twin, Twin boys.iStockphoto/ either of two young who are
simultaneously born from one mother. Twinning, common in many
animals, is of two biological kinds: the one-egg (monozygotic),
or identical, type and the two-egg (dizygotic), or fraternal,
type. The latter type is more usual and can be thought of simply
as a litter of two. In humans, psychological studies of sets of
identical twins, since they are genetically identical, have
provided much otherwise unobtainable information on the relative
effects of genetic endowment and environment. See also multiple
birth.