Brain Size

 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," Geary says.

The observation led the researchers to a radical conclusion: As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive. As Geary explains, individuals who would not have been able to survive by their wits alone could scrape by with the help of others—supported, as it were, by the first social safety nets.

*EQ is encephalization quotient, the ratio of brain size to body size.

Source

This is not really a radical conclusion.  Anyone familiar with economic tools or biological systems will realize that it makes perfect sense.  In a Malthusian environment, people who need less calories are more likely to survive, and big brains burn a lot of calories.  If you have a smaller brain that manages to perform the same necessary tasks, you have a competitive advantage

For the last 10,000 years, about 500 generations, humans have been selected for their ability to provide physical labor while consuming as few calories as possible.  Peasants who needed to eat too much, or who couldn't work, starved.  Intelligence and wisdom didn't really do much to improve one's reproductive success, so they simply were not selected for in most cases.

 

In fact, intelligence may have been actively selected against for most of the population.  Smart uppity peasants who question the social order are more likely to get killed than to be successful. 'The nail that sticks out gets hammered down.'  It is only recently, with the advent of the modern economy, that smart people can make themselves successful without using politics or an army.