Keith Humphreys points out that the number of Americans in prison has gone down the past few years. One reason you don't hear about this more:
Issue advocates, funnily enough, have an interest in downplaying news that the problem they address is lessening. When the Non-Profit Center to Combat X (where X is anything from hate crimes to spitting on the sidewalk) gives a quote to a reporter about their issue, they will virtually always say that things have never been worse/the problem is exploding/the window to act is closing rapidly etc. It’s not that advocates truly want their problem of interest to get worse, but that their fundraising and profile will suffer if the general public knows that the problem they address is declining in severity.
Kevin Drum singles out one cause of the crime drop: lead.
(Chart from the BJS (pdf))