marriage in which two or more women share a husband. Sororal polygyny, in which the cowives are sisters, is often the preferred form because sisters are thought to be more mutually supportive and less argumentative than nonsiblings. A typical rule for sororal polygyny is that the eldest girl in a family marries first and that as they come of age her younger sisters join her as cowives; this was a practice in at least 40 Native American cultures during the 19th century.

Polygyny has several economic, social, and health advantages over monogamy. In most cultures, women contribute significantly to the wealth of the household and can thus materially benefit from the labour of an additional spouse. Where mortality rates of men consistently exceed those of women, polygyny can be seen as a resolution to the “deficit” of males and the “surplus” of females.

Socially, cowives and their children may accrue enhanced status and prestige as members of a large (and therefore inherently prosperous) household. In societies that provide no institutionalized role for unmarried women, the status of a cowife may be preferable to that of a single woman.

Polygyny can also have a positive effect on maternal and child health. During postpartum recovery, for instance, cowives can usually rely upon each other to perform the most strenuous work of the household. By creating opportunities for sexual companionship among the other members of the marriage, polygyny also supports the once common expectation that women will remain sexually abstinent for two or more years beginning in the last months of pregnancy (or upon parturition). This practice fosters adequate birth spacing for the mother to recover from the physiological and emotional stresses associated with pregnancy, lactation, and the care of a young child.

Despite certain advantages to both sexes, polygynous families can be fraught with bickering and sexual jealousy. In order to dampen strife, many groups accord seniority to one wife, usually the first. Marital harmony may be additionally fostered by customs that valorize the institution, restrict polygyny to the sororal form, or—especially in matrilineal cultures—support easy and recrimination-free divorce.

In most polygynous cultures, some people choose monogamy. This is often explained as a way to avoid marital strife or the expense of supporting several cowives and a multitude of children or as a result of a dearth of eligible or willing women.