[YorkshireSeason]

> building a high trust society is more culture than demographics

This is the kind of question that benefits from a probabilistic analysis: does ethnic homogeneity, on average, help or hinder building and maintaining a high trust society?

Many conflicts in world history have an ethnic dimension. Many substantial social cleavages, even if they are not ethnically based, over time may evolve an ethnic dimension. For example some religions have an ethnic dimension, most religions require members to marry only within the own religion etc. Moreover, people are more likely to procreate with people they spend a lot of time with. So even linguistic differences can lead to ethnicisation, since it's easier to socialise with people you can easily communicate with.