Global Patterns of Seasonal Variation in Human Fertility

This paper looks at seasonal patterns in births in populations around the world with a focus on two major issues. First, the authors examine the extent of systematic similarities and differences in seasonal patterns across populations, paying particular attention to the well-defined but quite different patterns in the southern United States and northern Europe. They also examine the extent to which seasonal birth patterns in the southern hemisphere are a mirror image of patterns in the northern hemisphe re. The second major issue addressed is the extent to which temperature explains seasonal birth patterns and the differences in these patterns across countries.

The authors find that the most pronounced seasonal patterns are in the southern United States, where births decline substantially in April and May, and in northern Europe, where births increase substantially in March and April. Although seasonal variatio ns in fertility were more pronounced in earlier agricultural populations, the authors show that seasonality has increased in this century in some high income, low fertility populations such as Sweden. They use data on monthly temperature to analyze the potential role of temperature in explaining the July-August trough in conceptions in the southern United States. They find little evidence, however, that temperature plays any role in explaining the pronounced June-July peak in conceptions in Sweden. Temperature also appears to be relatively unimportant in several other populations with substantial seasonal variations in births, suggsting that other factors play an important role in birth seasonality.