It usually comes down to "I'm a scientist and I specialize in this field, so my opinion is...". Most often it's some low grade researcher at a second rate university trying to push his/her latest book, without having done anything remotely similar to the scientific process to reach this hypothesis. Speaking of which, the scientist here has a book out in a couple months about child psychology. Go buy it.
The other end comes down to "the kid played a lot of videogames then killed a bunch of people", or when someone finally gets access to some killer's place they find a stack of videogames there somewhere. Of course, by this logic owning a toothbrush makes you a child molester and having a few dirty dishes in your sink may lead to genocide.