The incest taboo refers to the cultural prohibition of sexual activity or marriage between persons defined as "close" relatives - commonly called incest; the degree of which is determined by the society in which the persons live. Various theories exist to explain the origins and motivations of the incest taboo, and in particular, whether or not such a taboo exists universally or relatively.

Ethnographic research

The following excerpt from Notes and Queries, the most well-established field manual for ethnographic research, illustrates the scope of ethnographic investigation into the matter.

Incest is sexual intercourse between individuals related in certain prohibited degrees of kinship. In every society there are rules prohibiting incestuous unions, both as to sexual intercourse and recognized marriage. The two prohibitions do not necessarily coincide. There is no uniformity as to which degrees are involved in the prohibitions. The rules regulating incest must be investigated in every society by means of the Genealogical Method. The prohibition may be so narrow as to include only one type of parent-child relationship (though this is very rare), or those within the elementary family; or so wide as to include all with whom genealogical or classificatory kinship can be traced. The more usual practice is that unions with certain relatives only are considered incestuous, the relationships being regulated by the type of descent emphasized. In some societies unions with certain persons related by affinity are also considered incestuous.

What penalties fall on (a) the individuals concerned; (b) the community as a whole? Are such penalties enforced by authority, or are they believed to ensure automatically by all action of supernatural force? Is there any correlation between the severity of the penalty and the nearness of the blood-tie of the partners in guilt? Should children be born as the result of incestuous unions, how are they treated? Are there any methods, ritual or legal, by which persons who fall within the prohibited degrees and wish to marry can break the relationship and become free to marry?

As this excerpt suggests, anthropologists are interested in the gulf between cultural rules and actual behavior, and many ethnographers have observed that incest occurs in societies with prohibitions against incest. In these theories anthropologists are generally concerned solely with brother-sister incest, and are not claiming that all forms of incest are taboo (these theories are further complicated by the fact that in many societies people related to one another in different ways, and sometimes distantly, are classified together as siblings). Moreover, the definition restricts itself to sexual intercourse; this does not mean that other forms of sexual contact do not occur, or are proscribed, or prescribed. In these theories, anthropologists are primarily concerned with marriage rules and not sexual behavior. In short, anthropologists were not studying "incest" per se; they were asking informants what they meant by "incest," and what the consequences of "incest" were, in order to map out social relationships within the community.