No season-of-birth effect in schizophrenic patients from a tropical island in the Southern Hemisphere.

The distribution of birth rates was examined in 668 schizophrenic patients born in Réunion, a tropical French island in the Southern Hemisphere, and compared with that in the general local population. We failed to observe a significant season-of-birth effect, either in the total sample of schizophrenic patients or in subgroups categorized by gender, age, or family history of schizophrenia. Seasonal factors do not appear to affect the yearly distribution of births among schizophrenic patients on Réunion Island. It is nevertheless possible that environmental factors which are seasonal in countries with more contrasting climates have a continuous effect, throughout the whole year, in subtropical areas. Conversely, these findings provide some evidence against the hypothesis that there is an age-incidence artifact in seasonal studies from countries in the Northern Hemisphere.