The number of violent crimes in the United States dropped significantly last year, to what appeared to be the lowest rate in nearly 40 years, a development that was considered puzzling partly because it ran counter to the prevailing expectation that crime would increase during a recession.
....Criminology experts said they were surprised and impressed by the national numbers, issued on Monday by the Federal Bureau of Investigation and based on data from more than 13,000 law-enforcement agencies....“Striking,” said Alfred Blumstein, a professor and a criminologist at the Heinz College at Carnegie Mellon University, because it came “at a time when everyone anticipated it could be going up because of the recession.”
Actually, my recollection of the evidence is that recessions have a mixed effect on crime rates. On the other hand, the cohort effects from lead abatement efforts and the introduction of unleaded gasoline probably continue to make themselves felt, so this might not be as mysterious as it seems. More here and here. And while you're at it, this too.