Relative hand skill predicts academic ability: global deficits at the point of hemispheric indecision
Introduction
Language is the faculty which distinguishes the species. A clue to the mechanism was apparent from the observations of Dax [20]and Broca [6]—for most people speech is localised in the left hemisphere. Language it seems has evolved by a process of increasing hemispheric specialisation. For this function one hemisphere is dominant over the other—some component of language is confined to one side of the brain. The genes that influence the process could be those that have permitted the evolution of language—i.e., those by which Homo sapiens has speciated. But whereas for most individuals language is localised in the left hemisphere for a significant minority it is in the right. This population variation lends interest to handedness, the most accessible correlate of hemispheric specialisation for language.
That handedness is genetically determined is suggested by an increased rate of left-handedness in the children of left-handed parents 1, 34while the handedness of a child is unrelated to that of an adopting parent [9]. Transmission of handedness within families can be accounted for by the postulate of a single bi-allelic gene biasing the left hemisphere to be dominant and the individual to be right-handed, together with a random influence 1, 34. While more complex hypotheses may be required to explain the diversity associated with hemispheric specialisation (see below) this simple theory [1]explains how dominance can develop in either hemisphere while in the population there is directional preponderance to the left.
If the evolution of language has depended upon selection acting upon genes influencing cerebral dominance, and particularly if the selection is still in force, the genetic variation might be expected to be associated with variation for other aspects of cerebral function. Annett [2]suggested that variation associated with the gene for handedness and dominance (the right shift factor) represents a balanced polymorphism (heterozygote advantage) in that with respect to certain cognitive abilities heterozygotes (+⧹−) are advantaged relative to both homozygotes (−⧹− and +⧹+). Observations on the abilities of schoolchildren aged 7–11 years have been interpreted as consistent with this hypothesis [3], an interpretation that has been challenged [35]. Earlier claims of a relationship between handedness and cognitive ability 29, 36were not supported by more systematic investigation 5, 26, 44. Two recent studies 39, 40are interpreted as giving no more than equivocal support to the theory. Most studies have treated handedness as a simple dichotomy rather than as a continuous variable. However, degrees of handedness may be important. It has also been suggested 37, 45that failure to develop unequivocal dominance in one hemisphere predisposes to pathology, e.g., dyslexia.
Section snippets
Methods
We examined data recorded in the U.K. National Child Development Study [41]. This cohort of individuals born in the week 3–9 March 1958 was originally identified as the Perinatal Mortality Survey. These individuals, now aged 38 years, have been followed up at the ages of 7, 11, 16 and 23 years with assessments of physical and educational development. At 11 years, cohort members completed a series of tests of cognitive ability including assessments of verbal ability, non-verbal ability and
Results
For each of the abilities (Fig. 2) there were striking departures from the null hypothesis that relative hand skill and ability are unrelated. Although the general form of the deviation was similar for each ability, differences between abilities were observed that related both to degree of relative hand skill and to sex.
For verbal ability (Fig. 2) there was a substantial sex difference, females performing better than males, as previously established [30], but the relationship between verbal
Discussion
The findings yield decisive disconfirmation of the null hypothesis that relative hand skill and cognitive ability are unrelated. Across abilities and sex the relationship is consistent: whilst at the extremes of hand skill there are modest decrements that are sex and ability dependent, the most substantial deficits are close to the point of equal hand skill or hemispheric indecision. From Fig. 2, Fig. 3it appears that it is failure to establish dominance unequivocally in one or the other
Acknowledgements
—This study was supported by the U.K. Medical Research Council. We thank J. Bynner and P. Shepherd for their help with the NCDS data set and M. Annett, D.V.M. Bishop and I.C. McManus for comments on aspects of the analysis.
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