Incest is any sexual activity between close relatives (often within the immediate family) irrespective of the ages of the participants and irrespective of their consent, that is illegal, socially taboo or contrary to a religious norm. The type of sexual activity and the nature of the relationship between persons that constitutes a breach of law or social taboo vary with culture and jurisdiction. Some societies consider it to include only those who live in the same household, or who belong to the same clan or lineage; other societies consider it to include "blood relatives"; other societies further include those related by adoption or marriage. According to some studies, the most frequently reported type of incest is father-daughter incest. However, others studies suggest that sibling incest occurs as often, or more often, than other types of incest. Incest between adults and prepubescent or adolescent children is considered 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 has yielded estimates as high as twenty percent. although it is seen by some as a victimless crime, and thus, it is rarely reported. Most societies have prohibitions against incest. 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 practised among royalty. In addition, the Balinese and some Inuit tribes have altogether different beliefs about what constitutes illegal and immoral incest.

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