Genetic sexual attraction can also occur among relatives that know each other from birth or childhood. This is much rarer, however, because people raised together experience a kind of reverse sexual imprinting called the Westermarck effect, which prevents relatives from developing sexual feelings for one another.
Researchers believe a number of factors can cause genetic sexual attraction. Humans typically choose mates who share physical and mental similarities. Relatives share several traits due to heredity, so when adult relatives meet they may naturally feel drawn to one another.
If people who experience genetic sexual attraction act on their urges and enter a sexual relationship, they are partaking in something known as incest. Incest is frowned upon in society and is illegal in some nations. A child conceived through incest also has a higher risk of developing birth defects. Even if individuals do not act on feelings of genetic sexual attraction, they often feel guilty and depressed about these feelings, which lie outside societal norms.