I've read it in several places, but here are a couple articles with charts and references to other studies:

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.

Seems like "A Clash of Generations" is what the economist is referencing:

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1468-2478....

And does focus on political violence in particular (terrorism, rioting, and one more I couldn't pick out of the abstract). Would be interestig to read the whole thing, but thanks for following up! I wonder if the amount of young men in the US that were swayed by Russian influence operations over the last several years would be in-step with this logic.

Video games and movies and internet and junk food obesity have had a powerful impact on "defanging" people who might otherwise be bored and angry out in the streets. Now the aggression is channelled into shitposting online and only occasionally shooting up an office/mall/school

I would say free widespread online porn has much more of an affect than the other things you listed.

Those are internet movies.

The prevailing theory seemed to be lead but I also remember reading it doesn't explain every drop. I assume like often, it's a mixture of factors and many are quite subtle.