Etymology
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exogamy (n.)

"the custom among certain tribes which prohibits a man from marrying a woman from his own tribe," 1865, Modern Latin, literally "outside marriage," from exo- "outer, outside" + -gamy. Related: Exogamous (1865). Apparently coined by Scottish anthropologist John Ferguson McLennan (1827-1881) in "Primitive Marriage" (see endogamy).

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endogamy (n.)

"marriage within the tribe or group," 1865, from endo- on model of polygamy. Related: Endogamous (1865). Opposed to exogamy. Apparently both were coined by Scottish anthropologist John Ferguson McLennan (1827-1881) in "Primitive Marriage."

To this law, the converse of caste, forbidding marriage within the tribe, Mr. M'Lennan has given the name of exogamy: while, instead of caste, since that word involves notions unconnected with marriage, he has used the correlative word — endogamy. [review in The Lancet, March 25, 1865]
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