Nancy Wilmsen Thornhill states that the drive in humans to not reproduce or be attracted to one's immediate family is evolutionarily adaptive, as it reduces the risk of children having genetic defects caused by inbreeding, as a result of inheriting two copies of a deleterious recessive gene.
Edvard Westermarck said an aversion to marriage between blood relatives or near kin emerged with a parental deterrence of incest.
From a genetic point of view, aversion to breeding with close relatives results in fewer congenital diseases.
Men were obliged to seek wives from other groups, including marriage by capture, and exogamy developed as a cultural custom.
Morgan[7] maintains that exogamy was introduced to prevent marriage between blood relations, especially between brother and sister, which had been common in an earlier state of promiscuity.
According to this theory, groups that engaged in exogamy would flourish, while those that did not would all die, either literally or because they lacked sufficient ties for cultural and economic exchange, leaving them at a disadvantage.
In tribal societies, the dual exogamy union lasted for many generations, ultimately uniting the groups initially unrelated by blood or language into a single tribe or nation.