Some have proposed that populations differ in brain size because some populations have evolved to be smarter than others, and one biological mechanism by which a population can become smarter is to increase its brain size. Others have said that some populations evolved to have larger orbital volumes that others in order to see better, and this increased their over all brain size.
One paper making this argument is Pearce and Dunbar (2011). In this paper, Pearce and Dunbar find a strong correlation between a population's mean orbital volume and the latitude in which that population evolved. The authors theorize that populations distant from the equator evolved large vision related brain areas because of the lower levels of ambient light in such areas. The lead author of the study, when speaking to the press, concluded “we argue that having bigger brains doesn’t mean that high-latitude humans are necessarily smarter. It’s just they need bigger eyes and brains to be able to see well where they live.”
Wolf (2011) wrote a reply in which he took Peace and Dunbar's data set and combined it with national IQ data. He showed that the correlation between total cortical capacity and national IQ is larger than the correlation between orbital volume and cranial capacity. He took this to mean that population differences in orbital volume alone cannot explanation the relationship between national IQ and cranial capacity.
I decided to take this data set and produce a correlation matrix as well as a multiple regression in which orbital volume and cranial capacity are independent variables predicting latitude. I took national IQ data from Lynn and Vanhnnen (2012) and data on the IQ of Native Americans from Lynn (2015). Scandinavia's IQ was calculated by averaging the IQs of Sweden, Denmark, and Norway.