Predicting reading ability from handedness measures.

Word reading, hand skill on a peg-moving task and hand preference were measured in 203 children aged from 11-13 years. Differences in skill between the hands were largely attributable to variations in left-hand skill, consistent with the view that manual and cerebral asymmetry are due to the loss (pruning) of cells in the right hemisphere during development-a mechanism that may be dependent on the "right shift' (rs+) gene postulated by Annett (1985). There was only weak evidence for the curvilinear relation between differential hand skill and reading predicted by Annett's (1985) theory of balanced polymorphism, which postulates a heterozygotic advantage. Reading was better predicted by absolute measures of hand skill than by the difference between the hands, especially among boys. For the boys, word reading was predicted significantly from right-hand skill, while for girls left-hand skill predicted slightly more strongly than right-hand skill. This may be related to differential growth gradients in the two hemispheres, and different environmental influences for boys and girls during a period of right hemispheric growth.