Quantitative assessment of five behavioural laterality measures: distributions of scores and intercorrelations among right-handers.

Five behavioural laterality measures (degree of right-handedness, ear dominance, ocular dominance, line bisection performance, and lateral preference as assessed by preferred direction of conjugate lateral eye movements) were analysed in 1171 right-handers by merging data from 15 different samples. All laterality aspects were assessed quantitatively by using well controlled performance measures instead of questionnaires. The distributions and intercorrelations of the measures as well as their test-retest reliabilities were examined. Results show moderately high reliabilities for all measures, and clearly demonstrate considerable variability among dextrals in the direction and magnitude of the obtained behavioural laterality measures. Most importantly, independence of the different measures questions some common assumptions in laterality research. It seems unlikely, for instance, that handedness and other aspects of laterality may be a product of one single underlying mechanism of brain lateralisation.