There's been some speculation that one reason warmer areas have been less productive is that they were more conducive to having parasites around which reduced the average intelligence of the population (which could have also longer-lasting impact on culture and values after the parasites have been eradticated). Hookworm and Guinea worm seem to be the most commonly mentioned examples of this. The good news is that if this is correct then the ongoing projects to eradicate parasites should help raise the general intelligence of the population.

One thing to keep in mind here is that what matters most for people being really smart is the size of the far end of the tail of the distribution (since that's where the smart people who accomplish things lie). So a small shift in the bell curve can result in a large shift in the relative fraction of the population that is far right enough in the distribution.