Why did Europeans and North Americans come to dominate the modern world, overtaking peoples in places like China and India? The German sociologist Max Weber put it down to Protestant ethics and asceticism. Harvard academic Joseph Henrich has a different big idea: not marrying your cousins.

Beginning around 600AD the Roman church kicked off a millennia-long campaign to promote good Christian marriages, banning unions not merely between first cousins but distant relatives too. The result transformed the organisation and inner psychology of once-tribal European societies, giving birth in Henrich’s telling to the “weirdest people in the world” — where “WEIRD” stands for “Western, educated, industrialised, rich and democratic”.

WEIRD people, a group that includes most readers of the Financial Times, are indeed an odd bunch. They see themselves as autonomous individuals, rather than in relation to kin or caste. They are more trusting, patient, controlling and nonconformist than other humans. And they almost never marry relatives, which still account for a quarter of modern weddings in the Middle East and Africa. “Taken separately, each trait is uncommon,” as Henrich puts it in The Weirdest People in the World. “But in combination this package is extremely rare — WEIRD.”

Henrich is an eclectic academic, having held professorships in both economics and psychology while also working on human evolutionary biology. But by background he is an anthropologist who did fieldwork on distant Fijian islands and with Chilean tribal people. Unusually for his own tribe, however, he has a facility with numbers. This anthropology-meets-big-data approach is not merely innovative, but underpins a fascinating and creative book, brimming with provocative ideas.

This diverse intellectual background first led Henrich to notice how many academic studies in disciplines such as behavioural economics drew seemingly universal conclusions from experiments run on Americans, and more specifically US college students. Repeat these in China or South Korea and the results were often strikingly different. “They don’t actually tell us about human psychology, but merely reflect WEIRD cultural psychology,” Henrich argues. The result is a deep structural bias stretching from western researchers to novelists, who view their own mindsets as a model for the human condition, rather than an exception to it.

Henrich is a bold scholar. His last book, The Secret of Our Success, picked a fight with Darwin, arguing that cultural evolution, rather than natural selection, best explained why some societies prospered over others. Social groups compete with one another, learning as they go. Those with better “collective intelligence” — meaning norms, habits, and ideas — tend to grow more quickly. Along the way humans learn useful evolutionary cultural tricks, such as cooking with spices in hot climates, which protects against pathogens.

A similar story emerges through Henrich’s focus on cousin marriage. In 597AD, Pope Gregory the Great sent missionaries to the Anglo-Saxon Kingdom of Kent to win King Aethelberht round to the church’s strict matrimonial edicts. Rome adopted these in part as a form of competitive differentiation, marking out its teachings from the more permissive family rules held by the various cults, tribal deities and Roman religious leftovers that held sway at the time in much of Europe.

The resulting campaign, which lasted hundreds of years, also pushed back against arranged marriages, polygamy, and incest. This “extreme package of prohibitions” was not easy to follow, given it forced people to seek partners far outside kin groups. But by about 1,000AD the Anglo-Saxons had been persuaded. By 1,500AD, monogamous nuclear families had replaced tribal loyalty across much of Europe.

All this proved advantageous to the church, which surpassed other religions. More importantly, Henrich argues, it helped western Europeans prosper. He shows how WEIRD peoples evolved to become more co-operative, helping them to live as individuals in free towns and trade with strangers in markets. They also grew less violent, partly because men show lower testosterone levels when they form the kind of exclusive monogamous relationships Pope Gregory had in mind. “From the ruins of traditional social structures, people began to form new voluntary associations based on shared interests or beliefs rather than on kinship or tribal affiliations,” Henrich writes. And so the modern world was born.

This theory is far from uncontroversial, and in making it Henrich is now picking a second fight, this time with other sweeping social theories that seek to explain why societies rise and fall. He shares similarities with the political scientist Francis Fukuyama, for instance, who also examines how modern impartial states replaced kin-based “patrimonial” governance — a transition Fukuyama argues is much trickier than moving from dictatorship to democracy.

Henrich’s view echoes another big idea book of the past year, The Narrow Corridor by economists Daron Acemoglu and James Robinson. This also located the origins of western success in tribal 5th-century Europe, although for quite different reasons from those Henrich offers, focusing instead on the back-and-forth relationship between the state and civil society, and making no mention of cousins or marriage at all.

Perhaps the more useful question is what we in the west might now learn. Henrich notes overconfidence as another trait of WEIRD societies. Such bullishness seemed apt when the world appeared to be conforming to western models, an idea famously associated with Fukuyama’s “end of history”. Yet Henrich shows that WEIRD institutions have often simply been grafted on to other societies, as when China introduced western marriage norms by diktat in the 1950s. Underneath, differences in culture and values remain profound. It should perhaps not be surprising that western-style liberal democratic norms often struggle to take root elsewhere.

This in turn raises questions about the extent to which WEIRDness still confers social and economic advantages. Here the experience of Covid-19 should at least give pause, given the way so many western societies struggled when compared to eastern nations like China and Japan. More generally, as we westerners come to terms with our own gradually declining power and influence, we might benefit from a more modest conception of our own abilities too. Weird certainly, but not necessarily better.

The Weirdest People in the World: How the West Became Psychologically Peculiar and Particularly Prosperous by Joseph Henrich Allen Lane, £20, 680 pages

James Crabtree is author of ‘The Billionaire Raj’

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