Obviously there are confounders, but Kevin Drum’s decade old article positing that lead was a major contributor to the crime wave and subsequent reduction is pretty compelling. [1]
EDIT: aside, but maybe one of the most depressing things I’ve ever learned is the high levels of lead in the ink used to wrap various Mexican candy. [2] and this 2014 article [3] on lead levels in Mexican kids where the mean levels exceed the upper acceptable level in the US.
There are few things that cause me more distress than the failure to comprehensively tackle this issue worldwide.
[0] https://chicagohealthatlas.org/indicators/lead-poisoning
[1] https://www.motherjones.com/environment/2016/02/lead-exposur...
[2] https://www.orlandosentinel.com/news/os-xpm-2004-05-14-04051...
[3] https://pubmed.ncbi.nlm.nih.gov/25459328/
“Our results indicate that more than 15% of the population will experience a decrement of more than 5 IQ points from lead exposure. The analysis also leads us to believe that lead is responsible for 820,000 disability-adjusted life-years for lead-induced mild mental retardation for children aged 0 to 4 years.”