Double descent

Kinship terminology is the system used in languages to refer to the persons to whom an individual is related through kinship. Different societies classify kinship relations differently and therefore use different systems of kinship terminology; for example, some languages distinguish between consanguine and affinal uncles (i.e. the brothers of one's parents and the husbands of the sisters of one's parents, respectively), whereas others have only one word to refer to both a father and his brothers. Kinship terminologies include the terms of address used in different languages or communities for different relatives and the terms of reference used to identify the relationship of these relatives to ego or to each other.

Anthropologist Lewis Henry Morgan (1818–1881) performed the first survey of kinship terminologies in use around the world. Though much of his work is now considered dated, he argued that kinship terminologies reflect different sets of distinctions. For example, most kinship terminologies distinguish between sexes (the difference between a brother and a sister) and between generations (the difference between a child and a parent). Moreover, he argued, kinship terminologies distinguish between relatives by blood and marriage (although recently some anthropologists have argued that many societies define kinship in terms other than blood).

However, Morgan also observed that different languages (and, by extension, societies) organize these distinctions differently. He proposed to describe kinship terms and terminologies as either descriptive or classificatory. When a descriptive term is used, it can only represent one type of relationship between two people, while a classificatory term represents one of many different types of relationships. For example, the word brother in English-speaking societies indicates a son of the same parent; thus, English-speaking societies use the word brother as a descriptive term. A person's male first cousin could be the mother's brother's son, mother's sister's son, father's brother's son, father's sister's son, and so on; English-speaking societies therefore use the word cousin as a classificatory term.

Morgan discovered that a descriptive term in one society can become a classificatory term in another society. For example, in some societies, one would refer to many different people as "mother" (the woman who gave birth to oneself, as well as her sister and husband's sister, and also one's father's sister). Moreover, some societies do not group together relatives which the English-speaking societies classify together. For example, some languages have no one-word equivalent to cousin, because different terms refer to one's mother's sister's children and to one's father's sister's children.

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