The interaction of seasonality, place of birth, genetic risk and subsequent schizophrenia in a high risk sample.

Birth occurring in winter months, which are high viral infection months, have been repeatedly shown to produce a slight excess of later-diagnosed schizophrenics. As a result, some researchers have speculated on the possible aetiological effect of viral infections on some forms of schizophrenia. The implications of the viral hypothesis were indirectly tested in the context of an ongoing prospective study of Danish children at high-risk (HR) for schizophrenia. A third-order analysis of variance interaction was hypothesized. Genetically vulnerable individuals, born in winter, in an urban environment (which increases the likelihood of the presence and transmission of viruses) would be more likely, as foetuses or neonates, to have suffered some CNS damage due to the infection; thus they would show higher rates of schizophrenia in the HR-urban-winter birth condition reached 23.3 per cent, considerably above population base rates (1 per cent) or rates for the HR subjects (8.9 per cent). Alternative explanations for the results were explored.