“…If artefacts of ascertainment and deviations in parental procreative patterns can be excluded it has to be considered that some environmental influence, present at or before birth, increases subsequent risk of psychosis. The possibilities which have been mooted (Bradbury & Miller, 1985;Torrey, 1987) include obstetric complications, vitamin deficiencies, intra-uterine infection and environmental temperature. Each of the first three seems likely to show greater variations with time and place than is suggested by the literature on seasonality of birth.…”