The more segregated a community is, the more likely certain groups will be collected into what are effectively ethnic ghettoes, where crime is likely to increase. This almost always happens in large cities. It's how you end up with black gangs, Italian gangs, Jewish gangs, and Mexican gangs all operating in the same cities, just in different neighborhoods. It's also how you end up with petty crime being tied to specific economically depressed communities.
In theory, a nation can avoid this particular path to crime with homogeneity, either by staying relatively ethnically pure, or by successfully integrating new arrivals into the already existing culture.
NB: even in a homogeneous nation, there are plenty of other paths to increased crime available; this only covers one specific path.
