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.