Abstract
In all societies for which data exist, there is seasonality in the incidence of births, a major cause of which in the UK appears to be season ality in sexual activity. In contrast to the findings of Cowgill for earlier periods in other places, there has recently been in Great Britain a greater degree of seasonality in legitimate than in illegitimate births and in upper-class than in lower-class births. Part of the explanation of the change in illegitimate seasonality could be the recent increase in the numbers of illegitimate births. The greater seasonality of upper-class births could be due to longer and more mobile holidays.

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