The Effects of the Weather on Fertility in Human Populations

This paper provides new evidence on the effects of weather on fertility. Monthly temperature data for a variety of states and countries are used to estimate the direct contribution of weather in explaining both the seasonal and non-seasonal variation in monthly births. There is significant seasonality in births in every population studied, suggesting an influence of weather on conceptions. The differences in seasonal patterns across countries, however, provide mixed evidence regarding weather based explanations of birth seasonality. Populations in the sourthern United States, for example, exhibit spring troughs in births, consistent with the hypothesis that summer heat depresses conceptions. Most populations in Northern Europe, however, display spring peaks in births. In order to explore these puzzles the authors look directly at the effects of monthly temperature on conceptions by estimating regressions in which weather variables are entered in a flexible form. Results indicate that the weather has a quantitatively important influence on both the seasonal and non-seasonal variation in births. In particular, summer temperature extremes exert a depressing effect on conceptions, an effect that is clearly important in explaining the summer trough in conceptions in the southern United States. Results also show, however, that there is significant seasonality in births even after accounting for weather. Flexible controls for monthly weather do not appear to explain the persistent spring peak in births in Northern Europe, suggesting that other factors play an important role.