One obvious candidate is parasite burden. As countries develop, public sanitation gets better, and public health improves. If it's public health (pathogens plus nutrition) and offering standardized schooling to all, you would expect to see an eventual plateau in developed countries, and the developing countries begin to follow their trend.
Any parasite which directly damages the brain is an obvious candidate as one causative agent. This is especially interesting when you read that up to one-third of people in, e.g. Peru, have radiographic evidence of neurocysticerosis - tapeworm damage in the brain. This study shows that of people with evidence of the disease, 18.2% of them in childhood have IQ < 70. Starting to connect dots, we can start making an estimate of IQ improvement from eradication of neurocysticercosis alone.
- Let's assume that (as the Peruvian study showed) 33% of people have neurocysticerosis.
- Let's assume that of the people with neurocysticercosis, 18.2% (4% of the total population) have IQ < 70, the mean IQ is 69. This is obviously simple and actually quite conservative, but the higher we make the mean number for this subgroup, the more modest the effect of eradicating neuroscysticercosis.
- Let's also assume that the 70 and above IQ folks are evenly distributed between 70 and 100. Also simplifying, but I doubt neurocysticercosis makes many people smarter.
- With those assumptions, then a 3-point IQ increase in the general pouplation could be brought about by a one-third decrease in NC cases.
- Of course the 3-point IQ trend goes on for more than 3 decades (when all three-thirds of would-be neurocysticercosis patients were prevented from getting it) so it can't just be that.
To test the hypothesis, we could look at average IQ increases going forward in developing countries currently getting de-wormed. You could also look at existing Flynn Effect curves for the developed world and compare it against the % population getting on clean public water supplies. Of course it's hardly controversial that lower parasite burden would correlate with better outcomes, and indeed the de-worming projects have already shown an improvement in school attendance in participating areas. And while parasite diseases cause massive human suffering, this is still interesting even just by purely pragmatic reasons: a country's economic well-being is linked to the average IQ.