The West won out when cousins stopped kissing

When the Church banned cousins marrying in the Middle Ages it may have led to some very unexpected results, from rising individualism to more generous blood donation, according to new research.

It seems, perhaps counterintuitively, that the Roman Catholic Church’s rules on marriage have produced not only more children, but also more independent thinking and less conformity among the broader Western population.

If these findings appear a little weird, that’s appropriate. The research is focused on the impact of the Church’s rules on marriage in Western, educated, industrialised, rich and democratic societies – a segment known by the acronym WEIRD.

Jonathan Schulz and colleagues from George Mason University and Harvard University in the US wanted to know why people from WEIRD societies are, well, different.

“Western Europeans and their cultural descendants in North America and Australia tend to be more individualistic, independent, analytically minded, and impersonally prosocial (e.g., trusting of strangers) while revealing less conformity, obedience, in-group loyalty, and nepotism,” the authors write in the journal Science.

Psychological testing has documented the prevalence of these traits in Western society. But just how they came to the fore is poorly understood.

The researchers theorised that the Western Catholic Church’s prohibition on cousins marrying during the Middle Ages had a big impact on European social structure and development, and in turn on the thinking and behaviour of Westerners.

Their theory goes like this: until Rome got involved, marriage among cousins – and occasionally even siblings – was common practice, a part of life in large, extended families with strong kinship ties. Such kinship groups were the fundamental pre-modern social unit, and were characterised by respect for elders, conformity and obedience.

But from the early Middle Ages the Roman Catholic Church condemned such marriages, even disapproving of marriage between people as distantly related as sixth cousins. The Church, say the researchers, became “obsessed” with incest. It also promoted the idea of marriage by choice instead of arranged marriages.

These new rules broke down the old kinship networks and instead produced small, nuclear households, with weak extended family ties and high mobility. And this in turn fostered individualism, non-conformity and a greater tendency to engage with and trust people outside the family group.

At the same time, the Constantinople-based Eastern Orthodox Church was less concerned with incest and policing rules relating to cousins marrying.

To test their theory, the researchers built a database calculating the spread of influence of the Western and Eastern churches in every country of the world, and more finely in 440 regions of Europe.

They then tallied this data with Vatican records of cousin-cousin marriages, and findings from a range of previous tests comparing 24 different psychological characteristics and behaviours across countries and regions.

Their results showed a clear correlation between the length of time that the Western Church had been active in a geographic region, the decline of kinship structures, and the prevalence of characteristics such as analytical thinking, independence and individualism.

People from areas where the Western Church had held sway longest were also significantly more likely to donate blood, reflecting a greater tendency towards fairness and cooperation with strangers.

The researchers also tested these characteristics against other possible influences such as wealth, education and food sources, but found these did not provide as robust a relationship.

“This is important because most efforts to understand human behaviour presume either that little important psychological variation exists across populations or that any variation that does exist represents merely shallow responses to current material incentives, governmental institutions, or ecological conditions,” they write.

“Our work, by contrast, suggests that contemporary psychological patterns, ranging from individualism and trust to conformity and analytical thinking, have been influenced by enduring family structures, particular religious prohibitions, and deep cultural-evolutionary processes.”