Incest

Incest consists of sexual relations between closely-related blood relatives. While the boundaries of incest vary from one culture/jurisdiction to another (first cousins? third cousins), the core forms of incest are agreed on universally: sex between parent and child, or between siblings.

It is sometimes believed that incest is and has always been abhorred everywhere and illegal in any culture developed enough to have laws. This is far from the case. In ancient Egypt, for example, sibling marriage of and reproduction by rulers was normal and institutionalized.

Genetic risk of inbreeding

The genetic deterioration and substandard offspring caused, or facilitated, by incest are well known in animal husbandry. The same phenomenon in European royal families, who even if not closely related had a limited pool of (biologically) related mates to choose from, has also caused degeneration in offspring, as in the seventeenth-century Spanish Hapsburgs. This biological fact is used to stigmatize and criminalize incest, although incest, like other types of sexual intercourse, does not necessarily lead to offspring.

Adult incest

Incest between individuals who are both over the age of consent, and which does not lead to reproduction, has become increasingly tolerated in Western countries. It is the subject of several Web sites and has been appearing in pornography more and more frequently since 2010. Prosecutions are rare, and there is an identifiable "pro-incest" movement (although this label is used by those opposed to incest). Prosecutions are rare.

Incest with minors

Incest between minor siblings is generally ignored, especially if they are close in age. However incest between parent and child is often viewed with the same horror as are other types of sex between adult and child. Its extent is unknown and has not been studied; what is known about it comes from legal cases, in which one or more other adults learn of it and go to the authorities, who categorize it as serious child abuse and usually results in the parent's incarceration and the destruction of tge family.