Geographical and secular changes in the seasonal distribution of births

The long-term variations of seasonal distribution of births in three places distant from one another, Osaka City in Japan, the northern U.S.A. (Massachusetts and Missouri) and Görlitz in East Germany were investigated, using several kinds of historical records.

The birth periods covered were 1755–1975 for Osaka, 1741–1941 for the northern U.S.A. and 1675–1816 for Görlitz, respectively. The sample size of births before the era of modern vital statistics were 6536 for Osaka, 4132 for northern U.S.A. and 41,507 for Görlitz, respectively.

The birth patterns in the mid-20th century are known to be different among the three areas in the following manner: early spring peak in Japan, fall peak in the U.S.A. and spring peak in Europe.

However, this observation of the long-term variation of birth seasonably has revealed that there had been secular changes of the seasonal distribution of births in all three areas, and that a pattern of alternation had been occasionally alike and synchronous.

It seems that both the spring-peak and fall-peak birth patterns seen recently in Europe, Asia and the U.S.A. were not fixed for certain geographical locations, and that the interchangeability of these patterns could be seen rather commonly.

Based on the various phenomena collected to date about the seasonal distribution of human births, a hypothesis—the epidemic seasonally-infertile factors hypothesis—on the causality of the seasonal distribution of births was put forth.