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.

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Marian Annett &
M. Manning

Reading and a balanced polymorphism for laterality and ability: bibliographical excerpts


"The idea that reading ability might vary with right-left hand skill, such that children with mild biases to the right hand have advantages for learning, while children at both left and right extremes have disadvantages, was supported by findings for a large sample of primary schoolchildren. Poor readers tended to have weak left hand skills, giving strong right hand preferences in many cases. Those with additionally poor right hand skills tended to be of low intelligence. Good left hand skills and raised incidences of mixed and left hand preference were found only in a small subgroup, "bright dyslexics"." (Annett M, Manning M (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychol Psychiatry 31(4):511-29)

"Assessments of hand preference and skill in pupils attending a dyslexia clinic (Annett & Kilshaw, 1984) found higher incidences of left- and mixed-handedness, in proportions expected for a distribution of right-left (R-L) skill which was less shifted to the right. However, the R-L mean did not differ from that of controls because some dyslexics were very strongly right-handed. That is, there was an excess of clinic children at both ends of the R-L continuum and a relative deficit in the centre." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 511)

"The above analysis raised the question as to what the costs of left hemisphere specialization for speech might be. The suggestion (Annett, 1978) that costs must be right hemisphere function has been supported by subsequent findings; degree of right-handedness is related to weakness of the left hand rather than strength of the right hand (Kilshaw & Annett, 1983, see Fig 3 below) and strong right-handers are rare among talented groups (Annett & Kilshaw, 1982; Annett, 1985). ... As expected for heterozygote advantage, the highest means were found in children with R-L differences just to the right of 0 (mild dextrals) in both sexes. The costs of the rs ++ genotype seemed to include risks to verbal as well as non-verbal abilities, that is, intelligence. This prediction has been confirmed for the present sample; scores for Raven's matrices and for several tests of educational progress was significantly poorer in strong dextrals than in mild dextral, the overall trends in all cases being a decline form left to right across the R-L continuum (Annett & Manning, 1989, 1990). (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 512)

"(4) Since strong biases to dextrality are associated with left-hand weakness and with lower intelligence, poor readers at the dextral extreme are expected to have poor left-hand skills and to be less intelligent than poor readers at the sinistral extreme. If this prediction were upheld, it would help to explain the greater prevalence of sinistrality in "dyslexia clinic" samples, since such clinics are likely to select more intelligent poor readers. (5) The above hypotheses are expected to apply to both sexes. However, evidence of sex differences in the risk of developmental reading problems, and also in R-L asymmetry, suggests that males might be relatively more frequent at the sinistral and females more frequent at the dextral side of the laterality continuum among poor readers." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 514)

"As predicted, poor readers are more frequent at the extremes than in the centre, while good readers are more frequent in group 2 and especially infrequent in group 4 (x2 (6) = 13.845, p=0.031). Thus, the main prediction that the distributions of good and poor readers would differ is supported at a clearly significant level of probability. The distribution of poor readers can be described as showing a U=shaped trend, and the distribution of good readers the opposite inverted U-shape across the laterality continuum. These patterns were present for stricter criteria also (RQ < 83 and 126+, giving the bottom and top 11% of the sample, x2 (6) = 12.111, p=0.059). (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 517)

"Tukey tests (HSD) confirmed that good readers were significantly better than poor and average readers with the right hand. Since good and poor readers differed significantly for matrices, further analyses of the relationship between hand skill and reading in the total sample used multiple regression." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 519)

"(ii) Poor readers. In contrast to the similarity of good readers, significant differences for hand skill were found between subgroups of poor readers, as shown in Table 4. First it should be noted that matrices percentiles differed between all groups, those poor by RQ only being of low intelligence (matrices percentile, 18), those poor by stresid only being relatively high (69) and those poor by both criteria intermediate (40, all differing significantly by Tukey-HSD). For left hand peg moving score, the three groups of poor readers did not differ (89-91); they were all equally bad with the left hand, in the same way that all good readers were good with the left hand, in spite of marked differences in intelligence. The poor readers did differ significantly, however, for right hand score, with the RQ only group being poor and the stresid only group being particularly good with the right hand. This was associated, of course, with marked differences in the R-L score, four for the RQ only and 22 for the stresid only subgroups (Tukey-HSD). Distributions in R-L groups differed markedly, with the majority of poor RQ being in groups 1 and 2 and the majority of stresid only being in groups 3 and 4. The poor RQ group included 50% non-right-handers and 29% left-handed writers, while the stresid only included only 12% non-right-handers and no left-handed writers. The comparisons for actual hand skills above show, however, that the relative sinistrality of the poor RQ group depended on poor right hand skills; their left hands were as weak as those of other poor readers." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 522)

"Table 5 compares the "bright" versus the "dull" dyslexics (matrices percentile, 50 + and below 50, respectively). There were differences in the direction expected in that the bright children were faster with the left hand; since there were no significant differences for the right hand, the R-L difference was very much smaller for the bright than dull children (p=0.01 by t-test). The distribution in R-L groups shows that 73% of the bright children are in the sinistral half of the R-L continuum (groups 1 and 2) but only 33% of the dull children. Comparison of the dull versus bright in groups 1 and 2 versus 3 and 4 is significant (x2 (1) = 4.717, p=0.03). The contrast between bright and dull above is true of the 19 male dyslexics; all of six bright males were in groups 1 and 2, while nine of 13 (69%) dull males were in groups 3 and 4 (Fisher's exact test, p=0.01). In females, about two-thirds were in group 3 and 4 , irrespective of ability. With regard to hand preference, 46% of the bright and 75% of the dull dyslexics (sexes combined) were consistent right-handers. Two (18%) of the bright children and one (4%) of the dull were left-handed writers." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 525)

"The expectation that good readers would also be frequent among mild dextrals and infrequent in strong dextrals has been supported on all the criteria used in Fig. 2 (a-c) and in all analyses of subgroups of good readers (Table 3). ... The pattern is as expected if asymmetry depends on chance (roughly as in groups 1 and 2) plus a factor which weighs the chances of dextrality by handicapping the right hemisphere (in groups 3 and 4). There is a clear parallel with the variability in size of the left and right plana temporale (Galaburda, Corsiglia, Rosen & Sherman, 1987); in symmetrical brains, large plana were found in both cerebral hemispheres, but in asymmetrical ones the smaller planum tended to be on the right. It appears that the mechanisms inducing human cerebral asymmetry operate in both cases by reducing the role of the right hemisphere. The disadvantages associated with strong dextrality (in the rs++ genotype) is that this reduction may go too far, leading to significant loss of right hemisphere function. ... Mild dextrals were expected to have advantages for intellectual development and, indeed 50% of the good RQ group of high intelligence (compared with 25% in the total sample) were mild dextrals. However, it is important to emphasize that the findings for reading do not depend on intelligence alone; the children reading above expectation for their relatively limited intelligence (good stresids with matrices mean, 28) also included a high proportion of mild, and low proportion of strong, dextrals." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 525)

"The proposition that children at the right extreme of the R-L skill distribution might be at increased risk for reading problems was an original and surprising aspect of the balanced polymorphism hypothesis. How well do the findings fit this prediction? A double dose of the gene (rs ++) is expected to be associated with poor left hand skill. All groups of poor readers (RQ, stresid and dyslexic) had poorer left hand scores than good and average readers. In terms of R-L difference, very large dextral biases were found for the poor stresid (22) and for the dull dyslexic (17) groups, in comparison with average readers (13). One of the groups with a small R-L difference, the poor RQ, also has poor left hand skill. That leaves the bright dyslexics as the sole exception to the rule that poor readers have weak left hand skills. Since all good readers showed good levels of left hand skill, there must be little room for doubt that efficiency of the left/right hemisphere is required for facility in learning to read. ... the poor RQ group had poor skills in both hands; they could be said to be "ambilevous", having two left hands, as might be expected in cases of pathological sinistrality (Bishop, 1980; Brain, 1945; Satz, 1972). However, it must be acknowledged that the right hand scores were not abnormal for most individuals in this group. The combined features of low intelligence, poor reading and relatively poor skill in both hands suggests at least borderline pathological status. ... This group [bright dysexics] could be said to fulfill expectations for the rs- - genotype; they have no impairment of the left hand and no general loss of intelligence." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 526)

"With regard to sex differences, the main prediction that findings would apply to both sexes has been strongly upheld. Examination of the possibility that males might be more frequent at the sinistral side of the distribution in poor readers and females more frequent at the dextral side was complicated by differences between subgroups of poor readers and the effects for intelligence. Trends in bright dyslexics were as expected, but numbers were too small for confident analysis." (Annett, M. & Manning, M. (1990) Reading and a balanced polymorphism for laterality and ability. J Child Psychology and Psychiatry 31 (4): pp. 527)

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