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Apparently the foraging life is more mentally demanding than is the farming life. Brain size rose during the forager era, but fell during the farming era. During the industry era brain size is rising again, yet another way we are returning to forager ways with increasing wealth.
Combined with social brain theory, that our brains are big to deal with complex social worlds, suggests farmer social worlds are less complex. Perhaps this is because stronger town social norms better discourage hypocritical norm evasion.
The data:
Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimeters to 1,350 cc, losing a chunk the size of a tennis ball. The female brain has shrunk by about the same proportion. … “This happened in China, Europe, Africa—everywhere we look.” … “I think the best explanation for the decline in our brain size is the idiocracy theory.”
When population numbers were low, as was the case for most of our evolution, the cranium kept getting bigger. But as population went from sparse to dense in a given area, cranial size declined, highlighted by a sudden 3 to 4 percent drop in EQ starting around 15,000 to 10,000 years ago. “We saw that trend in Europe, China, Africa, Malaysia—everywhere we looked.” … Skulls of Europeans dating from the Bronze Age, 4,000 years ago, to medieval times. Over that period the land became even more densely packed, … [and] the brain shrank more quickly than did overall body size, causing EQ values to fall. … in fact, [this] pattern … is even more pronounced. …
What may have caused the trend … is selection against aggression. In essence, we domesticated ourselves, … Some 30 animals have been domesticated … and in the process every one of them has lost brain volume—typically a 10 to 15 percent reduction compared with their wild progenitors. …
“Wild types and domesticates think differently.” … Wolves, with their larger brains, are more prone to flashes of insight, allowing them to solve problems on their own; dogs, with smaller brains, excel at using humans to help them. “Wolves seem to be a little bit more persistent than dogs in solving simple problems like how to open a box or navigate a detour,” Hare says. “Wolves persevere when dogs readily give up.” On the flip side, dogs leave wolves in the dust when it comes to tracking the gaze and gestures of their masters. …
He suspects that [bonobos] are domesticated chimps. … “Bonobos look and behave like juvenile chimps … They are gracile. They never show lethal aggression and do not kill each other. They also have brains that are 20 percent smaller than those of chimps.” Hare thinks bonobos became domesticated by occupying an ecological niche that favored selection for less aggressive tendencies. …
When … Richard Jantz … measured the craniums of Americans of European and African descent from colonial times up to the late 20th century, he found that brain volume was once again moving upward. (more)
Added 7p: Many suggest we explain this via lower farmer nutrition. But this would much better explain a sudden fall than a steady gradual decline.
Dumb Farmers
There is a gigantic driving force for smaller brains, the 1% or so maternal death that occurs in childbirth per pregnancy in the absence of medical C-section due to cephalopelvic disproportion.
I think what is being selected for is greater specialization in brain function, programmed in utero. A tribe needs experts in many different fields, not everyone with equal abilities in all fields. I think that maternal cephalopelvic disproportion has driven greater specialization of the human brain and that specialization has to occur in utero, when the major structures of the brain are being formed.
If this is correct, it explains why no one can find any "genes for intelligence". There aren't any. What is important for a successful tribe with successful genes is that it have multiple experts. The tribe with an expert tool maker, an expert rope weaver, an expert wood carver, an expert tracker, an expert wild-animal domesticator, where those experts can pool their expertise and teach the other non-experts how to make and use each of their expert-level products will do much better than a tribe where all its members have the same level of expertise and has no experts.
The “successful” genes that make the tribe “successful” and so are shared by the members of the tribe are the genes that create diversity in abilities. That diversity can't be genetic because it would then require genetic diversity to achieve phenotype diversity and a tribe consists mostly of related individuals. The successful genes must be those that create cognitive diversity without genetic diversity. They must get their “diversity” from somewhere, they must get it from coupling to the environment in utero at the level of noise via stochastic resonance and not from genetics.
This hypothesis explains the inability to find genes for intelligence, explains the Flynn effect, it explains why animal models are inappropriate (they didn't have hundreds of millennia of evolution driving diversity in cognitive abilities in limited brain sizes).
Australian Aborigines are not as subject to predation as other indigenous cultures nor as diverse an ecosphere.