Violence curve

This post at Crooked Timber, prompted by the terrible violence in Colorado, is titled "America is a Violent Country," which is fair enough given what the graph shows, but I find more noteworthy the parabolic shape of the graph, with violence soaring from about 1960 to 1980 and then declining since then (while remaining higher than the baseline of other OECD countries). America is a violent country, but how violent can and does vary dramatically over time. What caused that curve to shoot upward and then fall (not quite as fast) downward is a subject that's much more interesting (to me) than the particulars of whatever drove this latest murderer to do what he did.