Scottish lawyer and ethnologist whose ideas on cultural evolution, kinship, and the origins of religion stimulated anthropological research.

McLennan was admitted to the bar in 1857, and he became a parliamentary draftsman for Scotland in 1871. His interest in survivals of practice and behaviour from earlier cultures led him to develop a theory of social evolution, outlined in his book Primitive Marriage: An Enquiry into the Origin of the Form of Capture in Marriage Ceremonies (1865, reissued as Studies in Ancient History, 2nd series, 1896, and again as Primitive Marriage, 1970).

McLennan introduced the terms exogamy (marriage outside the group, as in bride capture between warring tribes) and endogamy (marriage within a specific group, leading to monogamy and determination of kinship through males, rather than females). He was critical of the views of the American anthropologist Lewis Henry Morgan on kinship terminology, which, McLennan contended, indicated degree of respect related to considerations of station and age rather than to consanguineous relationships. McLennan regarded totems (totemism) as survivals of an earlier worship of fetishes, plants, animals, and, in course, anthropomorphic gods. His views on totemism attracted the interest of Sigmund Freud and such social scientists as Émile Durkheim, Sir James George Frazer, and W. Robertson Smith. McLennan also wrote The Patriarchal Theory (1885).