You all ever heard about hookworm?
It's interesting, because on one hand I've read the actual original analysis (Bleakley) that gave the statistical argument for it, and it sounds sensible, but I also don't particularly like that it does amount to saying "well yes, Southerners were retards." I guess it's possible they MAY have been, I definitely don't see it in the world around me now.
Basically, hookworms are parasites that really like the soil and climate specific to the Lowland South, and they burrow up kid's bare feet and then suck all the life out of them, making them sluggish and difficult to concentrate. A large chunk of the population was infected with hookworms and didn't know it (if everybody is sluggish and slow at thinking, you don't think they're sick, you think they're just normal and a non-infected person seems vigorous and a quick-thinker). Eventually they went through and started building proper toilets and deworming kids and all that. But now some people argue it was the root behind the stereotype of Southerners as stupid and lazy.
I mean, I've seen hte evidence for it, but I just don't quite believe it. And I think if you go back to the colonial roots you find a massive superiority complex unlike anything else in Yankees from the very get go, has actually gotten better since then.