A well established fact about schizophrenia is that first degree relatives have an increased risk of the disorder. Few now doubt that schizophrenia has a genetic basis, yet its mode of inheritance has to be explained. Even the identical twin of a schizophrenic stands a better than 50% chance of escaping the illness.'Genetic factors are not the whole story. Kraepelin, who derived the concept of schizophrenia, considered that both heredity and organic brain disease were implicated, but somehow the organic aspects were neglected until the publication of a study using computed tomography by Johnstone et al in 1976.2 A decade of such research has confirmed that the cerebral ventricles or cortical sulci are enlarged in many schizophrenics. Such changes are non-specific and can follow head injury, intracranial infections, and alcoholism and other cerebral insults. 3 As they are present in the earliest stage of schizophrenia and are not progressive they may be the sequelae of earlier events of aetiological importance. But what events-and how early? The epidemiology of schizophrenia probably still holds the key. The disorder generally begins in early adult life, but the peak incidence in men is nearly a decade earlier than that in women. 4 The reason for this is unclear. An equally puzzling but equally consistent finding is the small excess of births of schizophrenics in the cold winter months. 5 This excess is not shared by the siblings of schizophrenics and is greater in those without a family history and in men with paranoid illness. 67 The most likely explanation is that some environ-mental factor associated with winter birth causes neural damage in the fetus or neonate. The cause could be a viral infection or a seasonal difference in other complications that occur during pregnancy or delivery. 89 Schizophrenics seem more likely than controls to have a history of obstetric complications,'0 11 and, despite the various events encompassed by the term, obstetric complications may increase the risk of schizophrenia. Furthermore, increased ventricular size and other abnormalities seen on computed tomography are more common in those schizo-phrenics witha history of obstetric complications." 2 Much research implicates the left rather than the right cerebral hemisphere in schizophrenia.'3 There is evidence that schizophrenics are more likely to be left handed than controls,'4 and possibly the normal development of lateralised cerebral dominance can be disrupted by premature birth with a resultant increase in left handedness.'5 What mechanism could explain the relation between obstetric complications, abnormalities on computed tomo-