Lead (and other heavy-metals and substances only partly explored) + other industrial contaminants, are associated with the permanent attenuation of learning capabilities, especially in formative years.
Leaving aside momentarily other areas of focus, perhaps larger in contribution (the psych. effect of a constant barrage of Ad-noise and other man-made hazards, especially to the nascent mind?):
What about all those miniballs, bullets etc. strewn about since ~ 1775? We tend to associate most 'progress'-created problems with the unnatural concentrations of substances which formerly were widely dispersed - or locked-up in relatively durable mineral salts.
Muricans been in-heat about boom-boom things since the first. What has been the effect of seeding lots of ground, later used to grow things..? It can't All be from the sweet-tasting paint being eaten by the candy-deprived young-uns.. now can it?
Ashton
while you're up, please get me a Grant.