The family-disruption theory, often associated with Bronislaw Malinowski,58 can best be summed up as follows: Sexual competition among family members would create so much rivalry and tension that the family could not function as an effective unit. Because the family must function effectively for society to survive, society has to curtail competition within the family. The familial incest taboo is thus imposed to keep the family intact.
But while this theory might explain why father-daughter and mother-son incest would need to be prohibited, why couldn’t societies have a rule that brothers and sisters could marry when they were adults? Hypothetically, brother-sister incest would not disrupt the authority of the parents if the children were allowed to marry when mature. And brother-sister marriages did exist in ancient Egypt. The family-disruption theory, then, does not explain the origin of the incest taboo.