History[edit]
While cousin marriage is legal in most countries, and avunculate marriage is legal in several, sexual relations between siblings are considered incestuous almost universally, regardless of legality. Still, sibling marriage is legally prohibited in most countries worldwide, with exceptions being Sweden, where marriages between half-siblings are legally permitted. Sibling marriage was historically practiced in ancient Egypt and Inca tribes.
Innate sexual aversion between siblings forms due to close association in childhood, in what is known as the Westermarck effect. Children who grow up together do not normally develop sexual attraction, even if they are unrelated, and conversely, siblings who were separated at a young age may develop sexual attraction. Thus, many cases of sibling incest, including accidental incest, concern siblings who were separated at birth or at a very young age.