Incest may be defined as the union, in sexual intercourse or in marriage, of two individuals of the opposite sex who stand in too close a relationship to each other. 

“Relationship” here means social relationship, regardless of biological considerations. 

“Too close” means any relationship that is deemed by society to be too close, and this conception will vary from one society or time to another; it may be between parallel cousins, first cousins, or persons belonging to the same clan or having the same surname. Each society provides its own definition, in specific terms, of incest.

 “Too close” means, also, that incest is always prohibited; there is no such thing as “permissive incest” among the ancient Hawaiians, Egyptians, Incas, or any other people. Incest is, by definition, a crime.

The concept of incest has meaning with reference to the human species only; the lower species, lacking the ability to symbol and to express themselves with articulate speech, can have no such concept, or behaviour proper to this term. 

Furthermore, incest and inbreeding are not synonymous by any means; there can be inbreeding without incest and incest without inbreeding. Incest is not simply a union between relatives, but between relatives who are too close to each other. And the lower animals, without language, have no means of classifying relatives or distinguishing degrees of relationship. Incest and its prohibition is not a factor, therefore, in the social life of the lower animals. But in the human species, the situation is different.

Every people on earth, so far as our knowledge goes, has both a conception of incest and customs or laws to prohibit and punish it. What is more, it is precisely among the most primitive peoples that the concept is most sharply defined and violations most drastically punished. This indicates clearly that the concept, together with rules pertaining to it, is not a recent development in social evolution, but is very primitive and remote in origin.