A common justification for prohibiting incest is avoiding inbreeding: a collection of genetic disorders suffered by the children of parents with a close genetic relationship.
Such children are at greater risk for congenital disorders, death, and developmental and physical disability, and that risk is proportional to their parents' coefficient of relationship—a measure of how close the parents are related genetically.
Numerous papyri and the Roman census declarations attest to many husbands and wives being brother and sister, of the same father and mother.
And second, the incest taboo also applies to non-procreative sex—for example, sex between infertile relatives, relatives of the same sex, or sex performed with birth control.
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Therefore, for example, an Egyptian could marry an aunt, but a Roman could not.
Despite the act of incest being unacceptable within the Roman Empire, Roman Emperor Caligula is rumored to have had sexual relationships with all three of his sisters (Julia Livilla, Drusilla, and Agrippina the Younger).