[derefr]

I don't know why you picked that list of countries in particular, but in most rankings of countries (by happiness, productivity, social welfare, GDP per capita, etc.) you tend to see several of the nordic countries, which are all quite culturally-homogenous.

I would highlight "social welfare" rankings in particular: people seem to vote more for socialist measures—measures that tax them to provide for others—when they care more about those others. If this correlation holds (between lack of cultural diversity and social-welfare leanings), then it would suggest that the cultures that care most about helping their "fellow man" are the ones in which one's "fellow man" is the least different from oneself.