The background to this phenomenon is rather simple. For several millions years up to ~200,000 years ago there was a study increase in hominin cranial capacities. I say hominin because it seems that this increase was evident in all branches of the human lineage. Neandertals were increasing in cranial capacity, just as African humans were. Then there was a leveling off and stabilization. Finally, over the past 15,000 years or so there has been a decline, from a median of 1,500 cubic centimeters (cc) to 1,350 cc.
You can read the article for an elaboration on the various hypotheses. But roughly, some think we’re getting less intelligent, while others believe that the brain is reorganizing its structure and development. Remember that the brain uses about ~20% of our caloric intake. It’s a metabolically expensive organ.
I would like to add that even if the median human intelligence is decreasing, the current generation has the largest absolute number of very bright people alive at any given time. This is a natural function of the large human population. If the stability of civilization rests not on the median human, but the coordination and mobilization of large numbers of cognitively gifted humans, then perhaps we should not worry too much in the short to medium term. Even with stabilizing world populations we’ll have a generation or two of large numbers of brights before differential fitness of the smart and dull really start eroding the numbers of the former.
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