hmm, not sure about only child one way or another.
Couple things off the top of my head, I would guess that the rate of absent fathers is higher with one child than multiple children. I also wonder if having a brother is more important than a sister. I seem to remember plenty of criminals with all women in their house (mom, grandmother, sisters, aunt, etc). I wonder if having a brother means more consequeces for infringing on anothers rights. In others words, you steal and you get punched (by a brother not a father) rather than lectured. Just speculating.
Okay, one last thing on the top of my head and the need for father figures. If you are a boy growing up today with no male role model, where does your perception of what men are like come from?
T.V.? Movies? Rap Music?
Why would we be surprised that such a situation would produce violent children. Add on to that, some degree of attachment deficit which is much more likely to occur when there is only one parent caring for a child and is more stressed, absent, etc. in order to provide for the child. Then add some culture issues (i.e. inner city culture or trailer park/white trash culture) where the value of working hard to suceed is not reinforced. Yikes.