I should also point out that the sort of violence I'm talking about is things like wars, civil disorder, gangs, terrorism, failed states, anarchy, and other group forms of violence, not individual crimes like premeditated murder. The latter has a lot of conflating factors (a sibling comment mentions lead, and there's also abortion, better policing tactics, economic growth, etc), but to get a critical mass of people who are so disaffected by their current situation in life that they want to burn the whole society down, you usually need some form of major demographic or environmental change.
https://www.theatlantic.com/international/archive/2014/03/an...
https://www.economist.com/news/special-report/21688587-young...
I should also point out that the sort of violence I'm talking about is things like wars, civil disorder, gangs, terrorism, failed states, anarchy, and other group forms of violence, not individual crimes like premeditated murder. The latter has a lot of conflating factors (a sibling comment mentions lead, and there's also abortion, better policing tactics, economic growth, etc), but to get a critical mass of people who are so disaffected by their current situation in life that they want to burn the whole society down, you usually need some form of major demographic or environmental change.