Lead level in blood is connected to crime in in state level inside the US and other countries. Different states and different countries had differentl levels of exposure and they stopped using leaded gas at different times.
There is also direct evidence of neurotoxicity of lead.
Years ago I had lead pipes replaced in the building where I lived. Zero difference... except for the fact that I'd actually drink tap water.
Surprisingly, it's not really a huge deal.
EDIT: I don't mean to be a dick or even argue, I'm just pointing out that these problems do affect people, just maybe not most people through an effective combination of factors that reduces risk (where you live, whether you can afford a filter, etc). I certainly grew up on tap water and I am mildly disturbed people are ok with it causing massive mental health issues.
EDIT 2: Finally, I don't filter water I cook with; I imagine lead would still be a problem there. I probably should filter that water too in Oakland....
Anyway, growing up middle class in the 80s and 90s, we had a filtered water spout on our sink. If you weren't poor, they were fairly common. Once lead was removed from the supply lines, we still continued to use them since it also stripped out chlorine etc.
My point is, work-arounds for leaded drinking water weren't impactful enough to cause crime themselves.
If you want to attribute a drastic decline in violent crime to drinking glasses filling slightly faster you're going to need some extraordinary evidence.
Ahh, I see your point. Places with high amounts of lead also have high numbers of old buildings predating lead paint laws, industrial runoff and/or dumping, and many other sources--see Fruitvale.
There certainly are other factors, but lead exposure seem like the strongest.
Not really, infrastructure changes of various sorts are often correlated with socio-economic factors. There's a reason Flint, MI's water system was wantonly destroyed and not that of a wealth Connecticut Suburb.
Anyway, there are still researchers doubting whether lead was the primary cause of the 80s-2000s rise and decline of violence. Just googling, see: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3829390/
But the main thing is single causes are hard in complex things like human society.