I thinking this is junk Science, I hate when researcher try to draw causality from information like this. The scientific method say that you introduce something then see what happens then you remove it and see if everything returns to normal. Well they can not undo things and they can not say some other factor was not going on at the same time since the can not keep everyone isolated or in a test tube. We have more violence since we have more people walking around who can not tell right from wrong or could care less.
Also do you know in the 30's to 60's we use to keep nut case locked up away from the public and today they are free to walk and drive among us.