Incest is sexual intercourse between family members and close relatives. The term may apply to sexual activities between: individuals of close “blood relationship”; members of the same household; step relatives related by adoption or marriage; and members of the same clan or lineage. The incest taboo is and has been one of the most common of all cultural taboos, both in current nations and many past societies. Most modern societies have laws regarding incest or social restrictions on closely consanguineous marriages.
In some societies, such as those of Ancient Egypt and others, brother–sister, father–daughter, and mother–son, cousin-cousin, uncle-niece, aunt-nephew, and other permutations of relations were practiced among royalty as a means of perpetuating the royal lineage.In addition, the Balinese and some Inuit tribes have altogether different beliefs about what constitutes illegal and immoral incest. However, parent-child and sibling-sibling unions are almost universally forbidden.
In countries where it is illegal, consensual adult incest is seen by some as a victimless crime. However, children born of close incestous unions have greatly increased risk of death and disability at least in part due to genetic diseases caused by the inbreeding. Studies with clearly defined terms, control groups, large sample sizes, non-clinical populations, and specific measures of consequences, and controlled for age and socioeconomic group, indicate that childhood incest experiences in lower-class families where the perpetrator has been prosecuted are associated with harmful effects; that occurrence of incest may predispose the individual to certain kinds of problems; that child victims of various reported sex crimes, in addition to incest, in lower-class families are likely to experience harmful effects; and that sibling sexual experiences in middle-class families have little influence on adult sexual functioning.