a1 Galton Laboratory, University College, London
It is suggested that dizygotic twinning rates and fecundability are closely related and that the recent declines in dizygotic twinning rates may have been accompanied by declines in fecundability in developed societies. Records on twinning rates for many countries are readily accessible in national vital statistics; demographers may therefore find that changes in dizygotic twinning rates may be a useful monitor of changes in fecundability. The question arises whether this decline in fecundability has been responsible, to any appreciable extent, for the decline in birth rates experienced in recent years by developed countries.
(Received October 02 1980)