To test the prediction of the (RS) theory that educational success would be lower at both extremes of the R-L continuum, and especially in strong dextrals (Annett, 1993c), the sample was divided into four quartiles for R-L hand skill differences in tracing lines (Ns = 136, 136, 135, 138) and frequencies of levels of school leaving qualifications (1 = low, 2 = medium, 3 = high) were calculated for the quartiles.
In quartile 1 (the most sinistrals) there is an excess of males (65 per cent males and 35 per cent females) and in quartile 4 (strong dextrals) a higher rate of females (34 per cent males and 66 per cent females).
Neither the expected quadratic relationship in form of an inverted U between R-L hand skill continuum and verbal abilities (Annett & Manning, 1990a) nor the prediction that strong dextrals are at a risk of lower ability (Annett & Manning, 1989) could be confirmed.
It seems her interpretation of the results is that there is a heterozygote advantage that accounts for the superiority of class 4 dextral males, but a specific superiority of the rs- - genotype among sinistral males, who scored higher than class 4 males.
McManus, Shergill & Bryden (1993) studied cognitive test performances in dextral participants.
The results for the spelling test showed superior performance of the most dextral individuals, poorest performance of the most sinistral, and intermediate performance by the groups with intermediate dextrality.
Since only dextral participants were studied, the homozygote advantage hypothesis straightforwardly predicts superior performances in the less dextral.
The decision to study only dextral persons was taken when we realized the difficulty and time vista required for securing adequate numbers of male and female volunteers to constitute adequate samples of the three sinistral hand preference classes in Annett's hand preference procedure.
based on their responses to the Annett Handedness Inventory participants were assigned to hand preference classes 1, 2, 3 or 4 in strict accordance with Annett's system for classifying dextral individuals.