Birthrates of schizophrenics following relatively warm versus relatively cool summers

Schizophrenics have consistently been found to be born with unusually high frequency in the early months of the calendar year. A previous study found significantly more schizophrenics to be born following warmer as contrasted with cooler summers, thus suggesting that summer temperature or a correlate of summer temperature may be an effective factor in the season of birth schizophrenia relationship. The current study re-tested the warmer vs. cooler summer finding, using both a local sample (N = 301) and a nationwide sample of Swedish schizophrenics (N = 13,440), each sample showing increased frequency of births in the early months of the year. In order to localize the possible effects of temperature to a more specific gestational period, the schizophrenics in each sample were further divided into sub-groups based upon which trimester of gestation likely occurred during the summer prior to the birth. The 71 years (1876-1946) during which the patients were born rank-ordered by mean summer temperature at representative geographical locations, and the rank-orders of years were divided into temperature quartiles. The rates of births for the total patient groups and the trimester sub-groups among both samples showed no positive linear or systematic relationship to temperature during the previous summer. Even the warmest among the 71 years were not followed by increased rates of births of schizophrenics. The results of the study did not corroborate the earlier finding.