The history and psychology of polygamy

A woman in ancient China might bring one or more of her sisters to her husband's home as backup wives. Eskimo couples often had cospousal arrangements, in which each partner had sexual relations with the other's spouse. In Tibet and parts of India, Kashmir, and Nepal, a woman may be married to two or more brothers, all of whom share sexual access to her. [...]

In other cultures, individuals often find such practices normal and comforting. The children of Eskimo cospouses felt that they shared a special bond, and society viewed them as siblings. Among Tibetan brothers who share the same wife, sexual jealousy is rare.

In some cultures, cowives see one another as allies, rather than rivals. In Botswana, women add an interesting wrinkle to the old European saying "Woman's work is never done." There they say: "Without cowives, a woman's work is never done." A researcher who worked with the Cheyenne Indians of the United States in the 1930s and 1940s told of a chief who tried to get rid of two of his three wives. All three women defied him, saying that if he sent two of them away, he would have to give away the third as well.

-  Stephanie Coontz, Marriage, a History