Incest refers to any sexual activity between closely related persons (often within the immediate family) that is illegal or a social taboo. 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.

The most frequently reported type of incest is father-daughter 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 by Russell (1986) and Wyatt (1985) has yielded estimates as high as twenty percent.

Consensual adult incest is rarely reported. 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.

Abuse of children

Main: child sexual abuse Incest perpetrated by an adult of either gender against a child is called "intrafamilial child sexual abuse." The most-often reported form of incest is of this inherently abusive form. Father-daughter and stepfather-daughter incest is most commonly reported, with most of the remaining reports consisting of mother/stepmother-daughter/son incest. Father-son incest is reported less often, however it is not known if the prevalence is less, because it is under-reported by a greater margin. Prevalence of parental child sexual abuse is difficult to assess due to secrecy and privacy; some estimates show 20 million Americans have been victimized by parental incest as children.