Monogamy (the union of one wife to one husband) is the prevalent form almost everywhere. Polygyny (or polygamy; having several wives at one time), however, has been a prerogative in many societies (see harem). It is commonly found where the value of women's labor is high and may be practiced as a way of acquiring allies: A man may cement his bonds with several other men by marrying their sisters or daughters. Polyandry (having several husbands at one time) is rare, having occurred infrequently in Tibetan society, among the Marquesas of Polynesia, and among certain hill tribes in India. People who enjoy only a marginal subsistence may practice polyandry as a way of limiting births. It is also practiced where brothers must work together to sustain one household; they share one wife. The custom of marrying a widow to her late husband's brother is known as levirate marriage and was common among the ancient Hebrews. In sororate marriages a widower marries his deceased (or barren) wife's sister. The levirate and the sororate occur in societies where marriage is seen to create an alliance between groups; the deceased spouse's group has a duty to provide a new spouse to the widow or widower, thereby preserving the alliance. In recent years many gay-rights groups have sought official recognition of same-sex couples that would be comparable to marriage (see gay-rights movement).