ELSEVIER Brother-Sister and Parent-Child Marriage Outside Royal Families in Ancient Egypt and Iran: A Challenge to the Sociobiological View of Incest Avoidance? Walter Scheidel Darwin College, Cambridge, United Kingdom Copious and unequivocal evidence of legally condoned and socially favored brother- sister and parent-child marriage among common people from Roman Egypt (first to third centuries, A.D.) and Zoroastrian Iran (fifth century, B.C. to 11th century, A.D.) can be taken to pose a challenge to the sociobiological case for universal evolved incest avoidance within the nuclear family, triggered by early childhood proximity (the Wes- termarck effect). Official census documents from Roman Egypt show a high incidence of full sibling unions with relatively small age gaps between the spouses and no indication of reduced marital fertility, sexual aversion, or increased infant and child mortality. Zo- roastrian religious tracts actively encourage nuclear family incest and extol its meritori- ous nature and supernatural benefits. A schematic assessment of the likely extent of in- breeding depression in such families under conditions of very high mortality through other causes makes reproduction at replacement level seem difficult to accomplish. But given the lack of information on the frequency of deleterious recessive genes in these populations, this reconstruction is fraught with uncertainty; pertinent ancient evidence is suggestive of some incidence of inbreeding depression but remains inconclusive. Aver- sion and revulsion between incestuous spouses proves a similarly elusive issue. Although these eases from antiquity do not clearly contradict the view of incest avoidance as an evolved mechanism that engenders sexual indifference and normally translates into cor- responding cultural norms, they demonstrate the need for a more comprehensive consid- eration of the available historical record in the testing of evolutionary rules. © Elsevier Science Inc., 1996 KEY WORDS: Incest; Inbreeding; Inbreeding depression; Westermarck effect; Human so- ciobiology. Received February28, 1996;revisedJune 14, 1996. Address reprintrequests and correspondenceto: WalterScheidel,DarwinCollege, CambridgeCB3 9EU, United Kingdom. Ethology and Sociobiology 17:319-340 (1996) © ElsevierScienceInc., 1996 655 Avenue of the Americas, New York, NY 10010 O162-3095196/$15.00 PII SO 162-309(96)00074-X