Which goes to say, diversity likely has very little to do with whether a society is low-trust or high-trust. It's more about politics and policy.
We have examples of homogeneous cultures that are high trust, and ones that are low trust.
We have examples of diverse cultures that are low trust, but none that I'm aware of that maintain high trust over time.
The best fitting hypothesis would be that homogeneity is necessary but not in itself sufficient for a high trust culture to be built.
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Which goes to say, diversity likely has very little to do with whether a society is low-trust or high-trust. It's more about politics and policy.