Widespread brother-sister incest in Roman Egypt and Zoroastrianism

The incest taboo is strong among humans, but in two cases, it has been overcome, or so it seems. The first case is in Roman Egypt, where for over 250 years, there was a tradition of full brother-sister marriages. The second case is in Zoroastrianism, where brother-sister (and other close-relative) marriages are considered a sacred duty called Xwedodah.

Roman Egypt needs no introduction, though Zoroastrianism might need some, so here's my summary:

1. There are two cosmic principles: good and evil, light and dark. They are equally powerful.
2. Ahura Mazda is the one true God that is all good. There are smaller gods, some working for Mazda and some against him.
3. Originally, the world was all good, as Mazda intended. But then evil appeared and since then has been a battle of good and evil.
4. Ahuras are good gods, and Daevas are evil gods.
5. Humans should choose to side with the good and reject the evil.
6. The Avesta are the sacred texts of Zoroastrianism.
7. Eventually evil will be eliminated and the world will be as good as Ahura Mazda intended. Evil creatures will either be destroyed or be purified (depending on who you ask).

Now let's look at the literature.

Strong, Anise K. “Incest Laws and Absent Taboos in Roman Egypt.”

Various types of close-kin marriage did exist elsewhere in the Mediterranean, although none appear to have been as widespread or to offer the historian as detailed evidence as the Egyptian instances.
For at least two hundred and fifty years, many men in the Roman province of Egypt married their full sisters and raised families with them. During the same era, Roman law firmly banned close-kin marriages and denounced them both as nefas, or sacrilegious, and against the ius gentium, the laws shared by all civilized peoples. In Egypt, however, Roman officials deliberately chose not to enforce the relevant marriage laws among the Greek metic, hybrid, and native Egyptian populations; the bureaucracy also created loopholes within new laws which tolerated the practice. This policy created a gap between the absolute theoretical ban in Roman law and the reality of common incestuous unions in Egypt.

In Ovid’s Metamorphoses, the character of Myrrha laments her incestuous passion for her father and refers to the contrast between Roman law and Egyptian custom:

Jealous men have established spiteful laws; and what nature allows, jealous judgments deny. Nevertheless it is said that there are peoples (gentes) in which the mother is joined to her son and the daughter to her father, and through this filial affection increases in a double bond. Wretched me, since I was not born in such a place!

Ager, Sheila L. “The Power of Excess: Royal Incest and the Ptolemaic Dynasty.” Anthropologica 48, no. 2 (2006): 165–86. https://doi.org/10/b6rn3x.

The ancient Greeks were certainly repelled by the notion of parent child or full sibling incest. There is plenty of evidence for such attitudes. But... first cousin-marriage was fully acceptable and very common; and in the city-state of Athens, at least, it was permissible for an uncle to marry his niece, and even for a half-brother to marry a half-sister, provided that they were children of the same father, not the same mother.

Shaw, Brent D. “Explaining Incest: Brother-Sister Marriage in Graeco-Roman Egypt.” Man 27, no. 2 (1992): 267–99. https://doi.org/10/bw6r65.

Proposes a socioeconomic explanation.

The case of the Egyptian colonial Greek settler class is a very specific and special one. Given the extreme social pressures dictated by their circumstances, we must see their decisions as to whom to marry as involving, on occasion, the question of just how close a kinsperson they would be willing to contemplate for the liaison. In a certain proportion of all cases (about one-sixth or so, according to our surviving data) we know that they were willing to collapse their traditional inhibitions against marriage between sibling

Scheidel, Walter. “Incest Revisited: Three Notes on the Demography of Sibling Marriage in Roman Egypt.” The Bulletin of the American Society of Papyrologists 32, no. 3/4 (1995): 143–155.

the incidence of brother-sister marriage in Arsinoe approached the theoretical maximum. In this scenario, nuclear family incest becomes the rule rather than merely a common choice. A population in which, for several generations, 80 to 90 percent of brothers married their own somewhat younger sisters is indeed a historical oddity of the first rank. There is no population on record, however small, that would even have begun to come close. 'Normative' sibling marriage is completely unknown in any other time or place.

The mean [coefficient of inbreeding] for the free city population would rise well beyond 10 percent... [exceeding] rates attested for any existing population. A mean rate of inbreeding of 2 to 3 percent is usually found in populations characterised by a high incidence of first-cousin marriage (accounting for one third to one half of all unions). Even in the most extreme genetic isolates on this planet that include certain religious sects, such as the Samaritans, small caste groups in India or the population of the forlorn Atlantic island of Tristan da Cunha the average coefficient of inbreeding always seems to remain in a range from about 5 to 7 percent.

Scheidel, Walter. “Brother-Sister and Parent-Child Marriage Outside Royal Families in Ancient Egypt and Iran: A Challenge to the Sociobiological View of Incest Avoidance?” Ethology and Sociobiology 17, no. 5 (September 1, 1996): 319–40. https://doi.org/10/bqfw3h.

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.)... 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. Zoroastrian religious tracts actively encourage nuclear family incest and extol its meritori- ous nature and supernatural benefits.

Concerning the Roman Egypt city of Arsinoe (modern Faiyum)

What is more, four census returns, as well as one other papyrus, document full brother-sister or half sibling marriage that continues through two generations.
There is no evidence for adoption.
he age difference between brothers and sisters married to each other is found to be somewhat smaller than for unrelated spouses
The majority of these marriages resulted in offspring
Only two sibling couples out of 24 got divorced, and childless couples of an advanced age are similarly rare... there is not a single case in the census returns of a full sibling couple who failed to arrange an analogous marriage for their own children if it was at all possible.

Curiously, in ancient Egyptian literature, calling your lover "brother" and "sister" is lovely. This causes problem in figuring out if their sibling marriages were emotional at all.

Owing to the use of the terms "brother" and "sister" as terms of endearment devoid of any specific biological meaning in private letters and other papyrus texts... it is impossible to decide whether individuals who address each other in this way are actually related whenever independent means of corroboration are lacking. Because of that, several papyrus letters containing longing and loving messages to an absent "brother" or "sister" that have been taken to reveal the feelings of sibling couples cannot be used for that purpose.

Concerning the Zoroastrian Iranians

[They] made themselves laws to take their sisters, daughters, and granddaughters to wife; some even go further and take their own mother to wife... They are called the Magians. And in all places and climates where they live, they live according to this law enjoined upon their fathers".
About sixty references to that custom come from the Greek and Roman world, Armenia, Arabia, and even India, Tibet, and China, ranging from the fifth century B.C. well into the Middle Ages.
But even more importantly, the extant copious religious writings of ancient and early medieval Zoroastrians contain at least 40 additional explicit references to this practice, which is normally translated as "next-of-kin marriage" (xwedodah).

the religious texts nonetheless offer unequivocal evidence for the desirability and occurrence of nuclear family incest. The motives behind this practice are said to be "a desire to preserve the purity of the race, to increase the compatibility of husband and wife, and to increase affection for children, which would be felt in redoubled measure for offspring so entirely of the same family". The puzzling intricacies of an incestrous family are openly acknowledged but presented in a favorable light: "blessed is he who has a child of his child... pleasure, sweetness, and joy are owing to a son that a man begets from a daughter of his own, who is also a brother of that same mother; and he who is born of a son and mother is also a brother of that same father; this is a way of much pleasure, which is a blessing of the joy ... the family is more perfect; its nature is without vexation and gathering affection

Next-of-kin marriage is also imbued with heavy religious and magical properties... "For it is revealed that the first time a man has intercourse, 1,000 demons and 2,000 sorcerers and witches die; the second time 2,000 demons and 4,000 sorcerers and witches die; the third time 3,000 demons and 6,000 sorcerers and witches die; the fourth time both man and woman become manifestly blessed". Perhaps the most intriguing reference that turns usual conventions upside down is a passage from the Zoroastrian precursor of Dante's Inferno, the Book of Arda Wiraz, where a visitor to hell enquiring about the sin committed by a woman entangled in a serpent that crawls into her mouth is given the explanation, "This is the soul of that wicked woman who violated the next-of-kin marriage" (who, in other words, refused to have sex with her father, brother, or son and therefore wound up in hell)

Zoroastrian incest may have been concentrated in polygamous and upper-class families that were able to cope with any unpalatable side effects.

Even Zoroastrians were uncomfortable with incest.

Not only hostile outsiders but the Zoroastrian sources themselves betray uneasiness about brother-sister and parent-child marriage. It also remains uncertain to what extent incest became popular among common people of Iran.

Remijsen, Sofie, and Willy Clarysse. “Incest or Adoption? Brother-Sister Marriage in Roman Egypt Revisited.” The Journal of Roman Studies 98 (2008): 53–61. https://doi.org/10/cpdpbg.

adoption and marrying an adopted son to a natural daughter were common practices in the eastern Mediterranean and that adoption was probably more frequent in Egypt than our sources suggest. We doubt, however, that marriage between natural and adopted children explains brother-sister marriage... ancient authors considered brother-sister marriage as a peculiarity of the whole Egyptian population... papyrological sources... suggest that full brothers really married their sisters.