This article is about the variable social/legal/religious/cultural infraction of human sexual relations with close kin.For the biological act of reproducing with close kin, see inbreeding.
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.
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.
The English word incest is derived from the Latin incestus, which has a general meaning of "impure, unchaste".
It was introduced into Middle English, both in the generic Latin sense (preserved throughout the Middle English period Before the Latin term came in, incest was known in Old English as sib-leger (from sibb 'kinship' + leger 'to lie') or mǣġhǣmed (from mǣġ 'kin, parent' + hǣmed 'sexual intercourse') but in time, both words fell out of use.
Terms like incester have been used to describe those interested or involved in sexual relations with relatives among humans, while inbreeder has been used in relation to similar behavior among non-human animals or organisms.