Free: Incest

Because close relatives share a high percentage of their genetic material, they are more likely to carry the same recessive mutations. While these mutations are often harmless when paired with a dominant, healthy gene, mating with a relative increases the chance that the child will receive the mutated gene from both parents. This can lead to a higher incidence of birth defects, genetic disorders, reduced fertility, and higher infant mortality rates. This biological reality is often cited as the evolutionary origin of the "Westermarck effect," a psychological hypothesis suggesting that people raised together in early childhood develop a natural sexual aversion to one another.

Incest refers to sexual relations between people who are closely related by blood. The definition of what constitutes close relatives can vary significantly between cultures and legal systems. Generally, incest involves sexual relations between first-degree relatives, which include parents, siblings, and children. Some definitions may also extend to include sexual relations between individuals related by blood in the second degree, such as aunts, uncles, nieces, nephews, and half-siblings. Incest