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I'd be really interested in how you separate this out.
Just off the top of my head you've got to find control countries/societies which have both comparable economic health/lack of as well as war time experience.
Very much being an armchair phycologist here, but surely something like the violence and trauma from Vietnam much have had a knock-on for the levels of violence in the US?
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This is a useful list I think https://www.motherjones.com/kevin-drum/2018/02/an-updated-lead-crime-roundup-for-2018/
I've certainly read a few books that say this as gospel. Hasn't even crossed my mind that it might not be true! Will have to do some proper reading on it now.