I seem to recall seeing some UK-based research that suggested that the clearest correlation for crime statistics in general (dating back over many years, rather than just cherry-picking recent data) was with youth unemployment. The point is though that with data that has varied by as much as the US violent crimes figures (see below for US homicides), it is highly unlikely that
any single factor will explain variations. Putting the recent decline in the US violent crime rate down to 'concealed carry' is simply untenable, without an explanation for why the rate varied so much before the relevant legislation was introduced. The data is all over the place, and accordingly one has to assume that it wouldn't have stayed steady without the new legislation.
(chart from here
http://www.americanthinker.com/2012/...est_media.html Original data from US government figures)