This article mentions, but undersells, the lead factor. Lead exposure causes cognitive problems in children, especially related to impulse control. The drop in crime correlates very well with lead cleanup. Kevin Drum (a left-leaning US journalist) has been making this argument: http://www.motherjones.com/kevin-drum/2013/01/lead-and-crime...
http://www.parliament.uk/business/publications/research/olym...