I wonder if there’s a correlation between Japan’s lack of violence and their strict immigration policies?...
I think that one of those observations you're not supposed to make, or at least say out loud.
They allow very little immigration and only a few guest workers. They have a fair number of Filipinos who work in old folks homes. I'm no expert on Japan though. I just watch Japanese news pretty often and have heard about Japan from coworkers.
I believe their societal uniformity reduces violence in their culture and it is usually the mentally ill who create most of the violence. Unity is strength, not diversity.