According to the neurodevelopmental hypothesis of schizophrenia, maturational events in the brain at puberty interact with congenital defects to produce psychotic symptoms. As girls reach puberty at a younger age than boys, we predicted that (i) females would show earlier onset of psychotic illness arising around puberty, and (ii) onset of psychosis in females would be related to menarche. Analysis of epidemiological data regarding admission to psychiatric units in (a) England over the period 1973-1986, (b) France over the period 1975-1980, as well as examination of 97 psychotic adolescents referred to an adolescent unit over a 14 year period, supported both these propositions.