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 against 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 immediate 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, 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 only permit sexual relations within the bounds of marriage--hence, their rules regarding marriage are the same as their rules regarding sex. And in other societies, where sexual relations are permitted outside marriage, persons prohibited to marry are most often (but not always) 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.
Children of incestuous relations have an increased risk of having genetic abnormalities due to inbreeding. The likelihood of genetic abnormalities increases considerably with the closeness of the relation.
All 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 universally included. Most societies restrict other close relatives, but these extensions vary.
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.
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.
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 cousins is illegal in some states, but not in others, and sociologists have classified marriage laws in the United States into to categories. One, used mainly in southern states, in which the definitions of incest are taken from the Bible, and which frowns upon marriage within ones lineage but less so on one blood relatives, and the other known which frowns more on marriage between blood relatives (such as cousins), but less on one's linage.
Within the West, sexual relations between parents and their children, and between brothers and sisters are almost universially 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.
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.
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.
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.)
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 the dynasties of Ancient Egypt.
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 it by some anthropologists, it prohibits sexual relations between aunts and nephews but not between uncles and nieces.
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 choses. 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.