Temperature and the Seasonality of Births

This paper considers the relation between temperature and the seasonal fluctuations in births. There is significant seasonality in births in every population for which data are available, and the presence of a seasonal pattern in any activity strongly suggests the influence of temperature. The differences in seasonal patterns across countries, however, provide at best ambiguous support for any temperature based explanation of birth seasonality. Thus, while temperature is probably one important factor in determining birth seasonality, it is clear that other factors are also at work. The paper has two purposes. The first is to survey existing evidence on the relation between temperature and the seasonality of births. The second is to provide new evidence on possible effects of temperature on fertility by using actual monthly temperature data to estimate temperature's direct contribution in explaining the seasonal variation in monthly births. Results show that temperature does have a quantitatively important influence on the seasonal variation in births in most of the populations considered. In particular, there is a consistent tendency for temperature to depress summer time conceptions. Even after controlling for the effects of temperature, however, there is still significant seasonality in births in all populations, specifically a global spring peak, a local September peak, and a global winter trough. Thus, controlling for the effects of temperature resolves some of the cross country differences in birth seasonality, but it does not fully explain this seasonality. There is clearly some other variable or combination of variables influencing seasonal fluctuations in births.