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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.


Incest taboos throughout human society

Anthropologists have found that all societies place restrictions on whom 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, siblings—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. This is true in some communities in India too.

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.

Cultural variations

The description and analysis of incest, and beliefs concerning incest, are complicated by the fact that the definitions of "close family member" and "sex" vary widely across cultures. For example, Trobriand Islanders prohibit both sexual relations between a man and his mother, and between a woman and her father, but they describe these prohibitions in very different ways: relations between a man and his mother fall within the category of forbidden relations among members of the same clan; relations between a woman and her father do not. This is because the Trobrianders are matrilineal; children belong to the clan of their mother and not of their father. Thus, sexual relations between a man and his mother's sister (and mother's sister's daughter) are also considered incestuous, but relations between a man and his father's sister are not. Indeed, a man and his father's sister will often have a flirtatious relationship, and a man and the daughter of his father's sister may prefer to have sexual relations or marry.

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 century Great 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. According to the interpretation given to it by some anthropologists, it prohibits sexual relations between aunts and nephews but not between uncles and nieces.

In many areas of the Arab world and southern India, it is estimated that as many as 50% of marriages are between first cousins or even closer relatives (primarily uncle-niece couples). This is considered abnormal in northern India, however historically it has had very few health consequences.

Modern views

In most of the Western world incest generally refers to forbidden sexual relations within the family. However, even here, definitions of family vary. Within the United States, marriage between (first) cousins is illegal in some states, but not in others, and sociologists have classified marriage laws in the United States into two categories. One, used mainly in southern states, in which the definitions of incest are taken from the Bible, and which frowns upon marriage within one's lineage but less so on one's blood relatives, and the other known which frowns more on marriage between blood relatives (such as cousins), but less on one's lineage. Many American states recognize two separate degrees of incest, the more serious degree covering the closest blood relationships such as father-daughter, mother-son and brother-sister, with the less-serious charge being pressed against more distantly-related individuals who engage in sexual intercourse, usually down to and including first cousins. Despite the strong stigma attached to incest, even the more serious charge is generally prosecuted as the least severely-punishable class of felony (in New York state, for example, the maximum penalty is four years in prison), and the less serious charge is usually only a misdemeanor (curiously, many incest laws do not expressly proscribe sexual conduct other than vaginal intercourse - such as oral sex - or, for that matter, any sexual activity between relatives of the same gender, so long as neither party is a minor).

Within the West, sexual relations between parents and their children, and between brothers and sisters are almost universally forbidden. Incest is most frequently engaged in by parents of both sexes and their children. And while it is usually perceived as an act engaged in by a father and his daughter, this is yet another myth surrounding the practice. Historically, the most important forms of incest were maternal incest (see also Oedipus complex). And while surveys do not indicate a high rate of maternal incest, this can be seen as a reflection of the difficulty of collecting information about illegal sexual acts with children rather than its rare occurrence. Incest is also an important part of sexual exploration by children, especially in families with children of the same age. A study by Floyd Martinson found that 10-15% of college students had had a childhood sexual experience with a brother or sister (see child sexuality).

The degree to which even the topic of incest is forbidden varies between societies. In the United States incest is rarely described in books or movies and then usually as a very traumatic and perverse experience. Meanwhile in Japanese manga and anime the topic of incest is covered relatively often and in a more neutral and tolerant way.

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. See Genetic sexual attraction.

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.)

Incest in ancient Egypt

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.

Incestuous unions were frowned upon and considered as nefas (a violation of the natural and social order) in Roman times, and were explicitly forbidden by an imperial edict in AD 295, who divided the concept of incestus into two categories of unequal gravity: the incestus iuris gentium, who was applied to both Romans and non-Romans in the Empire, and the incestus iuris civilis who concerned only the Roman citizens. Therefore, for example, an Egyptian could marry an aunt, but not a Roman.

Incest in royal dynasties

Adult incest has been notable in royal dynasties, probably in order to help concentrate wealth and political influence within the family (historical evidence suggests that this practice actually weakened the genetic makeup of elite society family lines, resulting in abnormally high occurrences of rare genetic defects and diseases). Although the marriage unions were often not consensual, with young adults or children forced to marry close relatives, this does not imply the sex was non-consensual. Best known for this practice, which included brother-sister marriages, are some of the dynasties of Ancient Egypt (as explained above), ancient Hawaii, and the Precolumbian Mixtec.

Incest in mythology

Examples of incest in mythology are rampant. Zeus and Hera are brother and sister as well as husband and wife. They were the children of Cronus and Rhea who were also brother and sister as well as husband and wife.

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 Bible also contains a number of references to incest. This subject is examined in the Wikipedia article Biblical references to incest.


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 birth control is rarely 100% effective the behavior of human male/female incest should still be restricted.

Genetic implications of incest

Incest may be a form of inbreeding, and some have suggested that the incest taboo is meant to reduce the chances of congenital birth-defects that can result from inbreeding. Scientists have generally rejected this as an explanation for the incest taboo for two reasons. First, in many societies partners with whom marriage is forbidden and partners with whom marriage is preferred are equally related in genetic terms; the inbreeding argument would not explain the incest taboo in these societies. Second, the inbreeding argument oversimplifies the consequences of inbreeding in a population. Inbreeding leads to an increase in homozygocity, that is, the same allele at the same locus on both members of a chromosome pair. This occurs because close relatives are more likely to share more alleles than nonrelated individuals. If an individual has an allele linked to a congenital birth-defect, it is likely that close relatives also have this allele; a homozygote would express the congenital birth defect. If an individual does not have such an allele, a homozygote would be healthy. Thus, the frequency of a defect-carrying gene in a population may go up, or down, when inbreeding occurs. The overall effect of inbreeding depends on the size of the population.

Thus, in small populations this dynamic would lead to an initial increase in birth defects. But if health care is limited, it is likely that such children would not reproduce; consequently, the frequencies for the allele in question would go down. Ultimately the result would be a population with a large number of homozygotes and a small number of congenital birth defects. In large populations with good health care, however, it is likely that there will be consistently high levels of heterozygosity despite periodic inbreeding. Consequently the alleles linked to congenital birth defects will remain in the population, with a significant chance of a homozygote with the linked allele.

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.

See also

Main: Westermarck effect, sexual morality, Taboo, Incest taboo, Inbreeding, Kinship and descent, Genetic sexual attraction

Sexual behavior: Human sexual behavior, Human sexuality, Ageplay, Subconscious mind, Inhibited sexual desire, Oedipus complex

Law: Lawrence v. Texas, Hustler Magazine v. Falwell, Sex crime, Babylonian law, Consensual crime, Child sexual abuse, Pedophilia

Lists: List of sex-related topics, List of sexology topics

Other: Edgeplay, Eugenics, Family Affairs, Marriage, Religious prostitution, Bonobo, Santorum controversy, Tiy

External links

Controversial issue of first cousins incest

Last updated: 10-24-2004 05:10:45