EXOGAMY. In every human society there are certain regu lations which control the relation of the sexes and the selection of a mate. Intercourse between close blood-relations as brother and sister, father and daughter, and mother and son, is almost every where condemned. But among very many tribes it is forbidden both on grounds of consanguinity and because two individuals are members of the same social group. This prohibition, based upon common membership of a social group, is the law of exogamy. If the group is a territorial unit, e.g., a village, the exogamy is local. More commonly, membership of the group concerned is determined by kinship, real or fictitious, as in the clan (see RELA TIONSHIP SYSTEMS). Hence exogamy is often loosely used to indicate clan exogamy.
That exogamy prevents the marriage of all near relatives is true only if membership of the exogamous group is determined by descent reckoned through both parents, but this is very rare. Normally descent is traced through only one parent and it is therefore inevitable that certain close blood-relations will not be long to the same group and will therefore be possible mates for each other. Thus if a tribe is patrilineal a man can select wives from among his mother's sisters, her brother's daughters and her brother's son's daughters, and, were exogamy the only marriage prohibition, even his mother would be available to him. On the other hand, clan exogamy does prevent unions between people bearing no relationship to each other, since membership of a clan is dependent upon fictive, not blood relationship.
The rigidity with which the law of exogamy is observed varies considerably. Among some people a breach of it is regarded as incest ; among others, though marriage is forbidden, extra-marital relations between clan members are tolerated; while in some cases even marriage can be condoned.
McLennan, who first coined the word, regarded it as the out come of female infanticide which, by limiting the number of women available within the group, forced tribesmen to capture their wives from their neighbours, but increased knowledge of the facts has made this theory untenable. Another suggestion is that the horror of incest, supposedly innate, has extended to all those women whom, under the classificatory system of relation ship, a man addresses by the term for "sister." On another view the original form was local exogamy arising from a natural dis taste on the part of those who have been reared together to cohabit. Others believe it originated with Totemism (q.v.), while the diffusionists consider that it developed under special conditions in one place and spread thence throughout the world.
(See ENDOGAMY.) (C. H. W.)