Both Richard Lynn and Jayman have argued that cold winters helped select for racial differences in IQ, but both of them also noted that the cold winter explanation was incomplete. For example, the most cold adapted race are the arctic people yet these appear to be somewhat less intelligent than Whites even though their brains are supposedly much bigger than those of Whites.
Richard Lynn resolved this paradox by arguing that because of their low population, Arctic people did not have genetic mutations which increased brain efficiency. So even though Arctic people faced more selection for high IQ than whites did because of the cognitive demands of their colder winters (making clothes, shelter, fire & hunting etc), evolution could mostly just select for bigger brains, while in Whites, selection was weaker but had more ways of making people smart, so more cognitive evolution occurred.
Although this theory makes sense, it appears to be wrong. Davide Piffer’s data shows that Native American (a proxy for Arctic people) score way below whites on polygenic scores for education (a crude proxy for IQ), even though the SNPs used are common in all races.
Jayman on the other hand argued that cold winters selected for brain size (via thermoregulation) but that only those cold adapted big brained races that acquired civilization would evolve high IQ.
The problem with this theory is that if civilization selected for IQ, it would also have also likely selected for brain size (though to a much lesser degree than if brain size were directly selected by the cold) and that doesn’t seem to have happened. Also, if civilization had selected for IQ, then people today would be better at drawing (a crude proxy for IQ) than they were in Upper Paleolithic Europe, and that doesn’t seem to have happened either.
Thus I’ve been forced to propose a third theory. Cold winters both selected for IQ directly (survival skills) and indirectly (big brains keep you warm) but the ratio of direct to direct selection was higher in Whites than in Arctic people because Whites lived in more population dense areas, where resources were running out. By contrast Arctic people (and Native Americans) were had a whole continent to themselves so there was less competition for survival skills, but you still need big heads to keep warm.