The rate of twinning, both MZ and DZ, as determined by the Weinberg formula, remained fairly static in Scotland up to 1958, when it fell (Fig. 1), due to a fall in the DZ twinning rate. These figures are based on the returns from the Registrar General for Scotland.

As age has a marked influence on the incidence of twinning, it was thought that the difference might be due to a reduced number of older women having babies. However, this does not seem to be the whole answer (Fig. 2). The most marked change would be expected in the women over 35, but this did not occur. It was not possible to look at other factors such as parity, social class and height in the figures for Scotland. The details of all births in the City of Aberdeen are known because of the very high hospital delivery rate, and more detailed studies have been made on the twinning rates for Aberdeen.

These figures, again, showed that from 1959 onwards there was a fall in the twinning rate in Aberdeen and this was due to the fall in the number of DZ twins. The fall in incidence occurred both in Primigravidae and in multigravidae (Tab. I), and more in older than in younger Primigravidae (Tab. II), but there was little difference by age in the fall in multigravidae (Tab. III).

Another factor which is related to the incidence of twinning is the height of the mother. This, in turn, reflects the social class, as tall women are more commonly from the upper, and small women from the lower social classes. The rate of twinning in Aberdeen was highest in the tall women before 1954 (Tab. IV). The rates in all women fell from 1954 onwards, but most dramatically in the tall women. The rates from 1959 onwards show very little difference according to height.