The term "incest" encompasses sexual behavior with a variety of patterns, variations, causes, types, relationships, and effects. Although incest is illegal in all states in the United States, the laws vary from state to state regarding the behavior and degree of relatedness considered incestuous. In general, incest laws prohibit marriage, cohabitation, and sexual relations (usually defined as sexual intercourse) between individuals who are closely related by blood, marriage, or adoption.

Most social scientists believe that the primary purpose of the prohibition, often called the incest taboo, is to protect the nuclear family from the consequences of sexual rivalry and jealousy. The taboo is linked with the rule of exogamy (marriage outside of one's kinship group, usually for the purpose of social alliance between groups). Besides reinforcing the incest prohibition, this rule prevents families from becoming culturally ingrown through continuous endogamy (marriage within a kinship group). Highly inbred populations...