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 the pitfalls of studying them to understand human nature in general. They are not necessarily the same as people elsewhere.
Two decades ago, a group of social scientists concluded “not only that WEIRD people were one population within a spectrum of cultural variation but that they were often unusual, outliers anchoring the ends of global distributions.” They diverge from other populations in many areas of life, including sensory ability, economic preferences, personality structure, morality and cognition (Henrich, 2024).
Northwest Europeans are so distinct because they have adapted to an atypical environment of weak kinship, strong individualism and “impersonal pro-sociality,” i.e., social interactions that are less personal and less emotionally intense but extend much further than friends and family.
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).
Most authors see WEIRDness as a legacy of Western Christianity, the form of Christianity that 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 cousin marriage was increasingly restricted by the early Western Church:
Roman times: only first-cousin marriages were banned.
7th century: the ban was extended two degrees further when the Western Church adopted the anti-incest prohibitions of the Visigothic Code.
Early 9th 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 measure forced almost everyone to marry outside their clan, 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).
Others, 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 7th century ban was taken from the Visigothic Code, and the new kinship calculation method, adopted in the 9th 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 9th century, when the Church’s most extreme ban on cousin marriage came into effect. French households were already 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 ancient DNA
We can look for earlier evidence of cousin marriage avoidance by examining 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. The first analysed 57 genomes from the 1st century BC to the 1st century CE in Southern England:
Y chromosome diversity is high … and patterns of ROH imply that these were relatively large outbreeding communities. (Cassidy et al., 2025)
The second analysed 248 genomes from the 3rd to 8th 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)
The third analysed 722 genomes from the 7th to 8th 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 [the Mödling site] and [the 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)
The fourth analysed 424 genomes from the 2nd to mid-9th centuries in Hungary:
We 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
Mentally and behaviorally, northwest Europeans are an outlier. In comparison to other humans, they are more individualistic, have weaker kinship ties and are more impersonal in their social interactions. Apparently, this has long been the case.
As a result, they have been better at transcending the limitations of kinship and creating larger forms of social organization, notably the state and the market economy. They have also been more inclined to think in terms of a potentially universal morality, an inclination that perhaps made them more receptive to the Christian faith. In such a 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 much more a moral judgment than a simple rejection of the “Other.”
This kind of moral system has, in turn, favored certain mental and behavioral adaptations:
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).
This evolutionary trajectory is usually attributed to the Western Church’s ban on cousin marriage. Yet if we look at DNA from human remains, specifically runs of homozygosity between spouses, we see that northwest Europeans were avoiding cousin marriage long before Christianity.
However, the zone in which cousin marriage was rare seems to have included not only northwest Europeans but also populations further east. The studies from Austria and Hungary were done not only on DNA from indigenous Europeans (probably Slavic peoples) but also on DNA from Avars—a Turkic people who entered the Carpathian Basin in the 6th century.
Avoidance of cousin marriage may therefore be part of a broad North Eurasian adaptation, which then evolved into a more specialized one among northwest Europeans. This point is actually made, in part, by the authors of one of the studies mentioned above: “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 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, particularly with the spread of Islam.
In addition, neither of the above studies 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.
Avenues for future research
Regardless of how we define avoidance of cousin marriage, it seems to be an imperfect yardstick for measuring the evolution of the entire mental and behavioral package of northwest Europeans—not only individualism but also high levels of affective empathy and guilt proneness, which led to the creation of a unique moral system. Some of these adaptations may have originated only in northwest Europe, while others may have arisen earlier within a broad North Eurasian zone.
To track how these adaptations evolved, we look for possible indications in ancient DNA. 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 be used to see 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 (Chandelier, 2021, p. 224, my translation).
Peter Frost has a PhD in anthropology from Université Laval. His main research interest is the role of sexual selection in shaping highly visible human traits. Find his newsletter here.
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References
Benedict, R. (1946 [2005]). The Chrysanthemum and the Sword. Patterns of Japanese Culture. First Mariner Books.
Blöcher, J., Vallini, L., Velte, M., Eckel, R., Guyon, L., Winkelbach, L., … Burger, J. (2025). Historic Genomes Uncover Demographic Shifts and Kinship Structures in Post-Roman Central Europe. bioRxiv 2025.03.01.640862; https://doi.org/10.1101/2025.03.01.640862
Caesar, J. (1915). De Bello Gallico & Other Commentaries of Caius Julius Caesar (Translated by W.A. Macdevitt). London: J.M. Dent.
Cassidy, L. M., Russell, M., Smith, M., Delbarre, G., Cheetham, P., Manley, H., ... & Bradley, D. G. (2025). Continental influx and pervasive matrilocality in Iron Age Britain. Nature, 637(8048), 1136. https://doi.org/10.1038/s41586-024-08409-6
Ceballos, F. C., Gürün, K., Altinişik, N. E., Gemici, H. C., Karamurat, C., Koptekin, D., ... & Somel, M. (2021). Human inbreeding has decreased in time through the Holocene. Current Biology, 31(17), 3925-3934. https://doi.org/10.1016/j.cub.2021.06.027
Chandelier, J. (2021). L’Occident médiéval. D’Alaric à Léonard. 400-1450. Mondes anciens, Paris: Belin. https://www.belin-editeur.com/loccident-medieval
Frost, P. (2017). The Hajnal line and gene-culture coevolution in northwest Europe. Advances in Anthropology, 7, 154-174. https://doi.org/10.4236/aa.2017.73011
Frost, P. (2020). The large society problem in Northwest Europe and East Asia. Advances in Anthropology, 10(3), 214-134. https://doi.org/10.4236/aa.2020.103012
Gnecchi-Ruscone, G. A., Rácz, Z., Samu, L., Szeniczey, T., Faragó, N., Knipper, C., ... & Hofmanová, Z. (2024). Network of large pedigrees reveals social practices of Avar communities. Nature, 629(8011), 376-383. https://doi.org/10.1038/s41586-024-07312-4
Hajnal, J. (1965). European marriage pattern in perspective. In: D.V. Glass and D.E.C. Eversley (eds). Population in History. Essays in Historical Demography. London, Arnold.
Hallam, H.E. (1985). Age at First Marriage and Age at Death in the Lincolnshire Fenland, 1252-1478. Population Studies, 39, 55-69. https://doi.org/10.1080/0032472031000141276
Hartman, M.S. (2004). The Household and the Making of History. A Subversive View of the Western Past. Cambridge, Cambridge University Press. https://doi.org/10.1017/CBO9780511818134
hbd chick (2014). Big summary post on the Hajnal Line. October 3. https://hbdchick.wordpress.com/2014/03/10/big-summary-post-on-the-hajnal-line/
Henrich, J. (2024). WEIRD. Open Encyclopedia of Cognitive Science. https://doi.org/10.21428/e2759450.8e9a83b0
Howard, G.E. (1904). A History of Matrimonial Institutions, Volume 1. Chicago: The University of Chicago Press.
ICA (2020). Research Themes—Marriage Patterns. Institutions for Collective Action. https://web.archive.org/web/20190329070516/http://www.collective-action.info/_THE_MarriagePatterns_EMP
Kirkegaard, E. (2025). Why did NW Europeans become WEIRD? Just Emil Kirkegaard Things, January 29.
MacDonald, K. (2019). Individualism and the Western Liberal Tradition: Evolutionary Origins, History, and Prospects for the Future. Amazon. https://www.amazon.ca/Individualism-Western-Liberal-Tradition-Evolutionary/dp/1089691483
McCann, C.A. (2010). Transgressing the Boundaries of Holiness: Sexual Deviance in the Early Medieval Penitential Handbooks of Ireland, England and France 500-1000. Theses, South Orange, NJ: Seton Hall University. https://scholarship.shu.edu/theses/76
Policy Tensor. (2021). The Church’s crusade against cousin-marriage did not create the Western nuclear family. May 7
Ringbauer, H., Novembre, J., & Steinrücken, M. (2021). Parental relatedness through time revealed by runs of homozygosity in ancient DNA. Nature Communications, 12, 5425. https://doi.org/10.1038/s41467-021-25289-w
Schulz, J.F., Bahrami-Rad, D., Beauchamp, J.P., & Henrich, J. (2019). The Church, intensive kinship, and global psychological variation. Science, 366(707), 1-12. https://doi.org/10.1126/science.aau5141
Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe. London: Verso. https://archive.org/details/millenniumoffami0000secc/page/n3/mode/2up
Tacitus (1970). Agricola, Germania, Dialogus. Loeb Classical Library (Translated by M. Hutton & W. Peterson). Cambridge, MA: Harvard University Press.
Wang, K., Tobias, B., Pany-Kucera, D., Berner, M., Eggers, S., Gnecchi-Ruscone, G. A., ... & Hofmanová, Z. (2025). Ancient DNA reveals reproductive barrier despite shared Avar-period culture. Nature, 1-8. https://doi.org/10.1038/s41586-024-08418-5
Wendt, F. R., Warrier, V., Pathak, G. A., Koenen, K. C., Stein, M. B., Krystal, J. H., ... & Polimanti, R. (2022). Polygenic scores for empathy associate with posttraumatic stress severity in response to certain traumatic events. Neurobiology of Stress, 17, 100439. https://doi.org/10.1016/j.ynstr.2022.100439