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Incest is sexual activity between family members who are forbidden to marry due to their close kinship. It constitutes a cultural taboo in most societies and is often prohibited by law. Which family members constitute those covered by the incest prohibition varies according to the culture. Generally, however, close members such as siblings or parents and children are included, while prohibitions on more distant relatives, such as cousins, vary. Whatever the extent of the prohibition, however, incest is almost universally condemned and viewed with horror.
Many reasons underlie the incest taboo: Biological explanations of the dangers of inbreeding support restrictions to avoid genetic disorders; psychological theories suggest that childhood experiences with one's immediate family, or others with whom one lives in close proximity, lead to a natural aversion to considering such people as potential mates; and anthropologists have noted that many societies have rules of endogamy or exogamy regulating marriage partners, and that these serve to maintain the identity of the group or solidify alliances with others. Violating the incest taboo, however, has repercussions beyond legal or social sanction, it breaks the structure of family relationships, causing pain and confusion for the individuals involved, and threatening the harmony and happiness of human society.
Incest is defined as sexual activity between family members who would be forbidden (either legally or socially) to marry. It derives from the Latin incestus or incestum, the substantive use of the the adjective incestus meaning "unchaste, impure," which itself is derived from the Latin castus meaning "chaste."
Incest constitutes a cultural taboo in most current nations and many past societies. In many areas, incest is also prohibited by law. Which family members constitute those covered by the incest prohibition is determined by the society in which the persons live. Some societies consider it to include only those related by birth or those who live in the same household; other societies further include those related by adoption, marriage, or clan.
The term "incest" can include sexual activity between family members of any gender and can include family members of any age. When one of the family members involved is a minor, incestuous activity has also been called "intrafamilial child sexual abuse."
Consensual incest between similar-age brothers and sisters is not uncommon, according to a study by Floyd Martinson, who found that 10-15 percent of college students had childhood sexual experiences with a brother or sister. However, only 5-10 percent of those included intercourse; and therefore most probably represent a form of childhood sexual curiosity and experimentation.
In most of the Western world, while incest generally describes forbidden sexual relations within the family, the applicable definitions of family vary. Within the United States, marriage between first cousins is illegal in some states, but not in others. Sociologists have classified marriage laws in the United States into two categories: One in which the definitions of incest are taken from the Bible, which frowns upon marriage within one's lineage but less so on one's blood relatives; and one that frowns more on marriage between blood relatives (such as cousins), but less on that within one's lineage.
Many states prohibit marriages between first cousins, and anothers permit them only under special circumstances. Utah, for example, permits first cousins to marry only if both spouses are over age 65, or at least 55 with evidence of sterility; North Carolina permits first cousins to marry unless they are "double first cousins" (cousins through more than one line); Maine permits first cousins to marry only upon presentation of a certificate of genetic counseling. Other states with some, but not absolute, limits on first-cousin marriage are Arizona, Illinois, Indiana, and Wisconsin.
It is generally accepted that incestuous marriages were widespread at least during the Graeco-Roman period of Egyptian history. Numerous papyri and the Roman census declarations attest to many husbands and wives as being brother and sister.
Incestuous unions were frowned upon and considered as nefas (against the laws of gods and man) in Roman times, and were explicitly forbidden by an imperial edict in 295 C.E., which divided the concept of incest into two categories of unequal gravity: The incestus iuris gentium, which was applied to both Romans and non-Romans in the Empire, and the incestus iuris civilis, which concerned only the Roman citizens. Therefore, for example, an Egyptian could marry an aunt, but a Roman could not. Despite the act of incest being unacceptable within the Roman Empire, Roman Emperor Caligula is rumored to have had open sexual relationships with all three of his sisters, (Julia Livilla, Drusilla, and Agrippina the Younger) and to have killed his favorite (Drusilla) when she became pregnant with his child.
Customs have changed in the United Kingdom, with incest at one time apparently being normal practice, at least in the south of the country. When Julius Caesar invaded Britain for the second time in 54 B.C.E., he noted the customs of the Britons, remarking, "Wives are shared between groups of ten or twelve men, especially between brothers and between fathers and sons; but the offspring of these unions are counted as the children of the man with whom a particular woman cohabited first."
Incestuous relations between adults, such as between an adult brother and sister, are illegal in most parts of the industrialized world. 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. 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 Offenses against the Person," released in November 1996. This particular proposal was later withdrawn by the committee due to a large public outcry. Defenders of the proposal argue that the outcry was mostly based on the mistaken belief that the committee was intending to legalize sexual relations between parents and their minor children.
The laws of many U.S. states recognize two separate degrees of incest, the more serious being 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 to and including first cousins and sometimes half-cousins. Many incest laws do not expressly proscribe sexual conduct other than vaginal intercourse—such as oral sex—or any sexual activity between relatives of the same sex (though if either party is a minor, it may be punishable otherwise).
In the wake of the Lawrence v. Texas (539 U.S. 558 2003) decision by the U.S. Supreme Court, striking down laws criminalizing homosexual sodomy as unconstitutional, some have argued that by the same logic laws against consensual adult incest should be unconstitutional. Some civil libertarians argue that all private sexual activity between consenting adults should be legal, and its criminalization is a violation of human rights. In Muth v. Frank (412 F.3d 808), the 7th U.S. Circuit Court of Appeals interpreted the case applying to homosexual activity, and refused to draw this conclusion from Lawrence, however—a decision that attracted mixed opinions. The Supreme Court refused to hear the case.
Inbreeding increases the frequency of homozygotes within a population. In some species, most notably bonobos, sexual activity, including that between closely related individuals, is a means of dispute resolution or greeting. Depending on the size of the population and the number of generations in which inbreeding occurs, the increase of homozygotes has positive or negative effects.
The concepts of incest and inbreeding are distinct. Incest describes socially taboo sexual activity between individuals who are considered to be too closely related to enter into marriage. In other words, it is a social and cultural term. Inbreeding describes procreation between individuals with varying degrees of genetic closeness, regardless of their relative social positions. It is a scientific term, rather than a social or cultural term.
In many societies, the definition of incest and the degree of inbreeding may correlate positively. For example, sexual relations between people of a given degree of genetic closeness is considered incestuous. In other societies, the correlation may not be as obvious. Many cultures consider relationship between parallel cousins incestuous, but not those between cross cousins, although the degree of genetic relationship does not differ. Relationships may be considered incestuous even when there is no genetic relationship at all: Stepparent–stepchild relationships, and those between siblings-in-law, have been considered incestuous, even though they involve no risk of inbreeding.
Inbreeding leads to an increase in homozygosity (the same allele at the same locus on both members of a chromosome pair). This occurs because close relatives are much more likely to share the same alleles than unrelated individuals. This is especially important for recessive alleles that happen to be deleterious, which are harmless and inactive in a heterozygous pairing but, when homozygous, can cause serious developmental defects. Such offspring have a much higher chance of death before reaching the age of reproduction, leading to what biologists call "inbreeding depression," a measurable decrease in fitness due to inbreeding among populations with deleterious recessives. Recessive genes, which can contain various genetic problems, appear more often in the offspring of procreative couplings whose members both have the same gene. For example, the child of persons who are both hemophiliac has a 25 percent chance of having hemophilia.
Leavitt argued that inbreeding in small populations can have long-term positive effects: "Small inbreeding populations, while initially increasing their chances for harmful homozygotic recessive pairings on a locus, will quickly eliminate such genes from their breeding pools, thus reducing their genetic loads." However, other specialists have argued that these positive long-term effects of inbreeding are almost always unrealized because the short-term fitness depression is enough for selection to discourage it. In order for such a "purification" to work, the offspring of close mate pairings must be either homozygous-dominant (completely free of bad genes) or homozygous-recessive (will die before reproducing). If there are heterozygous offspring, they will be able to transmit the defective genes without themselves feeling any effects. This model does not account for multiple deleterious recessives (most people have more than one) and multi-locus gene linkages. The introduction of mutations negates the weeding out of bad genes, and evidence exists that homozygous individuals are often more at risk to pathogenic predation.
Animals have been observed to inbreed only in extremely unusual circumstances: Major population bottlenecks and forced artificial selection by animal husbandry.
Evolutionary psychologists have argued that humans should possess similar psychological mechanisms. The Westermarck effect, that children who are raised together during the first five to ten years of life have inhibited sexual desire toward one another, is one strong piece of evidence in favor of this. This attraction stems from desensitization to one another's presence and is a sort of reverse imprinting. (Imprinting occurring when animals learn the qualities of an attractive mate.) In what is now a key study of the Westermarck hypothesis, anthropologist Melford E. Spiro demonstrated that inbreeding aversion between siblings is predictably linked to co-residency. In a cohort study of children raised communally (as if siblings) in the Kiryat Yedidim kibbutz in the 1950s, Spiro found practically no marriages among his subjects as adults, despite positive pressure from parents and community. The social experience of having grown up as brothers and sisters created an incest aversion, even though the children were genetically unrelated.
Further studies have supported the hypothesis that some psychological mechanisms cause children who grow up together to lack sexual attraction to one another. Spiro's study is corroborated by Fox (1962), who found similar results in Israeli kibbutzum. Lieberman found that childhood co-residency with an opposite-sex sibling (biologically related or not) was significantly correlated with moral repugnance toward third-party sibling incest. It is not unusual for biological siblings who did not know each other in childhood to be attracted to each other when meeting as adults.
One of Sigmund Freud's most famous theories was the oedipus complex, which involves incestuous desires on the part of the child. In this complex, children wish for the death of their same-sex parent and wish to engage in sexual relations with their opposite sex parent (the version experienced by girls is known as the Electra complex). According to Freud, this complex arises when boys become aware of female genitalia and think of females as castrated by their father. The boy, not wishing to be castrated, comes to resent his father.
Anthropologists have found that marriage is governed, though often informally, by rules of exogamy (marriage between members of different groups) and endogamy (marriage between members of the same group). The definition of a group for purposes of exogamy or endogamy varies considerably between societies. In most stratified societies, one must marry outside of one's nuclear family—a form of exogamy—but is encouraged to marry a member of one's own class, race, or religion—a form of endogamy. In this example, the exogamous group is small and the endogamous group is large. But, in some societies, the exogamous group and endogamous group may be of equal size, as in societies divided into clans or lineages.
In most such societies, membership in a clan or lineage is inherited through only one parent. Sex with a member of one's own clan or lineage—whether a parent or a genetically very distant relative—is considered incestuous, whereas sex with a member of another clan or lineage—including the other parent—is not be considered incest (although it may be considered wrong for other reasons).
For example, Trobriand Islanders prohibit both sexual relations between a man and his mother and those 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. Anthropologists have hypothesized that, in these societies, the incest taboo reinforces the rule of exogamy, and thus ensures that social ties between clans or lineages will be maintained through intermarriage.
Some cultures include relatives by marriage in incest prohibitions; these relationships are called affinity rather than consanguinity. In medieval Europe, standing as a godparent to a child also created a bond of affinity. More recently, 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 the United Kingdom in the nineteenth century, involving, among others, Matthew Boulton.
The Book of Leviticus in the Bible lists prohibitions against sexual relations between various pairs of family members. Father and daughter, mother and son, and other pairs are forbidden, on pain of death, to have sexual relations. It prohibits sexual relations between aunts and nephews, but not between uncles and nieces. Christians interpret it to include the latter by implication, though Jews traditionally do not.
The Qur'an mentions incest in the Surat An-Nisa, which prohibits a man from having sexual relationships with his mother, daughter, sister, paternal aunt, maternal aunt, and niece. Relations with wet nurses are also prohibited. But on the other side, Islam allows marriage with cousins and other more distant relatives. Only in case of marriage does Islam allow sexual relations between cousins and other distant relatives.
Hinduism speaks of incest in highly abhorrent terms. Hindus were greatly fearful of the negative effects of incest and, thus, practice, to date, strict rules of both endogamy and exogamy, that is, marriage in the same caste (varna) but not in the same family tree (gotra) or bloodline (Parivara).
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