If you're looking for "proof" that any one thing is the "cause" of a complex social issue, I'm afraid you're going to be disappointed. There isn't a great deal of academic literature on the topic, but perhaps two key illustrations:

1. The 1954 "Toward an Understanding of Juvenile Delinquency" by Lander, which showed that the rate of "delinquency" rose for both whites and blacks as the ratio between the two reached 50%, and proportionally fell in areas where either whites or blacks held the majority.

2. The 1982 "Population Heterogeneity and the Sociogenesis of Homicide" by Hansmann and Quigley, which recognizes that though the issue is complex, their findings support the the idea that population heterogeneity is a "significant causal factor in homicide".

And of course, unacademically off the top of your head, it's likely that the lowest-crime places you can think of are generally ethnically homogeneous.

> If you don't, please take a second to reflect why you're pointing at this specific difference.

Hey, I'm not the one who held up Scandinavia (>90% white) as a model. I myself am neither white nor black and immigrated to the US, where I would much rather prefer to live, warts and all, than in Scandinavia.