It's been suggested that the removal of lead from fuels, paints, etc. may explain the fact that our IQs have risen since about the same time, and since leveled off somewhat.
Say what you will about environment, but the lead explanation seems to match historical data in all the ways we'd expect it to. Find something that lead does, and we find a trend showing its decline since we started eliminating lead.
http://en.wikipedia.org/wiki/Flynn_effect
Say what you will about environment, but the lead explanation seems to match historical data in all the ways we'd expect it to. Find something that lead does, and we find a trend showing its decline since we started eliminating lead.