A family is a domestic group of people, or a number of domestic groups linked through descent (demonstrated or stipulated) from a common ancestor, marriage, or adoption. Families have some degree of kinship.
In Western culture, a family is defined specifically as a group of people affiliated by blood or by legal ties such as marriage or adoption. Many anthropologists argue that the notion of "blood" must be understood metaphorically; some argue that there are many non-Western societies where family is understood through other concepts rather than "blood."
According to sociology and anthropology, the primary function of the family is to reproduce society, either biologically, socially, or both. Thus, one's experience of one's family shifts over time. From the perspective of children, the family is a family of orientation: the family serves to locate children socially, and plays a major role in their enculturation and socialization. From the point of view of the parent(s), the family is a family of procreation the goal of which is to produce and enculturate and socialize children. However, producing children is not the only function of the family. In societies with a sexual division of labor, marriage, and the resulting relationship between a husband and wife, is necessary for the formation of an economically productive household. In modern societies marriage entails particular rights and privilege that encourage the formation of new families even when there is no intention of having children.
The structure of families traditionally hinges on relations between parents and children, between spouses, or both. Consequently, there are three major types of family: matrifocal, consanguineal, and conjugal. (Note: these are ideal families. In all societies there are acceptable deviations from the ideal or statistical norm, owing either to incidental circumstances, such as the death of a member of the family or infertility, or personal preferences).
A matrifocal family consists of a mother and her children. Generally, these children are her biological offspring, although adoption of children is a practice in nearly every society. This kind of family is common where women have the resources to rear their children by themselves, or where men are more mobile than women.
A consanguineal family consists of a mother and her children, and other people -- usually the family of the mother. This kind of family is common where mothers do not have the resources to rear their children on their own, and especially where property is inherited. When important property is owned by men, consanguineal families commonly consist of a husband and wife, their children, and other members of the husband's family.
A conjugal family consists of one or more mothers and their children, and/or one or more spouses (usually husbands). This kind of family is common where men desire to assert control over children, or where there is a sexual division of labor requiring the participation of both men and women, and where families are relatively mobile.
Non-scholars, especially in the United States and Europe, use the term "nuclear family" to refer to conjugal families. Sociologists distinguish between conjugal families that are relatively independent of the kindreds of the parents, and of other families in general, and nuclear families which maintain relatively close ties with their kindreds.
Non-scholars, especially in the United States and Europe, also use the term extended family. This term has two distinct meanings. First, it is used synonymously with consanguinal family. Second, in societies dominated by the conjugal family, it is used to refer to kindred (an egocentric network of relatives that extends beyond the domestic group) who do not belong to the conjugal family.
These types refer to ideal or normative structures found in particular societies. In any society there is some variation in the actual composition and conception of families. Much sociological, historical, and anthropological research is dedicated to understanding this variation, and changes over time in the family form. Thus, some speak of the bourgeois family, a family structure arising out of 16th and 17th century European households, in which the center of the family is a marriage between a man and woman, with strictly defined gender roles. The man typically is responsible for income and support, the woman for home and family matters. In contemporary Europe and the United States, people in both the academy, politics, and civil society have called attention to single-father-headed households, and families headed by same-sex couples, although academics point out that these forms exist in other societies.
Hawaiian: the most classificatory; only distinguishes between gender and generation.
Sudanese: the most descriptive; no two relatives are referred to by the same term.
Eskimo: has both classificatory and descriptive terms; in addition to gender and generation, also distinguishes between lineal relatives (who are related directly by a line of decent) and collateral relatives (who are related by blood, but not directly in the line of descent). Lineal relatives have highly descriptive terms, collateral relatives have highly classificatory terms.
Iroquois: has both classificatory and descriptive terms; in addition to gender and generation, also distinguishes between siblings of opposite sexes in the parental generation. Siblings of the same sex are considered blood relatives, but siblings of the opposite sex are considered relatives by marriage. Thus, one's mother's sister is also called mother, and one's father's brother is also called father; however, one's mother's brother is called father-in-law, and one's father's sister is called mother-in-law.
Crow: like Iroquois, but further distinguishes between mother's side and father's side. Relatives on the mother's side of the family have more descriptive terms, and relatives on the father's side have more classificatory terms.
Omaha: like Iroquois, but further distinguishes between mother's side and father's side. Relatives on the mother's side of the family have more classificatory terms, and relatives on the father's side have more descriptive terms.
Societies in different parts of the world and using different languages may share the same basic terminology; in such cases it is very easy to translate the kinship terms of one language into another. But it is usually impossible to translate directly the kinship terms of a society that uses one system into the language of a society that uses a different system.
Some languages, such as Japanese, add another dimension to some relations: relative age. There are different words for "older brother" and "younger brother."
It is generally assumed that the mother's husband is also the genitor. In some families, a woman may have children with more than one man or a man may have children with more than one woman. Children who share one parent but not another are called "half-brothers" or "half-sisters." Children who do not share parents, but whose parents are married, are called "step-brothers" or "step-sisters."
If a person is married to the parent of a child, but is not the parent of the child themselves, then they are the "step-parent" of the child, either the "stepmother" or "stepfather". Children who are adopted into a family are generally called by the same terms as children born into the family.
Typically, societies with conjugal families also favor neolocal residence; thus upon marriage a person separates from the nuclear family of their childhood (family of orientation) and forms a new nuclear family (family of procreation). This practice means that members of one's own nuclear family were once members of another nuclear family, or may one day become members of another nuclear family.
Members of the nuclear families of members of one's own nuclear family may be lineal or collateral. When they are lineal, they are referred to in terms that build on the terms used within the nuclear family:
Grandfather: a parent's father
Grandmother: a parent's mother
Grandson: a child's son
Granddaughter: a child's daughter
When they are collateral, they are referred to in more classificatory terms that do not build on the terms used within the nuclear family:
When separated by additional generations (in other words, when one's collateral relatives belong to the same generation as one's grandparents or grandchildren), these terms are modified by the prefix "great".
Most collateral relatives were never members of the nuclear family of the members of one's own nuclear family.
Cousin: the most classificatory term; the children of aunts or uncles. Cousins may be further distinguished by degree of collaterality and generation. Two persons of the same generation who share a grandparent are "first cousins" (one degree of collaterality); if they share a great-grandparent they are "second cousins" (two degrees of collaterality) and so on. If the shared ancestor is the grandparent of one individual and the great-grandparent of the other, the individuals are said to be "first cousins once removed" (removed by one generation); if the shared ancestor is the grandparent of one individual and the great-great-grandparent of the other, the individuals are said to be "first cousins twice removed" (removed by two generation), and so on.
Distant cousins of an older generation (in other words, one's parents' first cousins) are technically first cousins once removed, but are often classified with "aunts" and "uncles".
Similarly, a person may refer to close friends of one's parents as "aunt" or "uncle," or may refer to close friends as "brother" or "sister". This practice is called fictive kinship.
Relationships by marriage, except for wife/husband, are qualified by the term "-in-law". The mother and father of one's spouse are one's mother-in-law and father-in-law; the spouse of one's son or daughter is one's son-in-law or daughter-in-law.
The term "sister-in-law" refers to three essentially different relationships, either the wife of one's brother, of the sister of one's spouse, or the wife of one's spouse's sibling. "Brother-in-law" is similarly ambiguous. There are no special terms for the rest of one's spouse's family.