Family is a group consisting of parents and children living together in a household. It is a basic unit of social structure. 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.
George Murdock, an early and influential observer of families, describes the term in this way:
The family is a social group characterized by common residence, economic cooperation and reproduction. It contains adults of both sexes, at least two of whom maintain a socially approved sexual relationship, and one or more children, own or adopted, of the sexually cohabiting adults
Nuclear Family:
The nuclear family or elementary family is a term used to define a family group consisting of a pair of adults and their children. This is in contrast to a polygamous family, single-parent family, and to the larger extended