The most frequently reported type of incest is father-daughter incest. Incest between adults and prepubescent or adolescent children is a form of child sexual abuse that has been shown to be one of the most extreme forms of childhood trauma, a trauma that often does serious and long-term psychological damage, especially in the case of parental incest. Prevalence is difficult to generalize, but research has estimated 10-15% of the general population as having at least one incest experience, with less than 2% involving intercourse or attempted intercourse. Among women, research by Russell (1986) and Wyatt (1985) has yielded estimates as high as twenty percent.
Consensual adult incest is very rare. Consensual incest between adults is criminalized in most countries, although it is seen by some as a victimless crime.
Most societies have some form of incest avoidance. The incest taboo is and has been one of the most common of all cultural taboos, both in current nations and many past societies, with legal penalties imposed in some jurisdictions. Most modern societies have legal or social restrictions on closely consanguineous marriages. However, in some societies, such as that of Ancient Egypt, brother–sister, father–daughter, and mother–son relations were practiced among royalty. In addition, the Balinese and some Inuit tribes have altogether different beliefs about what constitutes illegal and immoral incest.
Research indicates that 46% of children who are raped are victims of family members. (Langan and Harlow, 1994.) The majority of American rape victims (61%) are raped before the age of 18; furthermore, 29 percent (29%) of all forcible rapes occurred when the victim was less than 11 years old. Eleven percent (11%) of rape victims are raped by their fathers or step-fathers, and another 16 percent (16%) are raped by other relatives.
Emotional incest occurs when a parent relates to a child as a substitute for an adult partner. That child may become emotionally bonded to, and codependent with, the parent. Emotional incest usually occurs before physical parent-child incest. Even without physical sexual contact, the consequences to such "bonded" children include a lifetime of partnership difficulties, according to Martyn Carruthers who wrote that this is a socially accepted form of child abuse in many countries.
A study of victims of father-daughter incest in the 70s showed that there were "common features" within families prior to the occurrence of incest: estrangement between the mother and the daughter, extreme paternal dominance, the mother's inability to fulfill her traditional parental role and reassignment of some of the mother's major family responsibility to the daughter. Oldest and only daughters were more likely to be the victims of incest. Furthermore, it was stated that the incest experience was psychologically harmful to the woman in later life, frequently leading to feelings of low self-esteem, unhealthy sexual activity, contempt for other women and other emotional problems.
Adults who were incestuously victimized by adults in their childhood often suffer from low self-esteem, difficulties in interpersonal relationships, and sexual dysfunction, and are at an extremely high risk of many mental disorders, including depression, anxiety, phobic avoidance reactions, somatoform disorder, substance abuse, borderline personality disorder, and complex post-traumatic stress disorder.
The Goler clan is a specific instance in which child sexual abuse in the form of forced adult/child and sibling/sibling incest took place over at least three generations. A number of Goler children were victims of sexual abuse at the hands of fathers, mothers, uncles, aunts, sisters, brothers, cousins, and each other. During interrogation by police, several of the adults openly admitted to engaging in many forms of sexual activity, up to and including full intercourse, multiple times with the children. Sixteen adults (both men and women) were charged with hundreds of allegations of incest and sexual abuse of children as young as five.
According to a study by Floyd Martinson, 10–15% of college students reported childhood sexual experiences with a brother or sister, mostly fondling of genitals rather than actual sexual intercourse. Of those, 30% reported negative reactions and 30% reported positive reactions; 25% of the reported experiences involved coercion and there was a correlation of coercion with the negative responses. A 2006 study showed a large portion of adults who experienced sibling incest have distorted or disturbed beliefs both about their own experience and the subject of sexual abuse in general. An observational study in 1993 found that 16 percent of the 930 adult women interviewed reported that they had been sexually abused by a sibling before they were 18 years old.
Sibling incest is most prevalent in families where one or both parents are often absent or emotionally unavailable, with the abusive siblings using incest as a way to assert their power over a weaker sibling and thereby express their feelings of hurt and rage. Absence of the father in particular has been found to be a significant element of most cases of sexual abuse of female children by a brother. The damaging effects on both childhood development and adult symptoms resulting from brother–sister sexual abuse are similar to the effects of father–daughter, including substance abuse, depression, suicidality, and eating disorders.
Proponents of incest between consenting adults draw clear boundaries between the behavior of consenting adults and rape, child molestation, and abuse. According to one incest participant who was interviewed for an article in The Guardian
"You can't help who you fall in love with, it just happens. I fell in love with my sister and I'm not ashamed ... I only feel sorry for my mom and dad, I wish they could be happy for us. We love each other. It's nothing like some old man who tries to fuck his three-year-old, that's evil and disgusting ... Of course we're consenting, that's the most important thing. We're not fucking perverts. What we have is the most beautiful thing in the world."
The Guardian article also states:
Voices in Action, a US support group for victims of incest, vehemently rejects these arguments: "These teens have been brainwashed into believing this behaviour is natural; it is not ... Sexual abuse is learned behaviour." But some political thinkers are prepared to support the distinction between abuse and consenting relationships. "
In Slate Magazine, William Saletan drew a legal connection between gay sex and incest between consenting adults. As he described in his article, in 2003, U.S. Senator Rick Santorum publicly derided the theory of the Supreme Court ruling to allow private consensual sex in the home (primarily as a gay rights move). He stated: "If the Supreme Court says that you have the right to consensual sex within your home, then you have the right to bigamy, you have the right to polygamy, you have the right to incest, you have the right to adultery." However, David Smith of the Human Rights Campaign professed outrage that Santorum placed being gay on the same moral and legal level as someone engaging in incest. Saletan argued that, legally and morally, there is essentially no difference between the two, and went on to support incest between consenting adults being covered by a legal right to privacy.
In most countries, marriage between cousins is legal, though some religious restrictions exist - for example marriage between first cousins is forbidden by the Orthodox Church and Roman Catholic Church. Many of the United States, however, prohibit such marriages as incestuous.
In ancient China, first cousins with the same surnames (i.e. those born to the father's brothers) were not permitted to marry, while those with different surnames (i.e. maternal cousins and paternal cousins born to the father's sisters) were.
Prior to the introduction of the Latin term, incest was known in Old English as sibbleger (from sibb 'kinship' + leger 'to lie') or mǣġhǣmed (from mǣġ 'kin, parent' + hǣmed 'sexual intercourse') but in time, both words fell out of use.
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 being brother and sister (Lewis, 1983; Bagnall and Frier, 1994; Shaw, 1993). In Hopkins (1980) this is conclusively demonstrated, and more recent scholars in the field have not questioned it. Some of these incestuous relationships were in the royal family, especially the Ptolemies (see the biography of Cleopatra VII, who married more than one of her brothers).
The fable of Oedipus, with a theme of inadvertent incest between a mother and son, ends in disaster and shows ancient taboos against incest as Oedipus is punished for incestuous actions by blinding himself. In the "sequel" to Oedipus, Antigone, his four children are also punished for their parents having been incestuous.
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 AD 295, which divided the concept of incestus 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 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). The taboo against incest in Ancient Rome is demonstrated by the fact that politicians would use charges of incest (often false charges) as insults and means of political disenfranchisement.
Additionally, many European monarchs were related due to political marriages, sometimes resulting in distant cousins (and even first cousins) being married. This was especially true in the Habsburg, Hohenzollern and Bourbon dynasties.
The Book of Leviticus lists prohibitions against sexual relations between various pairs of family members. Men are prohibited, on pain of death, to have sexual relations with their daughters, sisters, mothers, aunts, and various other relations, but it is silent on the subject of sex with a man's niece. (Father–daughter incest is covered by a prohibition on sexual relationships between a man and any daughter born to any woman he has had sexual relationships with, thereby prohibiting his incest not only with his own daughters but also with women who could be his stepdaughters by marriage.)
It is to be noted that the Book of Leviticus says nothing about the marriage of cousins and of uncles and nieces (see Cousin couple).
In some South Indian communities, where gotra membership passed from father to children, marriages were allowed between uncle and niece, while such marriages were forbidden in matrilineal communities, like Malayalis and Tuluvas, where gotra membership was passed down from the mother. A much more common characteristic of south Indian Hindu society is permission of marriage between cross-cousins (children of brother and sister). Thus, a man is allowed to marry his maternal uncle's daughter or his paternal aunt's daughter but is not allowed to marry his paternal uncle's daughter, a parallel cousin, who is treated as a sister.