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 taboo is

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The most accepted explanation for the incest taboo is that following rules of exogamy is adaptively advantageous. This means that it is evolutionarily advantageous to mate outside of one's immediate family, which helps to promote genetic diversity and reduce the likelihood of harmful genetic mutations. This hypothesis is supported by cross-cultural studies that have found that the incest taboo is nearly universal in human societies.

There Is No Simple or Universally Accepted Explanation for the Fact