As scholar Arthur Jensen & others argued, big heads are a huge liability because they are metabolically expensive,  increase the odds of babies and mothers dieing during child birth, burden the musculoskeletal system, require wider hips for child birth which slows down running, are a fatal target for an enemies club or missile, and overheat like a lightbulb in warm climates.  Despite being so maladaptive, cranial capacity roughly tripled as apes evolved into humans over the last 4 million years, because it offered the ultimate advantageous adaptation: intelligence, the ability to adapt any situation to your advantage.

As humans migrated north to novel and freezing environments, adaptability was more crucial than ever.  And yet 20,000 years ago brains began to shrink?  Why?  The ice age started ending & we simply didnt need to be as smart anymore to survive.  Intelligence was still an advantage, but as life was no longer challenging, it was negated by all the disadvantages of large heads, so evolution selected for smaller heads and IQ got lower as a result yet still remained high enough for us to invent agriculture when the climate became so hospitable it was hard to fail at farming.

With agriculture came massive population & freak genetic mutations that improved brain efficiency which compensated for our shrunken brains (which were suddenly shrinking even more because of the malnutrition of agriculture).  These brain efficiency mutations probably spread to all but the most isolated humans (i.e. hunter/gatherers) which might explain why arctic people have lower IQs than their brain size & challenging climate predicts

Another less likely possibility is arctic people are actually smarter than whites but our held back by poverty.