New economic research indicates that children exposed to lead in either paint or gasoline have violent tendencies. The article can be found at: http://www.msnbc.msn.com/id/19655897/
In Freakonomics, Steven Levitt asserts that crime fell in the 90'2 because of Roe v. Wade. Legalized abortion, explains Levitt, allowed mothers to abort unwanted childrend. These children are the ones most likely to commit a crime according to the book. The econ 101 explaination is that a mother knows more about her ability to raise a child than the state so she should have the choice to make decisions about whether to bear a child. In econ this is the classic asymmetrical information problem that leads to unintended consequences when a law is unilaterally enforced.
John R. Lott offered a rebuttal in his book, Freedomnomics, on why crime fell in the 90'2. Lott's explainations include the death penalty, better law enforcement, the right to bear arms. This book is a fast read and exceptionally well written.
The MSNBC link above offers a biological reason for abberant behavior. My law blog, http://www.fladdoglaw.blogspot.com/ has a link to a Newsweek article explaining how biology, life's experiences, and culture might link to aggression like Virginia Tech shooter Cho Seung-Hui. I wonder if this link isn't the association and causation problem.