The modern human brain is a bit smaller than the cro magnon brain.
One does not need as much intelligence to live in a civilized society.
Or domesticated...
Brains burn a lot of food for their weight. They are 2% of body weight yet consume 25% of the calories. So in lean food times, when there is nothing to hunt or gather no matter how clever, the people with big brains are at a calorie disadvantage. Another problem is the size of the mother's pelvis and birth canal. The head can be too big to fit out the factory door. These restrictions have recently been lifted: safe C-sections have been around for 100 years, and unlimited reliable food sources about 50.
The human gene pool is clearly going mongrel. Many descendants are getting more stupid, while a few are getter smarter than ever. The ugly side of evolution, the one no one likes to talk about, is that for evolution to lock in the winners, the losers must die an early death. For humans that's mostly accomplished by tribal warfare. War decides which forks in the human gene pool are improvements.