I came across a reference in the Sunday Times today to some research done by Harvard political scientist Robert Putnam - which he himself attempted to suppress - which indicates that societies with lower levels of immigration are happier.
What intrigues me about many of the countries of Central America and the Spanish Caribbean is that they are extremely diverse, at least in terms of ancestry; self-consciously so. Puerto Rico is said to have the world's most extensive mix of human genes from different continents.
These countries also feature consistently near the top of global 'happiness' surveys, such as they are. And this might be because they are having their cake and eating it: widespread, obvious miscegenation, and yet also a palpable sense of homogeneity.