Introductionfamily, a basic unit of social structure, the exact definition of which can vary greatly from time to time and from culture to culture. How a society defines family as a primary group, and the functions it asks families to perform, are by no means constant. There has been much recent discussion of the nuclear family, which consists only of parents and children, but the nuclear family is by no means universal. In the United States, the percentage of households consisting of a nuclear family declined from 45% in 1960 to 23.5% in 2000. In preindustrial societies, the ties of kinship bind the individual both to the family of orientation, into which one is born, and to the family of procreation, which one founds at marriage and which often includes one's spouse's relatives. The nuclear family also may be extended through the acquisition of more than one spouse (polygamy and polygyny), or through the common residence of two or more married couples and their children or of several generations connected in the male or female line. This is called the extended family; it is widespread in many parts of the world, by no means exclusively in pastoral and agricultural economies. The primary functions of the family are reproductive, economic, social, and educational; it is through kin—itself variously defined—that the child first absorbs the culture of his group.
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