Who counts as your family? The answer seems obvious until you compare societies. In some, your father's brother is also called "father". In others, your mother's brother, not your father, has the strongest say over what you inherit. Kinship, the web of relationships people recognize through descent and marriage, was the first great subject of anthropology, and it remains a window into how societies organize care, property, power and belonging.

Kinship is cultural

Biology gives every person a genetic mother and father, but societies decide what those ties mean and who else counts as a relative. Lewis Henry Morgan, who worked closely with Seneca members of the Haudenosaunee (Iroquois) confederacy in New York, noticed that they used kin terms very differently from English speakers: a mother's sister, for instance, was addressed with the same word as mother. In 1871 he published Systems of Consanguinity and Affinity of the Human Family, a vast survey of kin terms gathered from around the world. Morgan showed that kinship terms vary systematically across societies, falling into a limited number of patterns that reveal how people organize duties and rights.

Later anthropologists stressed that kinship is made socially as well as biologically. Adoption, godparents, and close family friends whom children call "aunt" or "uncle" are all examples of fictive kin: people treated as relatives without ties of birth or marriage. In many societies such bonds carry real obligations, from sharing food to caring for orphans.

Tracing descent

Societies trace descent in different ways. In patrilineal descent, membership in a family group passes through men, from fathers to their children. In matrilineal descent it passes through women, from mothers to their children. In bilateral descent, common in Europe and the Americas, people count relatives on both sides more or less equally. Groups that trace their links to a known common ancestor are called lineages; larger groups that claim descent from a distant or legendary ancestor are called clans.

Matrilineal does not mean that women rule. In the Trobriand Islands that Malinowski studied, land and group membership passed through the mother's line, and a Trobriand child's mother's brother held key authority over inheritance. The father was a loving and important figure in daily life, but his own property and rank passed to his sister's children, not his own.

Marriage and exchange

Every known society has some recognized form of union, but the forms vary. Monogamy joins one spouse with one other. Polygyny, one husband with several wives, has been permitted in many societies, though even where it is allowed most men have only one wife. Polyandry, one wife married to several husbands, is rare but documented, for example in some Himalayan communities where brothers share a wife, which keeps scarce farmland from being divided.

Marriage often involves exchanges between families. Bridewealth is a payment from the groom's family to the bride's family, common in parts of Africa, where it recognizes that her kin are giving up her labor and her children to another lineage. Dowry moves wealth the other way, from the bride's side, and has been common in parts of South Asia and historically in Europe. Rules about whom one may marry also vary: exogamy requires marriage outside one's own group, endogamy within it. The French anthropologist Claude Lévi-Strauss argued in 1949 that marriage rules create alliances between groups, turning potential strangers or rivals into in-laws with a stake in each other's welfare.

After marriage a couple must live somewhere: with the husband's family, called patrilocal residence; with the wife's, called matrilocal; or in a new household of their own, called neolocal, which is the norm in most industrial societies.

A household without a couple

The Mosuo, also called Na, of southwestern China offer a striking case. Many have practiced what is often translated as "walking marriage". In Mosuo walking marriage, partners live apart and children grow up in their mother's household: a man visits a woman at night and returns to his own mother's home by morning, and children are raised by their mother and her relatives, including her brothers. The practice has changed under tourism and state policy, and scholars debate how to describe it, but it shows that a household need not be built around a married couple.

Kinship rules decide who inherits land, who cares for elders and who may marry whom. They can protect people and also confine them, especially women and children. The Universal Declaration of Human Rights holds that marriage requires the free and full consent of both spouses, a standard that leaves room for many family forms while protecting each person within them.

Worked example: consent at a clinic

A clinic in a matrilineal farming region needs consent for a child's surgery. Staff ask only the father, as they would in their own bilateral, neolocal families, and are surprised when relatives object. An anthropologist working with the clinic explains that in this community the mother's brother traditionally has a recognized voice in serious decisions about children. The clinic does not hand him the legal decision; that still rests with the parents. But it changes its practice, inviting the mother, the father and the mother's brother to a single meeting. Understanding the kinship system turned a conflict into cooperation without giving up anyone's rights.