much less immigration (some at force) in these countries. However even with the amount you do have, this 'diversity' is starting to catch up. Look whats happening with the rioting from the radical muslim groups in france and norway. Murdering a cartoonist, not for sleeping with their wife, but for drawing... wait for it.. a cartoon.
EDUCATION or lack thereof is one of the largest precursors to violent crime. We have an over-abundance of people who fucked off in school and have no future. Their parents didnt require the school system be anything more than free daycare. Given the climate of all the violent crimes in the USA, a startlingly high percentage of them are committed by those you wonder have enough brain cells to even breath every day.