If that is all you've taken out of seeing human history and haven't seen a trend directly related to the use and intake of heavy metals and the sort, and in the modern age the difference between murder rates in the US, by population and police, compared to the rest of the western world, you haven't seen nearly enough of human history to understand anything.
Historically, the prevalence of just lead on its own boosted the aggression and sociopathic behavior of the population to ludicrous extents and these days, when that should be very little of a factor, the US still far outranks anyone else on the same developmental level by obscene levels.
It's a combination of the US being sucky about environmental pollution and having a serious societal problem with police being murderous asshats and people being OK with it.