Those living in the South have less stable molecules and are full of genetic mistakes.
One question once I tried to discover is the rate of defective births among the different races, but things like cousin marriage affect that rate. If the theory is right, defective babies should be rare in Norway and frequent in Equatorial Guinea. I have no data if that is true or not.
IQ or intelligence is also a sign of genetic health. Northern peoples should have more of it. Yet Southern Indian Brahmins and Parsis and of course Southern Chinese seem to have higher IQ levels. Hong Kong and Singapur are almost on the Equator and have people of very high quality. Slowly, also these questions are going to be solved and then become irrelevant, as we are turning ourselves into a synthetic life form.