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This paper documents the postwar pattern of seasonality of births, for the United States and its divisions, using a simple time-varying parameter model. Variations over time and over space in seasonality are analyzed. In general, the South exhibits the greatest seasonal variation, and the most dramatic changes in seasonality. In later sections the hypothesis that the spread of air conditioning is responsible for the postwar changes in seasonality is tested, and a link between hormones and seasonality is speculated upon. A final section has a short summary and several questions for future research.

Trend and variation in the seasonality of U.S. fertility, 1947–1976