Twinning is regarded as a very important phenomenon among the Yorubas in Western Nigeria. In this population, special ceremonies are performed to mark twin deliveries, and twin babies and their parents are given special names. Twins are therefore more readily identified, and data relating to twinning in families are likely to be fairly accurate.

Furthermore, the practice of polygamy in Western Nigeria provides a good opportunity for studying both maternal and paternal factors in the inheritance of twinning. The high incidence of twinning in the population (45‰: Nylander, 1969) is also an additional advantage, since it allows a large number of twin maternities to be studied in a short space of time.

Among 18 737 deliveries in the three major hospitals in Ibadan, Western Nigeria, between October 1967 and June 1969, there were 977 twin maternities. Every mother was interrogated immediately after delivery to find out whether she or her husband was a twin or had twins previously (including those delivered by the husband's other wives), and whether she or her husband had twin siblings. Obstetric data, including maternal age and parity, were collected for each patient, and zygosity of the newborn twins was determined by sex, placentation, blood groups, placental enzymes, haemoglobin types, and G6PD electro-phoretic studies, as will be described in a subsequent paper.