I should probably say "large groups of disparate people". Ethnic homogeneity makes everything easier. American gun laws probably don't help either, but there

I should probably say "large groups of disparate people". Ethnic homogeneity makes everything easier. American gun laws probably don't help either, but there are non-American cities - Sarajevo, South Africa, much of Europe after the Syrian migration crisis - that struggle with crime too, and there seems to be a correlation there.