I mean, you joke, but environmental pollution (lead) levels and certain nutritional deficits have a scientifically measurable effect on mental health disorder rates and general intelligence.

The use of leaded gasoline put a huge amount of environmental lead into our living spaces and breathing air. When it was completely phased out in the 1970's and environmental exposure dropped, murder rates dropped by the 1990s in what's known as the lead-crime hypothesis, where childhood exposure to lead of any level alters development negatively.