Agreed on hookworm in the American South. Add in malaria and 'tropical' diseases and we have an alternative hypothesis.

I got to thinking about this a few decades ago on my first trip to New Zealand; I asked a friend in Dunedin (South Island - southern hemisphere, so it gets colder as you go south). His explanation (in a strong Scottish accent) "Good Bracing Climate!". A lot of Scots settled there. Some of that was the lower health burden that comes with colder climate, and to be fair, the Māori mostly hadn't settled there as their starch crops wouldn't grow - and they weren't pastoralists (which gets us back to farmers vs hunters).