Abstract

PIP: A study has been made of twinning rates throughout the world over the last 2 decades. There has been a decline in age specific dizygotic twinning rates in almost all of the developed countries during this period. Exceptions to this trend are provided by the US, where the only decline in twinning rates was in births to older women in the 1960s, and by Czechoslovakia, Hungary, and Bulgaria, where an initial decline in twinning during the 1960s was followed by stable twinning rates. It is suggested that the cause may be some form of environmental pollutant, perhaps a pesticide, which has been the subject of restrictive legislaton 1st in the US and later in the 3 Communist countries. The cause of this decline may be identified by a study of such legislation. Low twinning rates found in Central and South America may indicate the genetic affiliations of their inhabitants with those (of Mongoloid origin) of countries in the Far East.