Seasonality of births in human populations.

Seasonal fluctuations in births have been observed in virtually all human populations. In this paper we re-examine the seasonality of births with two main objectives in mind. The first is to provide an overview of the basic facts about the seasonality of births, presenting new estimates of the seasonal patterns. Seasonality is an important if not dominant source of nontrend variation in births in virtually all populations, but there are dramatic and puzzling differences across countries and time periods in the pattern of seasonal variation observed in particular populations. The second purpose of the paper is to survey the leading hypotheses about birth seasonality that have appeared in the literature and to discuss the consistency of these hypotheses with observed seasonal patterns. Using our estimates of seasonal patterns along with other evidence in the literature, we conclude that no single explanation receives strong, consistent support from the data.