Rates of birth in the general population show seasonal fluctuations for reasons that are ill understood. Variations from these general population patterns have been reported for several psychiatric conditions and used as the basis for aetiological hypotheses. In this paper, the evidence for alterations in the expected seasonal fluctuation in birth dates of autistic people is evaluated. A national sample of 1435 autistic individuals and a clinic sample of 196 subjects are compared to general population figures and to 121 sibling controls. Compared with the general population, the national sample showed significant deviations from the expected rate of birth by month. In the clinic sample, differences from the anticipated monthly pattern were only evident when this sample was compared to the sibling controls. A variety of models for seasonal trends, including year quarters, temperature and sine wave forms, were fitted to these variations but no consistent picture emerged.
PIP: Evidence for alterations in season of birth of autistic children in 5 previous reports was tested using a national sample of 1435 autistic persons and those with Asperger's syndrome from the National Autistic Society (NAS) compared with 196 clinic patients from the Children's Department of Maudsely Hospital and their 121 sibling controls, both confined to births between 1947-80. The 5 previous reports of seasonality in birth of autistics were reviewed, and the rationale, either viral, nutritional, toxic, or hormonal, for seasonality of birth of several other disorders were reviewed. There is no sound theoretical rationale for seasonality in autistic births. Aggregated data showed significant peaks of autistic births between December-January, June-July and October in the NAS sample compared with the general population. By quarters, with winter defined as December-January, there was a significant deficit between March-May, but not when quarters were defined with winter as January-March. A smooth curve could be generated with peaks at June-July and December-January. No improvement could be obtained by a lagged quadratic temperature function, and spectral and autocorrelation analyses showed no evidence of seasonal pulses. The clinic sample showed no significant monthly variation compared with the general population, but suggestions of a pattern when compared with the sibling sample. Models were attempted using quarters, single pulse, 1- and 2-peak sine waves and lagged quadratic temperature functions. The only pattern seen was a slight deficit in autistic births in the September-November quarter, but not when the quarters were defined beginning in January. These inconsistencies and their inconsistencies with previous reports could not be explained. The value of season of birth studies to seek the etiology of rare diseases was discussed.