Analysts of the incest taboo who believe that cultural determinants alone are a sufficient explanation of human incest avoidance frequently cite alleged sibling marriages in the royal families of Egypt, Hawaii, and Peru as supporting evidence. If full-sibling incest were common in intact families in several populous societies, incest avoidance theories involving genetic components (and natural selection theory itself) would be seriously challenged because there would then exist successful societies that employed a relatively inefficient reproductive strategy. This review of historical sources regarding the actual practices of royal families reveals that full-sibling marriages were extremely rare, except during the Ptolemaic reign. Succession to the throne was almost never by an offspring of siblings. Brother-sister marriage was frequent among commoners in Roman Egypt during the 1st 2 or 3 centuries after Christ. It is concluded that this clear but solitary exception is an insufficient basis for rejecting the interactionist thesis.