Beat me too it, and better than I could say it also.
There was a program on PBS round the late 80s about the 10 safest countries in the world. Japan was one, as was Saudi Arabia, Ireland, Sweden and the USSR. They all were leaned homogeneous and with a common religion, the exception being the USSR in which political idealogy functioned as a religion, and people were taught they were the same, etc etc.
Witness the chaos that has wracked Russia since, nothing to bind them to the whole as was then, fragmentation along ethnic, religious, and class lines.
And not just ethnically similar, but family and social networks established over generations or even centuries. A more mobile society made of disparate people competing to establish themselves is bound to have more bumps, some ending in violence.
In 1977, a blackout tore NYC apart, with poorer people in slums who felt like outcasts really going bezerk, looting.
When another big blackout occurred in 2004, what happened was totally different. People who no longer felt so disenfranchised as they once had in the disintegrating city of yore plus I would suppose some of the effects of the unity that occurred after 9/11 witnessed folks hanging out in front of their houses and buildings, talking. The biggest concern was buying candles or batteries for flashlights before stores ran out.
So still a mix of peoples, still far from perfect, but more of a focus on our commonalities rather than our differences lent to a different atmosphere, a cultural shift.