Library of Excerpts

Heterochony and Balanced Polymorphism in Human Beings

This page contains a collection of excerpts from sources used to support Shift Theory, an alternative theory of human evolution. Click here for an introduction to this new and unique theory of evolution.

[abstract] "The incidence and typical symptoms of schizophrenia are similar in diverse human populations; it is likely that the same is true for manic-depressive illnesses. Relative constancy of incidence over time and place (referred to as the 'anthropo-parity principle') and absence of evidence for environmental causation suggest that these disorders are genetic in origin and in some way characteristic of the human condition. Continued high prevalence in the face of a fertility disadvantage requires explanation--why do the genes persist and what is their function? It is proposed that the origins of psychosis are closely linked to the evolution of the human brain. The capacity for language evolved by the process of hemispheric specialisation, a gene for asymmetry (the 'right shift factor' or cerebral dominance gene) playing a critical role. This gene, it is suggested, is represented in homologous form on the X and Y chromosomes and has been subject to sexual selection. Such a locus could explain sex differences in cerebral asymmetry, and in age of onset and outcome of psychosis." (Crow TJ (1995) A theory of the evolutionary origins of psychosis. Eur Neuropsychopharmacol 5 Suppl:59-63)

"Evidence that the rs - - might have advantages in motor skills has been found in the discovery that in the large control samples of students and school children to be used below, left handers were faster than right handers for absolute peg moving times, and mixed handers were of intermediate skill. (Kilshaw and Annett, 1983). (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 358)

"Marian Annett (Annett & Manning, 1990; Annett & Kilshaw, 1984) has hypothesized a balanced polymorphism in dyslexia that neatly fits with our theory of biological and cultural evolution. Shift theory predicts a specific structure of health and disease in humans. Heterochronic theory's descriptions of the operation of relative rate and timing changes of development and maturation are directly transposable to Annett's (1985) right-shift theory. It fact, superimposing Gould's (1977) clock model of heterochronic evolution directly over Annett's (1985) right-shift graph reveals the relationship between evolutionary time, the etiology of cerebral asymmetry, and neurological disease. Right-shift theory (Annett, 1985) states that there is a gene (+) that predisposes most people for language facility. Annett noted that there is a difference in the distribution of handedness between human and animal populations characterized by a right-shift in human beings. This right-shift makes clear that not all humans are equally well disposed to language use. People with a (- -) genotype (18-19 % of the population) evidence no predilection to specific handedness or cerebral asymmetry and so achieve a left- or right-handedness close to random. People with (+ +) (32%), or a strong predilection to right handedness and asymmetrical lateralization, are highly disposed to language usage, but at the expense of right hemispheric strengths. Annett believes the mixture of both genetic propensities, (- +), offers the advantages evidenced by 49% of the population belonging in this category. She characterizes these advantages as a balanced polymorphism (Annett 1984, 1990) when applied to overall strength in language facility. It is important to understand that changes from population to population are gradual, not clearly demarcated and that movements across this arc or spectrum from (- -) to (+ +) are incremental." (Lehman A & Bernsten M (1999) Evolution and the Structure of Health and Disease. web link)

"Population variation in handedness (a correlate of cerebral dominance for language) is part genetic and, it has been suggested, its persistence represents a balanced polymorphism with respect to cognitive ability. This hypothesis was tested in a sample of 12,770 individuals in a UK national cohort (the National Child Development Study) by assessing relative hand skill (in a square checking task) as a predictor of verbal, non-verbal, and mathematical ability and reading comprehension at the age of 11 years. Whereas some modest decrements were present in extreme right handers the most substantial deficits in ability were seen close to the point of equal hand skill ('hemispheric indecision'). For verbal ability females performed better than males, but the relationship to relative hand skill was closely similar for the two sexes; for reading comprehension males close to the point of equal hand skill showed greater impairments than females. Analysed by writing hand the relationship of ability to hand skill appeared symmetrical about the point of 'hemispheric indecision'. The variation associated with degrees of dominance may reflect the operation of continuing selection on the gene (postulated to be X-Y linked) by which language evolved and speciation occurred." (Crow TJ, Crow LR, Done DJ, Leask S (1998) Relative hand skill predicts academic abiltity: global deficits at the point of hemispheric indecision. Neuropsychologia : )

"The Mark II predictions (post hoc in the present case) depend on a more general interpretation of the balanced polymorphism hypothesis explained above. This suggests that there are risks to intellectual development for those at both extremes of the asymmetry distribution. Problems in learning to read may be associated with overexpression as well as with underexpression of the rs + gene. Support for the view that the rs + + risk intellectual deficits beyond the visuospatial and practical ones hypothesized above comes from two sets of observations. First, the analysis of absolute hand speeds in hand preference groups (Kilshaw and Annett, 1983) suggested that hemisphere specialization might be achieved not be some boost to the left hemisphere but rather by some handicap to the right hemisphere. Among those of rs + + genotype there could be some who risk significant impairment of right hemisphere efficiency and who must rely, therefore, on the left hemisphere for all major intellectual skills." (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 360)

"Female dyselics, however, tended to be slow with he non-preferred hand." (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 362)

"Dyslexics favouring the left hand (<0) tended to be slower than controls with both hands, but the slowness of the right hand was very much less marked than the slowness of the left hand for extreme right-handers." (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 367)

"For enjoyment in making things the reduction of L-R time was significant. Children who did not enjoy making things were more dextral than controls ... and also more dextral than dyslexics who did not enjoy making things..." (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 370)

"For example, when 60% or dyslexic males were assumed to be rs - - and 40% of rs + +, with means as for controls and standard deviation 8 for both genotypes, the D value was 0.035. The L-R distribution of the dyslexic sample is consistent with the Mark I predictions of the model that the rs - - genotype is overrepresented (perhaps 50-60% in comparison with about 18% in the general population). the unexpected finding, which led to the Mark II interpretation of the model, is that the rs + + genotype is also overrepresented (perhaps 40-50% compared with about 32% for the general population). There were too few female dyslexics to repeat this comparison with control females." (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 370)

"If there are two subgroups in the present sample, associated with each extreme of the L-R distribution, is it possible to distinguish the groups for other characteristics? It was anticipated that a subgroup including more rs - - genotypes would be likely to have advantages in practical skills and in mathematics, to have more affected relatives and to be less likely to have a history of perinatal stress than other dylexics. This last assumption depended on the possibility that some dyslexias might be associated with reduced expression of the rs + gene following pathological influence on early development. Trends toward a reduction of L-R mean were found for all these variables (Table 8), provided 'affected relative' is redefined as 'affected female relative'. The variable which most clearly separated the sinistral and dextral groups was 'enjoys making things'. Although it was expected, as explained above, that the rs + + risk deficits in practical skills, it was surprising to find that an assessment of clumsiness based on the simple question "Do you like making things?" would identify a small subgroup of males (16) who were significantly more dextral for L-R time than controls. The remaining males (93) were significantly less biased to the right than controls. This main group of males fulfilled the initial prediction for the whole sample of a reduction of right shift." (Annett, M. & Kilshaw D. (1984) Lateral preference and skill in dyslexics: Implications of the right shift theory. J Child Psychology and Psychiatry 25: 374)

"The prediction of the right shift theory that there are two types of dyslexia with different distributions of handedness was examined in a large cohort of school children. Dyslexics with poor phonology were less biased to dextrality than controls, while dyslexics without poor phonology tended to be more dextral than controls on measures of hand preference and hand skill. Relatives also differed for handedness, as expected if phonological dyslexics were less likely than nonphonological dyslexics and controls to carry the hypothesized rs + gene." (Annett, M., Eglinton, E., Smythe, P. (1996) Types of dyslexia and the shift to dextrality. J Child Psychology and Psychiatry 37 (2): pp. 167)

"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 excesss 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 "dysexia 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 famales, 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 wieghs 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 hemishere 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)