Basically, during the 20th Century, lead were used in a lot of products, including gasoline and paint. To shocking surprise, it turned out that this lead slowly ended up in our environment, and built up in our bodies.

The hypothesis is that because virtually the entire population got lead poisoned, we became more affected by "learning disabilities, decreased I.Q., attention deficit hyperactivity disorder, and problems with impulse control", which again leads to more crime.

Lead was eventually phased out (no thanks to the companies behind the products), and this correlated with a drop in US crime-rates in the early 1990s.

Since then there's been lots of discussion on why the crime rate dropped. The removal of lead from the environment is one (an alternative, but not mutually exclusive, hypothesis is the legalized abortion and crime effect[2]).

[Personally, I've been wondering how this also affects other qualities of the people affected... Would people born before the 1970s have been more intelligent than they are currently? Could this have given us better political leadership, and better secondary and tertiary effects?]

1: https://en.wikipedia.org/wiki/Lead–crime_hypothesis

2: https://en.wikipedia.org/wiki/Legalized_abortion_and_crime_e...

https://en.m.wikipedia.org/wiki/Lead%E2%80%93crime_hypothesi...

https://www.motherjones.com/environment/2016/02/lead-exposur...

Edit: Ah, apparently it was mercury, not lead. Cute anecdote anyway.