The Yoruba contribution to our understanding of the twinning process.

The Yoruba people of Nigeria possess a twinning rate more than fourfold that of Caucasian populations. As such, they provide extraordinary possibilities for the study of multiple birth. Percy Nylander clarified many of the factors that contribute to this high incidence of twinning. His analyses of Nigerian placentation and zygosity have shown the increased rate of multiple pregnancy to be a result of higher proportions of dizygotic twins and trizygotic triplets as compared to Caucasian populations. Whereas a well-established genetic influence upon dizygotic twinning exists throughout the rest of the world, there may be an environmental factor in Nigeria responsible for the increased incidence of multiple births. Nylander's study of the Yoruba also has provided important contributions to the studies of perinatal mortality in multiple gestation. In totality, much of Nylander's work can be applied to further the understanding of ethnic and racial differences in human twin placentation and zygosity.