Quick Take: The Christian Church Didn't Save the West from Cousin Marriage

The many ways in which an oft-cited Joseph Henrich book is wrong about this issue, and why his misleading claims matter

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Here’s two things that I care very much about: historical precision and the issue of cousin marriage.

That’s why I think it’s time to set the record straight about misleading claims made by Joseph Henrich on the role of the Christian Church and the Judeo-Christian tradition when it comes to regulations against first-cousin marriage in European societies.

These claims are in his best-selling book “The Weirdest People in the World: How the West Became Psychologically Peculiar and Particularly Prosperous” (Penguin Books, 2020), and have been cited all over the place. The Amazon page for his book has endorsements and praise from anyone from Nature Magazine to the Washington Post newspaper.

Political and scientific commentators like Steve Sailer, Razib Khan, Ed West and others have added to the love, almost always focusing on one specific claim that Henrich makes: that the Christian Church’s role on making cousin marriage a taboo in Christian lands was one of the key reasons why Europe eventually became the most powerful, influential and developed corner of the world, at least for a while.

This is a delicate and important subject: Henrich himself goes to great lengths to discuss scientific literature showing that cousin marriage is a terrible idea, as it lowers IQ and increases the chance of genetic diseases becoming rampant in a society. Even if one discounts those (huge) effects, cousin marriage favors tribalism and discourages social trust.

Yes, cousin marriage is an awful idea. That consanguinity is a terrible thing is evident in the fact that even the most happy-go-lucky societies in the world draw the line at letting siblings marry and have (almost always sickly) children, despite frequent proclamations that liberty involving what one does in the bedroom is the most sacred liberty of all. Well, not when it comes to brothers having intercourse with sisters, no sir, and most of us understand why without much explanation.

Late last May, a paper in the Oxford Journal of Law and Religion made a very straightforward call, in the headline, presenting “The Case for Banning Cousin Marriage”, citing Henrich himself, who has now become the go-to-guy to discuss cousin marriage:

The Economist recently ran an editorial titled ‘Cousin marriage is probably fine in most cases’ which suggested that all existing bans should be ‘stricken from the books’; yet, as Professor Joseph Henrich points out, the author/s simply ignored the overwhelming evidence to the contrary. It is deeply regrettable that a respected national broadcaster and magazine should publish material that trivializes and obfuscates the dangers for the very audiences that need to give them serious consideration.

Now, it’s important to understand where Henrich goes wrong in his book. He’s not wrong in claiming that cousin marriage is bad, or that low rates of cousin marriage have long been a competitive advantage of Western societies. Where he’s wrong is in claiming that the Christian Church is responsible for this taboo, and is the main reason why it’s been enforced. In reality, one could argue that the opposite is closer to the complex truth.

Throughout the book, Henrich uses the acronym MFP (“marriage and family program”) to refer to the set of Church policies helping the creation of the West’s nuclear family, including the ban on cousin marriage. Thus, Henrich writes (pp. 181-182):

The MFP is a mixture that peppers a blend of old Roman customs and Jewish law with Christianity’s own unique obsession with sex (i.e., not having it) and free will. Early Roman law, for example, prohibited close cousin marriage, though the law of the Roman Empire—where Christianity was born— permitted it without social stigma. Jewish law prohibited marriage (or sex) with some affines but permitted cousin marriage, polygynous marriage, and uncle-niece marriage. Roman law only recognized monogamous marriages, but basically ignored secondary wives and sex slaves (until Christianity took over).

The Church blended these customs and laws with new ideas, prohibitions, and preferences in creating the MFP. At the same time, other religious groups experimented with their own combinations of customs, supernatural beliefs, and religious taboos. Then, equipped with their different cultural packages and divine commitments, these groups competed for adherents. Winners and losers were sorted out in the long run (Chapter 4).

Zoroastrianism, a potent universalizing religion in Persia, favored marriage to relatives, especially cousins, but including siblings and other close relatives. Today, Zoroastrianism survives, but with only a few hundred thousand adherents. The other Abrahamic religions all build off Mosaic law in various ways. All permitted cousin marriage for centuries after the Church’s ban began, and some still permit it today. Cousin marriage is by far the most common form of kin marriage, so if you aren’t banning cousin marriage, you’re missing a pillar of intensive kin-based institutions. Similarly, both levirate and polygynous marriage were permitted in Judaism and Islam. This is interesting because it means that, although the Church’s policies also built on Mosaic law, the MFP overruled implicit biblical endorsements of levirate, cousin, and polygynous marriage.

There are several issues here. Even though Henrich’s book doesn’t contain the words “Indo-European” or “Aryan,” the taboo against cousin marriage is much more than an early Roman oddity that the Church later reinforced. It’s a widespread taboo of all Indo-European and Aryan peoples that left the Eurasian steppes between 5,000 and 3,000 years ago and settled all across Europe, plus large chunks of Asia like Iran, Armenia, the Kurdistan, Tajikistan, Afghanistan and India.

All steppe peoples shared the cousin marriage taboo, which is completely unrelated to Jewish or any Semitic law. We know this because it’s cited all over the ancient literature and in early law-codes. Livy reports that in ancient Rome marriages within the seventh degree of relationship were not permitted. The ban remained in place by the mid-Republic, reinforcing social cohesion as it decreased clannish propensions.

In fact, persistent aversion to cousin marriage appears to have been an Indo-European super-power; in India, Aryan invaders brought with them an aversion with cousin marriage that, faced with cousin-marriage tolerance among the older Dravidian population, only eroded over time: the Manusmriti, a compilation of Aryan laws first written down in around the 2nd century AD, forbids marriage between a man and the daughter of his maternal uncle or paternal aunt; Medatithi, a 10th century commentator on the text, stated that such cross-cousin marriages are against dharma; but Madhava, a 14th century commentator who apparently lived in a part of South India where such marriages were socially accepted, already objected against the ban, citing Vedic passages and custom.

In Mycenaean (pre-Classical) Greece, the Aegean Islands and Crete cousin marriage was still somewhat common by the mid-2nd millennium BC possibly because of a persistence of older customs, and was only wiped out when additional Indo-European invasions changed the region again from the late 2nd millennium BC.

That helps to explain what happened in Zoroastrian Iran, earlier mentioned by Henrich: as in India, Aryan invaders brought with them an aversion with cousin marriage that, faced with cousin-marriage tolerance among the older Dravidian population, slowly gave way to acceptance.

The persistence of the Vedic tradition, partly through the caste system, made that erosion slow in India. In Iran, however, where Aryans quickly merged with local Dravidians to the point that all Iranians (from the word for “Aryan”) spoke steppe languages by the first half of the 1st millennium BC, some of the early Mazdean texts already recommend “khvetukdas” (marriage between very close relatives) as highly meritorious.

In summary, Henrich's idea that cousin marriage was rampant before the Church came up with the novel idea that it was wrong is just not right. In fact, it was the Church itself that did much to weaken the taboo, by making cousin marriage legal — as it is in the Bible, its number one source of legislation and inspiration.

This is the context: the anti-Christian emperor Diocletian in the late 3^rd century forbade all marriages with close relatives, seeking to uphold ancient Roman mores, but this stance was quickly softened by his Christian successors. Thus, Constantine, the first Christian emperor, within a few decades had his four children marry first cousins, as Henrich himself notes (p. 169), not adding that this was so extreme an occurrence that there are no antecedents in Roman history (there are isolated cases, not four in a single family).

In December 396, a Roman law was enacted that banned men in “incestuous” first cousin marriages from giving property to their wives or children, but the taboo had already been weakened. In 401, the famous, wealthy Roman senator Symmachus asked for a rescript to allow a marriage between first cousins. These factors may have triggered controversy (the evidence is spotty) so that, in January 405 Christian-inspired changes were imposed to “ancient law” regarding such marriages, to protect their offspring in the same manner as other illegitimate offspring was eligible for filial rights, in the absence of legitimate brothers and sisters.

Later in the same year, the ban on first cousin marriages was lifted by the Christian Arcadius, for the most important half of the Roman Empire, the eastern one. This is all explained in “Law in the Crisis of Empire, 379-455 AD: The Theodosian Dynasty and Its Quaestors with a Palingenesia of Laws of the Dynasty,” by Tony Honore (Clarendon Press, 1998, p. 96). As Henrich notes (p. 173), in the Western Empire Honorius made a similar announcement in 409, allowing dispensations for Theodosius’ earlier ban on cousin marriage, in line with the changes of 405.

Cousin marriage remained legal, and more common than before, for almost two centuries, until Pope Gregory I arrived on scene. As the son of a Roman senator and likely descendant of earlier Popes with close ties with the Ostrogothic Kingdom and the Roman Empire, Gregory took a personal decision to definitely ban cousin marriage among Christians just one year after he became pope in 590, and this was not a decision based on Christian tradition or scriptures, even if he used them to justify it.

The ban required considerable contortions. Gregory, in fact, cited Leviticus 18:6 (“No one is to approach any close relative to have sexual relations”) in support of his ruling arguing that such first cousin marriages produce defective children.

This is a remarkable sleight of hand since Leviticus 18:7–18 provide explicit guidelines for acceptable partners that, in the Semitic tradition, make first-cousin unions, and indeed uncle–niece relationships, lawful (for details, I recommend “The background and outcomes of the first-cousin marriage controversy in Great Britain,” A H Bittles, International Journal of Epidemiological Advances, 2009).

So, to clarify: we have a pope who lied and manipulated to make an ancient tradition of his own people stick against the scripture that his religion held to be sacred.

Nobody should be surprised that the result was not a unified Church position against cousin marriage, but a mess of confusing, contradictory announcements that hardly amount to a policy. That mess is well explained for example here, with a focus on the work o the subject by German scholar Karl Ubl. As the author, Rachel Stone, eloquently puts it:

Ubl describes a complex story, a punctuated equilibrium where secular rulers and individual churchmen have an outsize role, rather than the steady programme of a collective “church”. For that reason, I suspect his results will often continue to be ignored even by research that cites him.

As a Southern European with a Catholic education, I find it amusing that the most influential person to openly challenge Gregory I was Martin Luther, a careful reader of the Bible, who almost a millennium later condemned the ban on first cousin marriages on the grounds that Leviticus 18 indeed has nothing against them. It wasn’t the case that Luther liked or recommended the practice, simply that he called out Gregory I for having made up a Christian justification for it.

This made the ban effectively null and void in many Protestant societies, the most MFP societies of all, for centuries. In addition, Leviticus 18 still had even more impact when Henry VIII cited it as grounds to annul his marriage to Catherine of Aragon around the same time; his biblical scholarship thwarted by the Vatican, Henry broke with the Catholic Church. And this is not the end of the issue.

Having embarked on this major religious realignment, and wishing to marry Catherine Howard, a cousin of his executed second wife Anne Boleyn, in 1540 Henry issued a statute that legalized marriage between all first cousins. Two decades later, Elizabeth I, Anne’s daughter, codified the consanguinity and affinity regulations as “The Tables of Kindred and Affinity of the Church of England.”

In 1839, protected by such laws, the famous scientist Charles Darwin married a first cousin. (Three of his ten children died young, but he got really lucky with the rest: their mean age at death was 77 years; three of the sons, George, Francis and Horace, were elected Fellows of the Royal Society of London for their scientific work; Leonard was a Member of Parliament from 1892 to 1895 and President of the Geographical Society from 1908 to 1911, as Steve Sailer has noted.)

Still, it’s not clear that the tolerance for first cousin marriage in Protestant Churches (except the Lutheran Church of Sweden, remarkably, according to A. H. Bittles) resulted in a significant increase in marriages of high consanguinity, and indeed every available piece of data indicates that these never reached the levels seen in Islamized societies. Which, again, argues for a social, rather than religious, taboo.

This is why I can confidently state that, when Henrich writes (p. 169), that in the Roman lands “cousin marriage in some form was socially acceptable (and) continued until the Church started its relentless opposition,” he is not right. The Church, as I just explained, was far from relentless in its opposition to a custom that had been rejected by Indo-European societies for millennia before even the Church itself was built.

To some, the above may appear to be an antiquarian discussion, with much recourse to ancient sources and documents and little relevance for us modern people of the 21st century. It’s not: at a time when the allegedly shared Judeo-Christian tradition is invoked to make people supportive of certain policies over others, it’s important to be able to state that oft-cited elements of such Judeo-Christian tradition are figments of collective imaginations.