In evolutionary biology, a spandrel is a phenotypic characteristic that is a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive selection—–Wikipedia, March 15, 2020

Commenter Mug of Pee writes:

arctic peoples have large brains for a very NOT just so story reason; the exact same adaptation is found in arctic mammals and in ice age humans and it’s very simple, a big round head is more insulating than a small or long head, allen’s rule.

Arthur Jensen agrees writing:

climate also influenced the evolution of brain size apparently indirectly through its direct effect on head size, particularly the shape of the skull. Head size and shape are more related to climate than is the body as a whole. Because the human brain metabolizes 20 percent of the body’s total energy supply, it generates more heat in relation to its size than any other organ. The resting rate of energy output of the average European adult male’s brain is equal to about three-fourths that of a 100-watt light bulb. Because temperature changes in the brain of only four to five degrees Celsius are seriously adverse to the normal functioning of the brain, it must conserve heat (in a cold environment) or dissipate heat (in a hot environment). Simply in terms of solid geometry, a sphere contains a larger volume (or cubic capacity) for its total surface area than does any other shape. Conversely, a given volume can be contained in a sphere that has a smaller surface area than can be contained by a nonspherical shape, and less spherical shapes will lose more heat by radiation. Applying these geometric principles to head size and shape, one would predict that natural selection would favor a small head with a less spherical shape (brachycephalic) shape because of its better heat dissipation in hot climates, and would favor a more spherical (dollichocephalic) shape because of its better heat conservation in cold climates….

From The g Factor page 436

So even if cold climates didn’t require any extra intelligence to survive in, they did require more brain mass just to keep warm, and given the moderate causal correlation between IQ and brain size, they would have selected for intelligence indirectly as a byproduct of thermoregulation.

There is also likely a causal correlation between IQ and brain sphericity (independent of size) because a sphere is the shape that minimizes the distance between neurons and thus presumably maximizes brain efficiency.

So it seems that not only could cold winters have selected for high IQ directly because of the intelligence needed to survive the cold, but they also may have selected indirectly via thermoregulation of brain size and brain shape.

The question for HBDers is how do we test these three potential causes to determine how big a role (if any) each played in population differences in IQ?