When did northwest Europeans become WEIRD?

Not when they became Christian

The countries in green are more individualistic than the world average. The ones in red are more collectivistic (Beugelsdijk & Welzel, 2018). This difference between northwest Europeans and other humans goes back at least a millennium. Was it due to Western Christianity or does the cause go farther back?

Joseph Henrich has argued that northwest Europeans are less clannish and more individualistic because Western Christianity banned cousin marriage—to a limited degree in the 7^th century and then much more in the 9th. Evidence from aDNA, however, suggests they were already avoiding cousin marriage before the ban.

WEIRD is an acronym for Western, Educated, Industrialized, Rich, and Democratic. It refers to people of northwest European descent and was coined to warn against using them in studies to understand human nature in general. They are not necessarily the same as people elsewhere.

Two decades ago, Joseph Henrich and other social scientists concluded that WEIRD people are outliers in many aspects of mind and behavior, including sensory ability, economic preferences, personality structure, morality, and cognition. They are divergent because they have adapted to an atypical environment of weak kinship, strong individualism, and “impersonal pro-sociality”—in other words, social interactions that are less personal, less emotionally intense, and extending much further than friends and family (Henrich, 2024; Schulz et al., 2019).

For at least a thousand years, this behavioral environment has prevailed north and west of a line running from Trieste to St. Petersburg (known as the Hajnal Line). It is characterized by certain longstanding patterns of behavior:

• Solitary living for at least part of adulthood, with many individuals remaining single their entire lives.

• Departure from the home upon reaching adulthood, either to form a new household or to circulate among unrelated households, typically as servants.

• Less loyalty to kin and greater willingness to trust strangers (Schulz et al., 2019; see also Frost, 2017; Frost, 2020; Hajnal, 1965; Hartman, 2004; hbd*chick, 2014; ICA, 2020; MacDonald, 2019; Seccombe, 1992, pp. 94-95, 150-153, 184-190).

Some authors, like Joseph Henrich, see WEIRDness as a legacy of Western Christianity, which arose in Western Europe after the fall of the Roman Empire and the rise of barbarian kingdoms. According to this theory, northwest Europeans became less clannish and more individualistic because the Western Church increasingly restricted cousin marriage:

1. Roman times: only first-cousin marriages were banned.

2. Seventh century: the ban was extended two degrees further when the Western Church adopted the anti-incest prohibitions of the Visigothic Code.

3. Early ninth century: the Western Church began to calculate degrees of kinship through the so-called “Germanic system,” thus doubling the number of forbidden marriage partners (Chandelier, 2021, p. 224; see Note).

The last restriction forced almost everyone to marry outside their local kin group, thus causing clans to disappear and making people more individualistic and less concerned about kinship ties (hbd*chick, 2014; McCann, 2010, pp. 57-58; Schulz et al., 2019).

Other authors, however, have argued that the cousin marriage ban was simply a Christianization of existing norms, specifically Germanic ones (Frost, 2020; Kirkegaard, 2025; MacDonald, 2019; Policy Tensor, 2021). As the Christian faith spread north and west, it absorbed local customs, including those relating to marriage:

During the period preceding the Teutonic invasion, speaking broadly, the church adhered to the Roman law and custom; thereafter those of the Germans, even when the marriage consisted in the formal sale and tradition [i.e., transfer] of the bride, were accepted. (Howard, 1904, p. 291).

Germanic provenance is evident in the bans themselves. The seventh-century ban was taken from the Visigothic Code, and the new kinship calculation method, adopted in the ninth century, was referred to as “Germanic” (Chandelier, 2021, p. 224; Frost, 2020; McCann, 2010, pp. 57-58; see Note).

Moreover, as shown by data from early medieval estates, northwest Europeans were already WEIRD in the ninth century, when the most extreme ban on cousin marriage came into effect. French households were small and nuclear, with 12% to 16% of adults not yet married and adults usually marrying in their mid to late twenties (Hallam, 1985, p. 56). High rates of delayed marriage seem to have long been common among northwest Europeans, as suggested by the writings of Julius Caesar and Tacitus on the Germanic tribes of Antiquity:

Those who have remained chaste for the longest time, receive the greatest commendation among their people: they think that by this the growth is promoted, by this the physical powers are increased and the sinews are strengthened. And to have had knowledge of a woman before the twentieth year they reckon among the most disgraceful acts.

Caesar, De Bello Gallico 6: 21

Late comes love to the young men, and their first manhood is not enfeebled; nor for the girls is there any hot-house forcing; they pass their youth in the same way as the boys.

Tacitus, Germania 20

Evidence from aDNA

For earlier evidence of cousin marriage avoidance, we can look at DNA from remains in ancient cemeteries, specifically the degree of genetic similarity between spouses as measured by runs of homozygosity (ROH).

To date, there have been four such studies from pre-Christian Western Europe. All four show that cousin marriages were already rare.

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A study of 57 genomes from the first century BC to the first century AD in southern England

“Y chromosome diversity is high …, and patterns of ROH imply that these were relatively large outbreeding communities.” (Cassidy et al., 2025)

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A study of 248 genomes from the third to eighth centuries in southern Germany

“The near absence of long (>12 cM) runs of homozygosity (ROH) and the lack of shared IBD segments (>8 cM) between spouses support strict incest avoidance, excluding relationships closer than the sixth degree.” (Blöcher et al., 2025)

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A study of 722 genomes from the seventh to eighth centuries in Austria

“Given that none of the newly reported individuals carry high amounts of runs-of-homozygosity genomic regions—the indication of inbreeding—as estimated by hapROH … , we infer that consanguinity was strictly avoided in both MGS [Mödling site] and LEO [Leobersdorf site] across six generations …. That was mainly achieved by exogamy: 17 of the 19 (90%) mothers buried in Leobersdorf with identifiable offspring have no ancestors buried on site; in the much larger community of Mödling, they are 46 out of 59 (78%). Many daughters seem to have left to be married elsewhere; between ages 7 and 17 years, the sex ratios of the deceased male to female individuals at LEO and MGS are about 1.5:1 and 1.7:1 respectively, and among adults, hardly any female individuals born by parents on site remain.” (Wang et al., 2025)

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A study of 424 genomes from the second to mid-ninth centuries in Hungary

“[W]e find no cases of biological consanguinity, based on the absence of long runs of homozygosity (ROH) segments in all analysed individuals …. We do not even detect ROH patterns consistent with more-distant consanguineous unions, such as at the level of second-degree cousins, despite a high occurrence of levirate and multipartner unions. Among Eurasian steppe peoples, intermarriage within the paternal line was permitted only after a certain number of generations, which could range between five and nine. Such rules would explain the absence of even distant biological consanguinity. It is intriguing that the only case we detected of reproductive partners being related was to the sixth degree (which would still be consistent with such rules) and involves the only non-exogamous female individual in RK. This further suggests the uniqueness of this single case.” (Gnecchi-Ruscone et al., 2024)

Discussion

Cousin marriage

The aDNA evidence points to a single conclusion: in pre-Christian times, cousin marriage was already rare in northwest Europe.

But it was also rare farther east. In the Austrian and Hungarian studies, the DNA came not only from indigenous Europeans (probably Slavic peoples) but also from Avars—a Turkic people who entered the Carpathian Basin in the sixth century. Avoidance of cousin marriage may therefore be part of a broader, North Eurasian adaptation.

This point is made, in part, by one of the above studies: “Among Eurasian steppe peoples, intermarriage within the paternal line was permitted only after a certain number of generations, which could range between five and nine. Such rules would explain the absence of even distant biological consanguinity” (Gnecchi-Ruscone et al., 2024).

In fact, cousin marriage may have been rare across all of Eurasia before becoming frequent in those regions where it is now common, i.e., the Middle East, North Africa, and Central/South Asia. This is the conclusion of two other studies, one of 411 genomes over the past 15,000 years and another of 1,785 genomes over the past 45,000 years (Ceballos et al., 2021; Ringbauer et al., 2021). Neither study, however, is sufficiently powered to tell us exactly when, where, or how cousin marriage became more frequent. Perhaps it began among the elites of early Middle Eastern civilizations and was later emulated by everyone else with the spread of Islam.

In addition, neither study distinguishes between avoidance of first-cousin marriage and avoidance of all cousin marriages up to the sixth degree. The latter may have reached higher levels within a smaller zone of Eurasia.

WEIRD morality

If avoidance of cousin marriage was not specific to northwest Europeans, we should seek the origins of WEIRDness in other mental and behavioral aspects, particularly those that have left traces in aDNA.

One such aspect is the moral system. Northwest Europeans are inclined toward a more universal and absolute conception of morality, which in turn is linked to certain heritable traits.

In this moral system:

• Rules are framed in universal and absolute terms, as opposed to the situational and relativistic framing of kinship.

• Help is willingly given to non-kin, as long as they belong to the same community of rule followers.

• Continual rule breaking leads to expulsion from the community. The line between insiders and outsiders is much more a line between the morally worthy and the morally worthless. Xenophobia is a moral judgment, rather than a simple rejection of the “Other.”

This system has favored—and been favored by—an increased capacity for affective empathy and guilt proneness:

• Affective empathy is extended from the mother-child relationship to all social relationships. Through this involuntary transfer of another person’s feelings to oneself, rule breaking is experienced emotionally as harm not only to others but also to oneself.

• Rule breaking is punished much more by guilt than by shame. A rule breaker feels guilty even when no one else has witnessed the rule breaking. In contrast, shame is felt only when there are witnesses. Everyone thus accumulates a burden of guilt, which can be reduced only through forgiveness, penance, confession, absolution, etc. (Benedict, 1946; Frost, 2017; Frost, 2020).

Like avoidance of cousin marriage, this moral system likely preceded the Christianization of northwest Europeans. In fact, the latter may have transformed the former, creating a branch of the Christian faith that is not only more fearful of cousin marriage but also more preoccupied with guilt, particularly the inevitability of guilt and the continual need to reduce one’s burden of guilt. This may be why the doctrine of original sin is more important in Western Christianity than in Eastern Christianity, and more important in Protestantism than in Catholicism.

To track how guilt proneness and affective empathy have evolved in northwest Europeans, we should look for indications in aDNA. For instance, changes to the mean population level of affective empathy could be tracked by examining the alleles associated with this trait, which is 52-57% heritable (Frost, 2020). These alleles have already been identified and used to calculate polygenic scores (Wendt et al., 2022). The same approach might show us how guilt proneness has varied over space and time.

Note

In the Roman Empire, marriage between close relatives was forbidden up to the third degree, that is, marriage was prohibited between brothers and sisters, parents and children, and between uncles and nieces or aunts and nephews, as well as between ascendants and descendants without limit of degree.

Beginning in the 8th century, the Church began to impose a broader restriction and forbade unions up to the seventh degree, while changing the calculation method to impose the tradition of Germanic origin whereby the degree no longer means the number of relatives between two individuals, but the number of generations before reaching a common ancestor. Thus, two first cousins are kin in the fourth degree under the Roman system, but only in the second degree under the Germanic system: their common ancestor is their grandfather or their grandmother.

The seventh-degree prohibition promoted by the Church therefore corresponds to the fourteenth degree under the old system. This considerably broadened the scope of prohibited unions. It was enough to have in common a great-great-great-great-great-great grandparent for a marriage not to be permitted (my translation) (Chandelier, 2021, p. 224).

Postscript: Discussion on X

After this article was published in Aporia Magazine on June 26, 2025, Joseph Henrich posted a reply on X, which I reproduce below. It seems to be a generic reply to critics of his work, rather than a specific reply to me. In reality, I don’t deny that a relationship of causality exists between WEIRDness and Western Christianity. Nor do I try “to clump Europeans together psychologically.”

I disagree with him only on the direction of the causality. Western Christianity didn’t create WEIRDness. Rather, the latter transformed the former by making the Western branch of Christianity more focused on the individual, more ideological, and more preoccupied with guilt as a means of behavioral control.

Joseph Henrich argues for the reverse: over time, the Western Church made northwest Europeans more and more WEIRD. As proof, he cites the correlation between WEIRDness and the time depth of Western Christianity in various regions. But this correlation is due largely to the expansion of Western European states into regions south and east of the Hajnal Line (notably southern Spain and southern Italy). As these states expanded, they imposed Western Christianity on populations that had previously practiced Islam or Eastern Christianity. So the correlation with time depth is really a correlation with the geopolitical expansion of Western Europe—an expansion that would eventually encompass the entire world.

I would also like to know about the aDNA evidence he mentions with regard to polygyny. The aDNA evidence I’ve seen indicates that Europe had a low polygyny rate even in pre-Christian times.

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Joseph Henrich’s comments

Peter misses the argument here, making the entire piece misleading. Once you understand the actual argument, the aDNA data is largely supportive.

The argument is that ONE branch of Christianity (not "Christianity") diffused a set of prescriptions and prohibitions that dissolved the pre-Christian kinship structures into monogamous nuclear families. It's NOT about "cousin marriage" per se.

These norms enforced monogamous marriage (ending common polygyny, concubinage, sex slaves) and tabooed levirate marriage along many types of unions, which included genetic cousins as well as spiritual kin (e.g., goddaughter) and all in-laws. Lots of non-close-genetic relatives.

The norms also altered post-marital residence and inheritance (by testament instead of customary). This is elaborated in great detail in my book.

These changes dramatically reduced what anthropologists call "kinship intensity", which captures the broad and tight social networks formed by kinship norms. (economists renamed these "kinship tightness"). The classic corporate "tight" kin group is the clan.

The aDNA strongly supports this view by confirming the prevalence of polygynous patrilineal clans (without marrying women) in pre-Christian Europe. There's even evidence for levirate marriage, which is the first thing the Church banned in Late Antiquity.

So, even if we take the claims about cousin marriage at face value (which I don't), the aDNA still supports a decline in kinship intensity... which is the key issue. The clans were dismantled.

In his effort to clump Europeans together psychologically, he ignores the application of this theory to explain the variation among Europeans today. E.g., knowing how long a population was under the medieval Church predicts their individualism, conformity & imp. Prosociality

The Church's exposure predicts psychological variation in Europe.

It also works globally

However, if Peter's reading of the aDNA is right, why does Church exposure predict rates of European cousin in the 20th century?

Curiously, after banning cousin marriage, the medieval Church realized that Europeans wanted to marry their kin so much, they monetized by selling dispensations...made big bucks. There was demand... why? Suddenly, people want to marry cousins?

Why does knowing the rates of cousin marriage among Italian provinces tell us so much about psychological variation within Italy?

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