Season of birth among siblings of schizophrenics. A test of the parental conception habits interpretation.

Many studies have shown schizophrenics to be born with unusual frequency in the period January-April, and one interpretation of this seasonality of birth is that the parents of schizophrenics have an unusually strong general tendency to conceive offspring who will be born in January-April. This parental conception habits interpretation may be tested by studying whether the siblings of schizophrenics are also born with increased frequency in January-April. The present study investigated the season of birth of 288 full siblings and 44 half-siblings in 91 families of male schizophrenics. Approximately 5% of the siblings were known to be schizophrenic; with the schizophrenic siblings excluded from the analyses, the full siblings showed a birth pattern much closer (nonsignificant) to that of the schizophrenics than to that of the general population. Half-siblings showed January-April birth rates at or below the population level. Although the mechanisms responsible for the current results still remain unclear, the study suggests that seasonality of births of schizophrenics may be related to the parents' conception habits as well as to an etiological or triggering effect on the development of schizophrenia in the offspring.