Homogeneity generally engenders more trust. More trust generally means less crime. This effect can be observed across the world. There are benefits to diversity, of course, but there are also downsides. Iraq, Afghanistan, India, Bosnia, and and Burma have all been riven by ethnic conflict as a result of their diversity, to give a few examples. The USA (and now also the UK, and some other Western European countries) are outliers when it comes to peaceful diversification, aka large-scale migration, and even there I would say the experiment has yet to deliver a conclusive result.