Incest involves sexual relations between parents and children or between siblings. Other relatives, such as first cousins, sisters-in-law or brothers-in-law may be included, depending on the law of the country involved. Incest may include homosexual or lesbian incest. Incest may cause genetic defects in any children of such a union; and in many societies where incest is taboo, its occurrence may cause family disintegration.
Incest was common among royal families, for example, in Egypt, Hawaii, Peru, Europe and sometimes led to severe genetic defects (such as with the Hapsburgs). If marriage between first cousins is considered incest, then it could be said to have been common among the European aristocracy until the 19th century. In English law it was not a crime until 1908.
Incest is taboo in most societies, primitive and developed. Where it does occur, it is usually under special circumstances, such as in the marriage of royalty (Azande tribe, Africa), or the treatment of incest as a sacred ritual act, or the taking of a forbidden relative as a sexual object by a tyrant to show his power. Other kinds of incest may occur clandestinely. In practice incest is almost always the sexual exploitation of a child by an older relative, usually by an adult male to a young female, often father to daughter. In the USA father-daughter incest accounts for three quarters of incest cases. In Peru about 60 percent of pregnancies in girls between 12 and 14 are the product of incest or rape.