Christopher Eppig writes about the effect of infectious disease on national IQ:
“In our 2010 study, we not only found a very strong relationship between levels of infectious disease and IQ, but controlling for the effects of education, national wealth, temperature, and distance from sub-Saharan Africa, infectious disease emerged as the best predictor of the bunch. A recent study by Christopher Hassall and Thomas Sherratt repeated our analysis using more sophisticated statistical methods, and concluded that infectious disease may be the only really important predictor of average national IQ.”
So, going back to the old “genetics versus environment” argument, what are we to make of this? There’s a certain genetic variation in disease susceptibility/resistance (for example, the gene causing sickle cell anemia may also provide some malaria resistance). Infectious diseases also have a very heavy environmental component.