Across human societies, incest appears to be a universal taboo. While different cultures define incest in different ways—some allowing marriage between cousins, others prohibiting even distant kinship ties—there is a broad consensus that unions between full siblings are deeply unnatural. The genetic consequences of inbreeding, as well as its disruption of familial roles, have made sibling marriage anathema to most civilisations.¹

Egypt, however, presents an anomaly. From Pharaonic times to the Roman period, marriage between brothers and sisters appears to have been not just accepted, but remarkably common.² This essay will explore what we can say about this custom, drawing on historical, literary, and papyrological sources to assess whether these marriages were genuine instances of incest or the result of alternative legal fictions.

Several Greek and Roman writers commented on the Egyptian practice of brother-sister marriage. Diodorus Siculus (1st century BC) noted that Egyptian law permitted such unions, attributing the custom to the divine example of Isis and Osiris.³ The Jewish philosopher Philo, writing in the early Roman period, contrasted Egyptian permissiveness with the strict prohibitions found in Mosaic law.⁴ Later, Seneca made a sardonic remark about Egyptian brother-sister marriage, assuming that it was common knowledge among his Roman audience.⁵ Pausanias and Sextus Empiricus further confirmed that such unions were a known aspect of Egyptian society.⁶

What is significant about these references is their matter-of-fact tone. Unlike later moralistic critiques of incest, these authors do not suggest that Egyptians viewed the practice as exceptional or shameful. Instead, they present it as an accepted part of Egyptian life, a custom that distinguished them from other Mediterranean peoples. The absence of shock or condemnation suggests that, within Egypt, sibling marriage was deeply entrenched.⁷

While literary sources provide a broad cultural picture, papyrological evidence from Roman Egypt offers statistical confirmation of the practice. Census returns from the Roman period frequently list husbands and wives who share both parents, described explicitly as ‘wife and sister from the same father and the same mother.’⁸ These documents leave little room for ambiguity: full siblings were marrying each other in significant numbers.

Papyrological sources also indicate that these marriages were legally recognised and recorded, suggesting state approval rather than mere social tolerance. Unlike the royal Ptolemies, who may have used incestuous marriages as a dynastic strategy, ordinary Egyptian families engaged in the practice without political motives.⁹ This raises the question: why was sibling marriage so common in Roman Egypt?

One possible explanation is economic. Egyptian society placed great emphasis on keeping property within the family. Brother-sister marriage would have been an effective way to prevent land from passing to outsiders through marriage.¹⁰ Similar motives explain cross-cousin marriage in many societies: keeping wealth consolidated within a kinship group.

Another factor may have been demographic. In a society with high mortality rates, sibling marriages might have served to ensure stable family units. If families lacked viable marriage partners outside their immediate household, marrying a sibling may have been a pragmatic solution.¹¹

Religious beliefs may have also played a role. The divine precedent of Isis and Osiris, who were both siblings and spouses, may have provided a mythical justification for the practice. The Ptolemies, seeking to align themselves with Egyptian traditions, adopted sibling marriage, reinforcing its status within elite culture. Over time, this royal precedent may have influenced broader social norms.¹²

A recent challenge to the traditional interpretation of brother-sister marriage in Roman Egypt comes from Sabine Huebner, who argues that these were not cases of true incest. Instead, she suggests that one spouse was often adopted into the family, creating a legal, rather than biological, sibling relationship.¹³

Adoption was a common strategy in the eastern Mediterranean, often used to secure heirs when natural sons were lacking. In this view, the phrase ‘wife and sister from the same father and the same mother’ was a juridical formula, disguising marriages between adopted and biological siblings. If true, this would mean that the incestuous unions recorded in Egyptian census returns were, in many cases, only nominally incestuous.¹⁴

However, papyrological evidence does not support this hypothesis. Analysis of census records shows no systematic correlation between adoption and brother-sister marriage. Moreover, if adoption were widespread enough to account for these unions, we would expect to find more explicit references to adoption in legal texts, which are largely absent.¹⁵ Thus, while adoption may have occasionally played a role, it does not seem to be the primary explanation.

While full-sibling marriage is rare, other societies have permitted close-kin unions. The Athenians allowed marriage between paternal half-siblings, while the Spartans permitted maternal half-siblings to marry.¹⁶ Persian elites engaged in sibling marriage, though whether this was widely practised or a royal prerogative remains debated.¹⁷

In Egypt, the practice was clearly not confined to the ruling class. It was a recognised, legal form of marriage for ordinary people. If there are parallels, they are faint. No other known society appears to have embraced full-sibling marriage at the same scale as Roman Egypt.¹⁸

Brother-sister marriage in Roman Egypt remains an enigma. The literary sources confirm that it was perceived as a distinctively Egyptian practice, while the papyrological evidence shows that it was widespread. Economic motives, religious precedents, and demographic pressures may all have contributed to its persistence, but no single explanation fully accounts for the phenomenon.

Attempts to explain away the practice through legal fictions such as adoption lack strong evidentiary support. Until more conclusive data emerges, we must take the census records at face value: Egyptians really did marry their siblings. This challenges our assumptions about the universality of incest taboos and forces us to acknowledge that cultural norms, even those that seem most fundamental, are not as immutable as we might believe.

Notes

1. Hopkins, K. (1980). ‘Brother-sister marriage in Roman Egypt’, Comparative Studies in Society and History 22, pp. 321–52.
2. Remijsen, S. & Clarysse, W. (2008). ‘Incest or Adoption? Brother-Sister Marriage in Roman Egypt Revisited’, Journal of Roman Studies 98, pp. 53–61.
3. Diodorus Siculus, Bibliotheca Historica 1.27.
4. Philo of Alexandria, De Specialibus Legibus 3.22–4.
5. Seneca, Apocolocyntosis 8.
6. Sextus Empiricus, Outlines of Pyrrhonism 3.205.
7. Pausanias, Description of Greece 1.7.1.
8. Census returns from Roman Egypt, as cited in Remijsen & Clarysse (2008).
9. Ibid.
10. Ibid.
11. Ibid.
12. Ibid.
13. Huebner, S. (2007). ‘Brother-sister marriage in Roman Egypt: A curiosity of humankind or a widespread family strategy?’, Journal of Roman Studies 97, pp. 21–49.
14. Ibid.
15. Remijsen & Clarysse (2008).
16. Ibid.
17. Ibid.
18. Ibid.

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