There is a theory that the Roman Catholic Church played a crucial role, as it could make decisions about what was good for society as a whole, as no one makes today.
The top science journal, AAAS Science, published research endorsing the theory:
We test these predictions at three levels. Globally, we show that countries with longer historical exposure to the medieval Western Church or less intensive kinship (e.g., lower rates of cousin marriage) are more individualistic and independent, less conforming and obedient, and more inclined toward trust and cooperation with strangers (see figure). Focusing on Europe, where we compare regions within countries, we show that longer exposure to the Western Church is associated with less intensive kinship, greater individualism, less conformity, and more fairness and trust toward strangers. ...It is still the case today that nuclear families are primarily found among those of Western European Christian descent.
This research suggests that contemporary psychological patterns, ranging from individualism and trust to conformity and analytical thinking, have been influenced by deep cultural evolutionary processes, including the Church’s peculiar incest taboos, family policies, and enduring kin-based institutions.
Update: I await the studies on how Chinese, African, Hindu, and other groups were genetically influenced by the religious and cultural forces that shaped them over the last millennia.