W HY SOME countries are rich and others are poor is an enduring debate in economics. Natural resources and friendly climates help only a bit. In contrast, robust political institutions and a steady rule of law seem essential. But why did these precursors evolve in just a few dozen states?

One oft-cited theory, advanced by Robert Putnam of Harvard University, is that the crucial ingredient is “social capital”, the affinity people feel for members of their society whom they do not know. Proxies for this sentiment, such as blood-donation rates or propensity to return a stranger’s lost wallet, closely track GDP per person.

Social capital can take centuries to amass. Mr Putnam has shown that parts of Italy that were ruled by a feudal monarchy around 1300 AD have low levels of social trust and are relatively poor today. In contrast, the Italian regions that formed city-states in that era, where citizens banded together for commerce and self-defence, are now unusually rich and well-run.

A recent study by Jonathan Schulz, Joseph Henrich and two other scholars proposes an explanation that delves even further back in time. They focus on family structure. Until recent human history, people lived in small groups and often married relatives. These habits reinforced family ties, but made people wary of outsiders.

In Europe this started to change around 500 AD , when the Catholic church began banning polygamy and marriages between cousins, or between widows or widowers and their dead spouses’ siblings. These edicts forced unmarried men to venture out and meet women from different social groups. The paper says that this reduced Christians’ “conformity and in-group loyalty”, and made them trust strangers more. By expanding the community beyond clans, it helped create the broad solidarity on which development may depend.

To show that Christian dogma caused this shift, the authors match historical data on the spread of religion with modern indicators. In places where Catholicism was generally the leading religion from 500-1500 AD , people score highly on measures of independence, impartiality and trust—such as agreeing to testify against a friend whose reckless driving killed a pedestrian. The same pattern occurs in countries settled mostly by Christian migrants, such as America. In contrast, social trust is lower and marriage between cousins is relatively common in areas whose populations do not descend from medieval Catholics.

This effect distinguishes Catholicism from other strands of medieval Christianity. Years spent before 1500 AD under Eastern Orthodoxy, which the authors say did less to police marriage within families, was a weaker predictor of “pro-social” survey responses than exposure to Catholicism was. Moreover, the trend holds up both between and within countries. Among Italian regions, those with high social capital (as measured by data like using cheques over cash) were influenced by Catholicism for longer than those lacking it were.

The study’s subject limits the strength of its findings. Barring an experiment to assign religions to countries at random and monitor them for 1,500 years, no one can prove whether incest bans built social trust or merely coincided with it. Nonetheless, the paper bolsters the case for studying ancient history to understand the present. ■

Source: "The Church, intensive kinship and psychological variation", by Jonathan Schulz and Joseph Heinrich et al., Science, November 2019