It is proposed that schizophrenia is a purely genetic condition that arises as a component of the variation generated in the speciation of Homo sapiens through the evolution of the faculty of language. A critical genetic change on the X and Y chromosomes allowed the two hemispheres to develop with a degree of independence. Thus Man's particular capacity for communication has evolved by a process of increasing hemispheric specialization on the basis of persisting diversity for the asymmetry ('right-shift)' factor. Sex differences in age of onset of psychosis, premorbid precursors and outcome are accounted for in terms of a mean difference between the sexes in cerebral asymmetry. The ultimate cause of this difference is the mechanism of sexual selection acting to retain different ranges of variation of the gene on the X and Y chromosomes. These ranges are expressed in terms of variation in hemispheric specialization and psychological function, a boundary of which represents the predisposition of psychosis. The hypothesis is developmental in the sense that the relevant gene has an influence in development and affects a critical aspect of higher nervous system function. But the key evolutionary process of selection of gene variants occurs late, by the mechanism of mate choice.