What stats do you have to back this crap up?
A number of things.
If you focus on reading comic books as opposed say Ulysses by James Joyce, the mind is focusing on low grade ideas meant to appeal to children. By placing the adult mind in focusing on minimal ideas you dumb down the mind.
It is the same value as watching WWF wrestling or a classic movie. Continual focus on ideas where violence predominates and fantasy prevails, just reduces mental competence in the long run.
It also promotes the idea that violence is the best and direct option to solve problems in a very subliminal way.
Most people who read comics don’t go down this road, but the fringe minds who also have other problems find easy harbor in fantasy and conflict resolution that comics offer. Over time these wackos can basically become failures at life or worse by acting out with the characters they read about, which again is the realm of children’s play.
So the adults never grow up and many failures ensue as the individual grows older.