This feels like a good time to bring up the lead-crime hypothesis (flaws and all). For those who don't know, there's a strong (if faulty) correlation between lead levels in preschool children and crime rates: https://www.vox.com/2016/1/14/17991876/crime-drop-murder-lea...
Regardless of what you think about the hypothesis, the growth and crunch in lead levels during the last many decades is astounding and probably still has many bad effects on IQ and related factors, at least in the US
https://en.wikipedia.org/wiki/Lead%E2%80%93crime_hypothesis
Regardless of what you think about the hypothesis, the growth and crunch in lead levels during the last many decades is astounding and probably still has many bad effects on IQ and related factors, at least in the US