Incest is illicit sex or marriage between persons socially or legally defined as related too closely to one another. All societies have rules regarding incest. Incest is conceptualized in four ways: as a proscribed or prescribed marriage form, as a taboo, as prohibited coitus, and as child abuse. The first three conceptualizations are most closely related to early scholars (mid-1800s to mid-1900s), who tended to overlap them. The last conceptualization has become prominent more recently.

Incest-as-marriage rules are usually proscriptive ("Thou shalt not"). Prescriptive ("Thou shalt") incestuous marriage rules have been documented for royalty in Old Iran and ancient Egypt and for Mormons in the United States (Lester 1972). Some groups historically encouraged brother–sister incest. Cases in point were the ruling families of Egypt and Polynesia, where preservation of family resources and ethnic identity took precedence over political alliances with other groups (Firth 1936, 1994).

That some groups...