Annett's theory that individuals heterozygous for the right shift gene are intellectually advantaged: theoretical and empirical problems.

Annett & Manning (1989; 1990a,b) have proposed that left-handedness is maintained by a balanced polymorphism, whereby the rs+/-heterozygote manifests increased intellectual ability compared with the rs-/- and rs+/+ homozygotes. In this paper we demonstrate that Annett's method of dividing subjects into putative genotypes does not allow the rs+/- genotype to be compared with the rs-/- genotype within handedness groups. Our alternative method does allow heterozygous right-handers to be compared both with rs+/+ and rs-/- homozygotes. Using this method in undergraduates we find no evidence that supposed heterozygotes are relatively more intellectually able than homozygotes on tests of verbal IQ, spatial IQ, diagrammatic IQ or vocabulary. Theoretical analysis of the balanced polymorphism hypothesis reveals additional limitations. Although estimation of the size of the heterozygote advantage suggests that it must be very large (21 or 45 IQ points) to explain the effects found by Annett & Manning, it nevertheless must be very small (3.4 IQ points) to be compatible with the known differences between right- and left-handers in social class and intelligence. Moreover power analysis shows that the latter effect size is too small for Annett & Manning to have found effects in their studies. Additional power analyses show that studies looking for effects in groups of high intellectual ability, such as university students, are incapable of finding significant results, despite Annett claiming such effects. If the Annett & Manning paradigm does demonstrate differences in intellectual ability related to skill asymmetry then those differences are unlikely to result from a balanced polymorphism, but instead probably reflect motivational or other differences between right-handers of high and low degrees of laterality.