Trusting a stranger is easy to take for granted when you live among institutions that make it practical. It isn't the only way people have organised their obligations. Families and extended kin groups have supplied security, work and mutual assistance in many societies.
One influential theory connects Western Europe's move away from intensive kinship with the medieval Church's rules about marriage. I find it interesting because it challenges the assumption that individualism and impersonal institutions simply appear once a society becomes sufficiently educated or wealthy.
The argument about kinship
In research published in 2019, Jonathan Schulz, Duman Bahrami-Rad, Jonathan Beauchamp and Joseph Henrich examined connections between exposure to the Western Church, kinship institutions and psychological differences.
Their argument is that restrictions on practices including cousin marriage helped weaken extended kin networks. Over time, people depended more on relationships and institutions beyond the family. That could contribute to differences in individualism and cooperation with unrelated people.
It is a theory supported through historical and comparative analysis. It isn't proof that a single ban created the Western mind, or that every modern act of generosity can be traced to a medieval wedding rule. Measuring a process across centuries creates difficult questions about causation and competing explanations.
Institutions shape expectations
The part I think matters for politics is that habits of cooperation have a history. If people can rely on an impartial court, a contract or an employer who isn't a relative, they have options that a person dependent on family protection may not have.
Those arrangements need maintaining. A rule that applies differently according to connections can undermine the expectation that strangers will be treated fairly. People may then have stronger reasons to rely on their own networks.
Cultural differences in those expectations deserve discussion, including when people move between societies. But ancestry can't tell us what an individual believes, and a kinship theory doesn't establish that migration inevitably destroys trust. What institutions require, and what people actually do, still need examining.
I don't regard liberal cooperation as something nature guarantees. The historical argument gives us a reason to take its supporting institutions seriously. We should be careful about weakening them, and equally careful about turning a disputed explanation of their origins into a claim that some people can never share them.