Multiple births are rare among human beings and other large mammals. More than 98 percent of all human pregnancies result in single offspring; multiple births, which account for the rest, occur with rapidly decreasing frequency the higher the multiple. An increase in the number of such births has been observed, however, since the introduction of so-called fertility drugs in the 1960s. These hormonal drugs tend to cause polyovulation in womenÑthe release of more than one egg at a time for fertilization. The increased use of in vitro fertilization techniques, in which up to four embryos are placed in the uterus, has also contributed to the increase in multiple births. Almost all higher-multiple pregnancies result in premature babies. Without drugs older women are more likely to have multiple pregnancies than younger women. For individual women such pregnancies are more likely to occur later in the birth order.
Multiple births may be identical, fraternal, or (in cases other than twins) some combination of these. Identical births are monozygoticÑthat is, they arise from a single fertilized egg (zygote) that at some stage in its development separates into two or more embryos.
(Siamese twins result from very late or incomplete division.) Such births share the same genetic material and are always of the same sex. Fraternal births are polyzygotic: they arise from two or more fertilized eggs and may be of either sex. The famous Dionne Quintuplets of Canada, born in 1934, were monozygotic. Almost all recent higher-multiple births, such as the McCaughey septuplets of Iowa, have been polyzygotic because they were caused by fertility drugs.
Because identical twins are genetically equivalent and are also relatively common, compared to higher-multiple births, they have been frequent subjects of study in such fields as _behavioral genetics and intelligence research. These studies probe the relative importance of cultural and genetic influences in determining the characteristics of individuals.