Another great metric is the volume of lead/mercury pollution in the surrounding air, water, and soil.

Brains are highly plastic, which cuts both ways. Building up brain capacity is onerous, but repeatable across populations. Money plays a big role in how rigorous and education you receive.

But breaking it down is comparably easy, given exposure to a well-known list of toxic particles. You can - and we routinely do - poison people into incompetence. Then we run out the old eugenics arguments to explain away the impact toxic waste exposure had on their ability to think.