According to the Merriam-Webster dictionary, incest is "sexual intercourse between persons so closely related that they are forbidden by law to marry; also: the statutory crime of such a relationship." Sociologists call incest "the universal taboo", due to the fact that every human society condemns it, regardless of their other beliefs. Incest almost always occurs among estranged relatives, due to a psychological effect called reverse sexual imprinting. Essentially, if someone is raised from birth around someone they know, instead of becoming sexually attracted to that person, they become protective of that person. Sons tend to be protective of their mothers, and fathers tend to be protective of their daughters for this reason. The reason the incest taboo is embedded so deeply into human instinct is probably because of the significantly higher risk of birth defects among conception between direct relatives. Between cousins, the probability is double the normal rate (of 4%)- if reproduction among relatives gets any closer than that, the probability spikes dramatically.