Schizophrenic illnesses occur with approximately the same incidence in all human populations with a characteristic distribution (slightly earlier in males) of ages of onset. Given that the predisposition (which presumably is genetic) is associated with a procreative disadvantage why do such illnesses persist? Here it is suggested that these conditions are a manifestation of genetic diversity in the evolution of the specifically human characteristic of language, an innovation that has occurred by a process of progressive hemispheric specialization-the establishment of dominance for some critical component of language in one or the other hemisphere. Individuals who develop schizophrenic symptoms show lesser anatomical and functional asymmetries than the population as a whole; such symptoms may reflect 'dominance failure' for language.