An interesting article appears in the current issue of The Atlantic, "How the Decline of Cash Makes America a Safer Country." The article relates the decrease in crime in the United States to the decrease in the use of cash, and cites some interesting research that supports this, leveraging a naturally-occurring experiment: a multiple time-series in the State of Missouri. I may have scooped The Atlantic in a way, as I blogged about this very topic several weeks ago, on February 4.
There are two other factors in the declining U.S. crime rate that rarely get mentioned, but I believe are significant: the growth of remote sensing security systems (i.e., CCTV and alarms), and he proliferation of cellular telephones. Not only is there less cash floating around the steal, but your basic thief is far more likely to trip an alarm or be caught on camera, and every single witness now has a hotline to the 911 center in his or her hand.