results make it clear that sex differences in the lateralization of verbal and nonverbal skills do not exist. The predictions from Buffery and Gray's or Levy's hypothesis are thus not supported. More importantly, these findings do not support the GBG model. The results of the present meta-analysis can thus be added to the evidence presented by BMB that contradicts predictions arising from the GBG theory." (Voyer, D. (1994) Anomalous Dominance, Sex, and Laterality . Brain and Cognition 26: 215)

"Handedness in family members was also tabulated in our study and, unexpectedly, asthmatic mothers, but not fathers, reported a particularly high percentage of their childen to be left-handed. This finding was corroborated in a study (Weinstein, Gurvitz, Greenberg, Weinstein, Solomon, Subbaiah, * Pieper, 1992) in which handedness was tested in 351 children of 139 asthamatic mothers. Left-handedness in offspring did not appear to be related to severity of asthma or to medication taken during pregnancy." (Weinstein, R.E., Lobocki, C.A., & Pieper, D.R. (1994) Allergy and the Geschwind-Behan-Galaburda Model. Brain and Cognition 26: 182)

"In a follow-up study, using more reliable measures of laterality, Grimshaw, Bryden, and Finegan (1993) found that increased prenatal testosterone levels were associated with stronger right-handedness and stronger right-ear effects on the dichotic listening test in girls. Both these findings are contrary to the predictions of the GBG model. [was than embryos T level or the mom's?] (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 52)

The general pattern that emerges is that autistic children show an increased incidence of left-handedness (18%, according to Fein et al.), at the same time, show a reduced degree of handedness such that (according to Fein et al.) fail to show a preference for either hand. ?, Soper, Orsini, Henry, and Zvi (1985b) and Soper, Satz, Orsini, ?, Zvi, and Schulman (1986) have make use of autistic children in developm? concept of "ambiguous handedness." They retested autistic subjects on a handedness battery, noting that many subjects were inconsistent in their hand useage for an activity across repeated testings and argued that these subjects really had not developed any hand preference. Overall, their data indicate 44% right-handed, 22% left-handed, and 36% without preference. McManus, Murray, Doyle, and Baron-Cohen (1992) have found that autistic children often indicate preference for the use of ? hand, but fail to show skill differences favoring the preferred hand. On the basis of these data, they suggest that hand preference precedes ? in hand skill." (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 145)

"However, Morley (1972) found that some 26% sample of 96 children with articulatory difficulties were left-handed and another 14% were poorly lateralized. An increased incidence of handedness, therefore, may be specific to children showing early articulatory problems, rather than being characteristic of all developmental language problems (see Bishop 1990a). (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 144)

"In general, then, there is at least reasonable evidence for associations between both handedness and reduced left-hemispheric speech lateralization in stutterers compared to nonstutterers. Lateralization and Tourette syndrome. TS, as mentioned earlier in connection with immune system and specific language-disability relations, is associated with an elevated frequency of dyslexia and stuttering and as such, might be considered to be a disorder involving language mechanisms. Although Shapiro, Shapiro, and Wayne (1972) found a very high incidence of left-handedness (35%) in TS patients, a much larger study by Comings and Comings (1987) failed to confirm this. If we use the Comings and Comings data on writing hand and combine the data from the two studies, the incidence of left-handedness in 195 TS patients is 14.6%, while Comings and Comings report a figure of 14.5% for their 47 control subjects. To summarize, in general there is at least modest evidence for an association between handedness and some developmental disorders of languge. Thus, left-handedness or ambiguous handedness appears to be more common in dyslexia, in autism, in articulatory disorders, and in stutterers and the normal anatomical asymmetries of the brain appear to be less common in developmental dyslexics, but there appears to be no elevation of non-right-handedness in Tourette's syndrome. To a large extent, the data reflect an increased incidence of poorly lateralized individuals, rather than an increase in the proportion of left-handedness. Such data may indicate a breakdown of the normal lateralizing mechanism rather than a true increase in left-handedness. Thus, while these data may be viewed as being at least moderately consistent with the GBG hypothesis, they can almost certainly be accommodated by other models. (e.g., Bishop, 1990a; Satz et al., 1985b) just as readily." (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 146)

"Swerdlow, Hattley, and Smith (1987) reported a lower incidence of left-handedness or ambidexterity in testicular cancer patients (11%) than in controls (16%). In addition, they reported a lower incidence of left-handedness (9%) in those with noncancerous cryptorchidism and/or hernia." (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 140)

[from abstract]"Information was obtained on the hand preference of 88 premature and 80 matched full-term children at 7-8 years old. These children were also evaluated for neurologic status, IQ, attention-deficit hyperactivity disorder, and learning disabilities. Although the difference on hand preference was not significant, 12% more of the premature children than the full-term children were left- or mixed-handed. Results showed that, among premature children, there is an association between non-right-handedness and cognitive and behavioral deficits and that left-handed children show relative clumsiness with the non-preferred hand." (Ross G, Lipper E, Auld PA (1992) Hand preference, prematurity and developmental outcome at school age. Neuropsychologia 1992 May;30(5):483-494)

[from abstract]"This paper is the first large-scale attempt to test Geschwind and Galaburda's (1985a, 1985b, 1985c) hypothesis that there should be a four-way association among neurodevelopmental disorders (NDs), special talents, non-right handedness, and immune disorders. In a sample of 11,578 children, several two-way associations were found, but not those most strongly predicted by the theory. For example, non-right handedness was not associated with NDs considered to be secondary to left hemisphere dysfunction (e.g., articulation disorder, reading disability, verbal aptitude deficits). Instead, non-right handedness was associated with NDs that involve generalized brain damage (e.g. cerebral palsy, mental retardation, and seizures). One immune disorder (asthma) was associated with one ND (attention deficit disorder); immune disorder was not associated with non-right handedness. Less than 1% of this sample manifested the co-occurrence of any three or four of these markers. In sum, there was little evidence in support of the syndrome suggested by Geschwind and Galaburda (1985a, 1985b,1985c)." (Flannery KA, Liederman J (1995) Is there really a syndrome involving the co-occurrence of neurodevelopmental disorder talent, non-right handedness and immune disorder among children? Cortex 31(3):503-515)

"Geschwind and Behan (1982) reported the very provocative finding of an association between autoimmune disorders and left-handedness. In two surveys carried out in London and Glasgow, they found that strongly left-handed individuals and their first- and second-degree relatives were more likely to suffer from various immune disorders than strongly right-handed people and their relatives. In addition, left-handers also reported a higher incidence of learning disorders such as developmental dyslexia and stuttering. In this survey, immune dirorders included celiac disease, dermatomyositis, Hashimoto's thyroiditis, myxedema, Crohn's disease, rheumatoid arthritis, thyrotoxicosis, ulcerative colitis, and uveitis. In a second study reported in the same paper, they found a greater incidence of left-handedness in patient populations with severe migraine or myasthenia gravis, but not in patients with rheumatoid arthritis, muliple sclerosis, or mixed-collagen vascular diseases. In a subsequent study, the same authors (Geschwind & Behan, 1984) reported higher incidence of migraine, allergies, dyslexia, stuttering, skeletal malformations, and thyroid disorders in left-handers. They also reported an elevated incidence of left-handedness in patients with Crohn's diseas, celiac disease, thyroid disorders, and ulcerative colitis and, by employing a one-tailed test of significance, in those with myasthenia gravis." (Bryden MP, McManus IC, Bulman-Fleming, MB (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 114-15)



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