A case-control study of the seasonality effects on schizophrenic births on a tropical island.

A substantial body of evidence from dozens of studies in many different countries suggests an excess number of individuals with schizophrenia are born in winter months. The presence of a seasonality effect in regions with year-round warm climate, however, has rarely been examined. The major purpose of this project was to better understand if the seasonality effect on schizophrenic births that has been reported in other, mostly cold regions of the Northern Hemisphere, also can be detected in a warm, tropical climate. We set out to study birth months as risk factors, quantifying the risk for being born with schizophrenia for every month of the winter season in terms of incidence rate ratios (IRRs) in the central region of Puerto Rico. We also analyzed climatic data in order to determine if there was any correlation between the rate of schizophrenic births (n=710) to births in the general population (n=101,248) and average rainfall and temperature for every month of the year in our period of study (January 1932-December 1967). Our results suggest that the risk of developing schizophrenia is 36.5% higher for people born in February than for people born in the other months of the year (95% C.I.=6.6-74.8%). We also found correlations between the rate of schizophrenic to control births for any given month, and rainfall 4 months earlier (r=0.66, p=0.010), and temperature 5 months earlier (r=0.64, p=0.013) that remained significant after correcting for multiple comparisons.