We know lead hinders brain development, and makes us a little bit slower. Some have argued that leaded gasoline was responsible for reducing the iq of generations of people. It has now been a while since leaded gasoline has been banned worldwide.
Do we see any non-negligible difference in levels of intelligence in people before and after the lead ban?
(I know IQ is an imperfect measure of intelligence it is just one data point. And I use it for a lack of a better metric)