The most distinctive pattern of Senegalese marriage is polygamy, a marital state in which a man is married to more than one wife. In principle a man may have as many wives as he wishes, although most polygamous males follow the Islamic rule that limits the number of wives to four. Senegal has the highest polygamy rate—the percentage of polygamous marriages among all married couples—in West Africa: Close to half of the women are married to polygamous husbands. Even for the youngest generations (fifteen to nineteen years old), polygamy rates are about 25 percent, suggesting that about one in four women first marries into a polygamous union (Pison et al. 1995).
The main social factors underlying the high polygamy rates are religion, pronatalism, high mortality, and levirate. Having more than one wife is authorized under Islamic law, and men often invoke their religious beliefs to explain their practice of polygamy. In 1997, about one-third of married Senegalese men had more than one wife, with one in four married men having two wives and one in ten men married to more than two wives (Ndiaye et al. 1997). The bulk of the Senegalese population (70%) lives in rural areas, with agriculture as their main economic activity. Polygamous marriage is, therefore, the expression of men's desire to have a large number of children to help on the farm. In such settings where overall mortality rates are extremely high, couples have many children in order to ensure that a few of them to survive to the productive years and, thus, serve as old-age social security for parents through intergenerational wealth transfers. Finally, levirate, the possibility of inheriting the wives of deceased brothers, is a common practice in Senegal, and this too has contributed to the observed high polygamy rates.
Although at the aggregate level, polygamy certainly contributes to larger family sizes, research has demonstrated that Senegalese women in polygamous unions have on average a lower number of children than their counterparts in monogamous marriages. Michel Garenne and Etienne van de Walle (1989), studying the Serer of central Senegal, explain these differentials by the fact that women in polygamous marriage exhibit lower than average frequency of sexual intercourse because a polygamous "husband has to distribute his sexual activity between his wives." Also, polygamous husbands are on average older and thus less fertile than monogamous men. It has been shown, however, that lower fertility of polygamous marriages may be due to infertility problems encountered by first wives. Ndiaye (1985) argues that monogamous husbands are often obliged to marry a second wife when the first wife is infertile.
Polygamy has had major influences on the living arrangements of Senegalese families. In rural areas, all wives usually live together with their polygamous husband in the same compound, a group of adjacent rooms with a common fence and entry. Typically, the husband spends a certain number of nights with each wife, and the wives rotate cooking and other household chores. These living arrangements lead to large average compound size (more than ten people per household) and to the cohabitation of several nuclear families. Almost one out of six compounds is formed by three cohabiting family members, and more than one-third of Senegalese families are neither the wife nor the children of the household head. In such households with more than one male, household headship is determined mainly by age—the elder male is usually designated as the head.