a1 MRC Mammalian Development Unit, University College London
During the first half of this century, the seasonal pattern of births in European countries showed a major peak in the spring and a minor peak in the autumn. In contrast, the pattern in the US was of a minor peak in spring and a major peak in autumn. Over the last 20 years, the pattern in England and Wales has changed to resemble the US pattern, and the same seems to be true of several other European countries. A hypothesis is offered to account for the difference between the European and the US patterns and for the change from one to the other in some countries.
The magnitude of seasonality correlates positively with latitude: it is suggested that this is partially consequent on variation in luminosity.
(Received November 29 1988)