The onset of psychotic illness, with constant incidence across populations, in the reproductive phase of life with a decrease in fecundity requires an evolutionary explanation: What is the survival value of the predisposing gene or genes? In morphological studies 3 findings -- (1) ventricular enlargement, (2) a decrease in cortical mass, and (3) loss of asymmetry -- each appear to apply to the group of patients with schizophrenia as a whole. The findings must be related -- schizophrenia is a disorder of brain size and shape. In developmental studies there is evidence that deficits in language and reading ability, and a failure of lateralisation for hand-skill precede the onset of psychosis by at least 15 years. It will be argued that schizophrenia represent a component of the variation generated in the evolution of language: the function that accounts for the great increase in brain size in homo sapiens.