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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?
And the removal of lead from petrol (mild lead poisoning of developing brains leading to violence later in life).
Possibly not factual...