Schizophrenic birth seasonality in relation to the incidence of infectious diseases and temperature extremes.

Previous research has indicated that schizophrenics are particularly likely to have been born during the winter months. In the present investigation, we studied the relationships of this birth-seasonality effect to year-to-year variations in the incidences of eight seasonal diseases and climatological temperature extremes in 3,246 schizophrenics. The winter birth-seasonality effect was greater in the years directly following those marked by high levels of infectious disorders than in years directly following those with low incidences of these diseases. Winter diseases (particularly diphtheria, pneumonia, and influenza) appeared to be more involved than others. These effects appeared among unmarried (presumably severe) schizophrenics but not among married patients, suggesting that the relationship is specific to process schizophrenia. The fact that most of the significant and near-significant relationships paired strength of birth seasonality to previous-year disease incidences suggested a prenatal rather than postnatal effect. Birth seasonality did not vary with winter or summer temperature extremes.