How far does the brain lateralize?: an unbiased method for determining the optimum degree of hemispheric specialization.

The relationship between measures (of size or function) on one side of the brain, in relation to the difference between the two sides on that measure, are important components of theories of hemispheric asymmetry. For example, it has been concluded that increasing lateralization (e.g., of hand skill or planum temporale area) occurs at the expense of the non-dominant hemisphere. Here it is demonstrated that such relationships could merely be a necessary consequence of relating components of a laterality index to the index (L - R)/(L + R) itself, or indeed to L - R. An alternative approach (using random data to exemplify the null hypothesis) is presented together with an application to data on hand skill from 12,782 11-year-olds in a cohort study. This demonstrates a symmetry hitherto undocumented of maximal hand skill in left and right hands in left- and right-hand writers respectively, the point of the maximum falling short of the population mean for relative hand skill in either case. If degrees of laterality are what is genetically determined, this suggests that selection is present for a function (perhaps language) associated with a greater magnitude of lateralization than is represented by hand skill.