Incest is sexual activity between close family members. It is a taboo in most societies and a criminal offence and an impediment to marriage in most countries, as well as being opposed by most modern religions. But the exact definition of what is a "close family member" varies widely: some jurisdictions consider only those related by birth, others also those related by adoption or marriage; some prohibit relations only with nuclear family members and ancestors or descendants, while others prohibit relations with aunts and uncles, nephews and nieces, and cousins as well.
The term is also sometimes used metaphorically and legally, to describe relationships between an authority figure and a pupil, such as teacher-student or troop leader and scout, or between those who are closely related in some other nonsexual way, as in an "incestuous relationship" between stockbrokers and mutual fund managers.
Anthropologists have found that all societies place restrictions on who one may marry. Although marriage should not be confused with sex, many societies in the past only permitted sexual relations within the bounds of marriage—hence, their rules regarding marriage were the same as their rules regarding sex. In some other societies, where sexual relations were permitted outside marriage, persons prohibited to marry were also prohibited to have sex. Sociology generalizes these marriage restrictions with the terms endogamy—the group within which one must marry—and exogamy—the group one must not marry.
All known societies have rules of exogamy, such as incest taboos, that specify ranges and categories of relatives who are forbidden as marriage (and sexual) partners. The most closely related biological kin—parents, children, brothers and sisters—are almost universally included. Most societies restrict other close relatives, but these extensions vary. Chinese society probably takes these restrictions the furthest, by banning relations between two individuals with the same surname, as they would theoretically be related.
Most societies also specify rules that encourage and sometimes force marriage within groups, frequently ethnic and religious ones. Even in modern Western societies, individuals consistently express preferences for mates from similar social class and educational backgrounds, and attempts to violate this endogamic principle can cause dramatic resistance from the associates of the violators, despite the society's pervasive emphasis on love and individual choice.
Some cultures cover relatives by marriage in incest prohibitions. For example, the question of the legality and morality of a widower who wished to marry his deceased wife's sister was the subject of long and fierce debate in 19th centuryGreat Britain, involving, among others, Matthew Boulton.
The Tanakh (Old Testament) contains prohibitions (primarily in Leviticus) against sexual relations between various pairs of family members. Father and daughter, mother and son, etc., are forbidden on pain of death to engage in sexual relations. An interesting aspect of the Tanakh's prohibition of incest is that, according to the interpretation given to it by some anthropologists, it prohibits sexual relations between aunts and nephews but not between uncles and nieces.
It is widely, but by no means universally, agreed that incest by parents is abuse and should be illegal. Some societies, notably India in the 1920s, consider incest an inescapable fact of life. In many societies some forms of sexual contact between close family members is socially (and sometimes even publicly) encouraged. For example, in Bali it was encouraged for mothers to sexually stimulate infants. This practice, among many others, is also common among certain tribes in Papua New Guinea, Polynesian and Melanesian islands. It is also common among the Japanese who claim to have no Oedipal complex "because the father is no competition" to the son.
Finally, there is also the much rarer phenomenon of consensual incestuous relations between adults, such as between an adult brother and sister. This is illegal in most places, but these laws are sometimes questioned on the grounds that such relations do not harm other people (provided the couple have no children) and so should not be criminalized. Artificial insemination and distant adoption have compounded these problems. There are known cases of people having romances, or even marrying, only to later find out they are closely related.
Proposals have been made from time to time to repeal these laws—for example, the proposal by the Australian Model Criminal Code Officer's Committee discussion paper "Sexual Offences against the Person" released in November 1996. (This particular proposal was later withdrawn by the committee, in spite of their own feelings on the issue, due to a large public outcry. Defenders of the proposal argue that the outcry was mostly based on the misunderstanding that the committee was intending to legalize sexual relations between parents and their minor children, which it did not.)
A controversial topic concerns the practice of incestual marriages in ancient Egypt; some scholars suspect this lies behind the otherwise puzzling passage in Genesis 26:7-11, where Isaac tells his wife Rebekah that she should claim to be his sister, rather than his wife, otherwise he might be killed.
Some experts claim that these incestual marriages were widespread at least during part of Egyptian history, such as Naphtali Lewis (Life in Egypt under Roman Rule: Oxford, 1983), who claims that numerous papyri attest to many husbands and wives as being brother and sister.
When instances of brother-sister marriages first began to appear in the papyri, they were greeted with great scepticism in some quarters, where doubt was expressed that any society would really have countenanced such common violation of the incest taboo... Such arguments [to otherwise explain the evidence] are ingenious, but they collapse completely in the face of the cumulative evidence of scores of papyri, official as well as private documents, in which the wife is unequivocally identified as the husband's "sister born of the same father and the same mother". (pp.43f)
Joyce Tyldesley (Ramesses: Egypt's Great Pharaoh: London, 2000), writing about the pre-Roman Egyptian period, expresses the opposite viewpoint. She states that within the royal family there was a tradition of hypergamy, where a king or his son might marry a commoner, but his daughter could not marry beneath herself, without the act be considered degrading herself. As a result, the royal princess often found herself either marrying her royal brother, or living her life without a spouse.
Even if brother-sister marriage was wide-spread in Roman times, it was explicitly forbidden by an imperial edict in AD 295.
In Icelandic folklore a common plot involves a brother and sister (illegally) conceiving a child. They subsequently escape justice by moving to a remote valley. There they proceed to have several more children. The man has some magical abilities which he uses to direct travellers to or away from the valley as he chooses. The siblings always have exactly one daughter but any number of sons. Eventually the magician allows a young man (usually searching for sheep) into the valley and asks him to marry the daughter and give himself and his sister a civilized burial upon their deaths. This is subsequently done.
The topic of inbreeding is one that is largely separated from incest for human beings. This is so since human beings are able to use birth control to have recreational, instead of procreational, sexual intercourse. Opponents of incest argue that since birthcontrol is rarely 100% effective the behavior of human male female incest should still be restricted.
Recent research on the mechanisms of human adaptive immunity suggests that there is a strong evolutionary pressure to maintain as diverse an array of antibody genes as possible. This may provide a biological explanation as to why opposite-sex siblings tend not to be attracted to one another and generally prefer to seek other partners. However, this has not yet been definitively ascertained.
The Universality of Incest, by Lloyd deMause () - author argues that incest is universal across all human societies; equates incest with incest with children; argues that sexual relations between children and third persons with parental knowledge or consent constitutes 'indirect incest'
Comment on "The Universality of Incest," by Andrew Vachss () - comments on deMause's article by well-known children's attorney and child protection consultant.