There is no simple or universally accepted explanation for the fact that nearly all cultures ban incest. However, the most accepted explanation for the incest restriction is

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Following rules of exogamy (marrying outside one's group) is considered adaptively advantageous because it promotes genetic diversity, which can lead to healthier offspring and reduce the risk of inheriting genetic disorders. This evolutionary perspective is widely accepted as a key reason for the incest taboo across cultures.

There Is No Simple or Universally Accepted Explanation for the Fact