Why Do Some Nations Have Lower IQ Scores?

Is that because for early humans to survive in cold climates they would have to have the intelligence to create fire and clothing?

It seems hard to control for climate in a study like this since it seems so closely linked to rates of infectious disease. Parasites like malaria are carried by mosquitoes that only survive in tropical climates. Also, water shortages and food-borne illnesses would be more prevalent in warmer climates.