Temperature and the seasonality of births.

The relationship between temperature and seasonal fluctuations in births is presented cross nationally. Previous literature which give some credence to this relationship is reviewed, but the authors caution that there is no singular reason for birth seasonality. The summary conclusion is that the evidence is inconclusive; the most consistent hypothesis is that summer heat depresses conceptions. The next section is concerned with a selected set of estimates for birth seasonality. The data description and methods are published elsewhere. Tests of statistical significance at the 1% level reject the null hypothesis of no seasonality. In the US birth seasonality if reflected in a September peak and an April/May trough with variation between states in amplitude. The southern state's pattern is compared to 3 regions in India and Israel, and found to be similar. A European pattern is discerned with a spring peak, a local September peak, and a trough during late fall and early winter. The September peak is the only similarity to the US The explanation for variations is difficult, particularly when the birth seasonality between Sweden and the US is different and the seasonal temperature patterns are the same. 2 explanations are posited and discussed: 1) temperature operates in a more complicated manner than by simply depressing conceptions during the period of summer heat; and 2) 1 other factor, in addition to temperature explains the observed seasonal patterns. An estimation strategy is outlined which utilizes a variety of temperature effects, such as the effect of temperature on coital frequency. The equations also allow for the possibility that temperature has no effect at moderature temperature, a negative effect an high temperatures, or that hot or cold temperatures suppress fecundity. The results strongly reject the null hypothesis, but are mixed in the monthly temperature explanation with significance at the 5% level for Georgia, New York, Kerala, Maharashtra, England, and France, at the 10% level in California, Israel, and New Zealand, and insignificant for the Punjab, Canada, Sweden and Australia. The trends show that there is a spring trough in births in warm climate populations (hot summers reduce conceptions); that there is a pattern without temperature explanation for a persistent September peak in births and a spring peak in births in northern populations. High temperatures reduce conceptions in a wide variety of populations, but the explanation is not apparent.