Thus defined, the nuclear family was once widely held to be the most basic and universal form of social organization. Anthropological research, however, has illuminated so much variability of this form that it is safer to assume that what is universal is a “nuclear family complex” in which the roles of husband, wife, mother, father, son, daughter, brother, and sister are embodied by people whose biological relationships do not necessarily conform to the Western definitions of these terms. In matrilineal societies, for example, a child may be the responsibility not of his biological genitor but of his mother's brother, who fulfills the roles typical of Western fatherhood.

Closely related in form to the predominant nuclear-family unit are the conjugal family and the consanguineal (consanguinity) family. As its name implies, the conjugal family is knit together primarily by the marriage tie and consists of mother, father, their children, and some close relatives. The consanguineal family, on the other hand, typically groups itself around a unilineal descent group known as a lineage, a form that reckons kinship through either the father's or the mother's line but not both. Whether a culture is patrilineal or matrilineal, a consanguineal family comprises lineage relatives and consists of parents, their children, and their children's children. Rules regarding lineage exogamy, or out-marriage, are common in these groups; within a given community, marriages thus create cross-cutting social and political ties between lineages.

The stability of the conjugal family depends on the quality of the marriage of the husband and wife, a relationship that is more emphasized in the kinds of industrialized, highly mobile societies that frequently demand that people reside (residence) away from their kin groups. The consanguineal family derives its stability from its corporate nature and its permanence, as its relationships emphasize the perpetuation of the line.