Genetic and Environmental Influences

review the literature on handedness, dichotic listening and anatomical asymmetry in schizophrenia and concluded that there was strong evidence for decreased cerebral lateralisation. As they point out, the implication is that finding the locus of the gene for cerebral dominance could unravel the genetic predisposition to schizophrenia. Procopio (2001) raises a number of points relevant to the findings of Sommer et al and concludes that both genetic and environmental factors have to be accounted for – ‘the right shift is still only a hypothesis’. Procopio’s comments focus on handedness and it is important to remind ourselves, as Sommer et al’s review makes clear, that handedness is no more than an indirect and developmentally labile index of anatomical asymmetry and language lateralisation; it is this last variable and its genetic determination (Crow, 1998a,b) that is of greatest relevance.