Incest is sexual activity between family members or close relatives. This typically includes sexual activity between people in a consanguineous relationship (blood relations), and sometimes those related by affinity, such as individuals of the same household, step relatives, those related by adoption or marriage, or members of the same clan or lineage.
The incest taboo is and has been one of the most widespread of all cultural taboos, both in present and in many past societies. Most modern societies have laws regarding incest or social restrictions on closely consanguineous marriages. In societies where it is illegal, consensual adult incest is seen by some as a victimless crime. Some cultures extend the incest taboo to relatives with no consanguinity such as milk-siblings, stepsiblings, and adoptive siblings. Third-degree relatives (such as half-aunt, half-nephew, first cousin) on average share 12.5% genes, and sexual relations between them is viewed differently in various cultures, from being discouraged to being socially acceptable. The children of incestuous relationships were regarded as illegitimate, and are still so regarded in some societies today. In most cases, the parents did not have the option to marry to remove that status, as incestuous marriages were, and are, normally also prohibited.
A common justification for prohibiting incest is avoiding inbreeding: a collection of genetic disorders suffered by the children of parents with a close genetic relationship. Such children are at greater risk for congenital disorders, death, and developmental and physical disability, and that risk is inversely proportional to their parents' coefficient of relationship—a measure of how close the parents are related genetically. But inbreeding is not the sole basis for the incest taboo for two reasons. First, most prohibitions on incest cover affinity relationships—that is, relationships created by marriage (for example, father-in-law and stepfather)—as well as relationships created by adoption. And second, the incest taboo also applies to non-procreative sex—for example, sex between infertile relatives and sex performed with birth control.
In some societies, such as those of Ancient Egypt and others, brother–sister, father–daughter, mother–son, cousin–cousin, aunt–nephew, uncle–niece, and other combinations of relations were practiced among royalty as a means of perpetuating the royal lineage. Some societies, such as the Balinese and some Inuit tribes, have different views about what constitutes illegal and immoral incest. However, sexual relations with a first-degree relative (such as a parent or sibling) are almost universally forbidden.