I've heard that there was a sharp increase from the early 20th century to the 70s-80s, then a small decline since then (maybe due to deleading gasoline among other things), and it seems to have happened in other countries too http://www.parliament.uk/business/publications/research/olym....

It's a bit hard to judge since there are only a few crimes where measurement has been consistent and good, e.g., homicide and violent crime. And there you have to take into account the decline in lethality due to better medical care:

https://twitter.com/RAVerBruggen/status/754446756805509120

https://twitter.com/RAVerBruggen/status/756512858331082752.

I'd like to see the trend in assaults, which would be most relevant to free-range parenting, but haven't been able to find it anywhere.