"The human brain has been getting smaller and smaller since the Stone Age - and no one is sure why".
Since 20,000 years ago the human brain has gone from 1,500cc in Cro-Magnon to about 1,350cc today, losing a chunk about the size of a tennis ball. There is no consensus why this is happening, but there are several theories:
1. We are just plain getting dumber. This is loosely based on the premise that we are "devolving" culturally and intellectually. The movie "Idiocracy" is used as an example of this phenomenon, in which the uneducated produce more offspring than the educated.
I can imagine a cultural devolution scenario - but loss of intelligence would have to be based on something more substantial than "dumb" parents.
2. We are the equivalent of a domestic cow. Just as domestic animals have developed smaller brains and are less robust than their wild form, so has man "domesticated" himself - with a similar outcome. Now living in goups and relying on each other for safety and daily necessities man, could no longer afford to behave like a "caveman". To preserve the cohesion of the group, the murderers and the very aggressive would be driven out or killled, and thus eliminated from the gene pool. Being a cooperative member of the group greatly increased the chance of survival. Having a large, robust body and an energy-hungry brain was no longer an advantage. The wild, cunning wolf was becomiing a tail-wagging puppy.
Dmitri Belyaev successfully bred the aggression out of wild foxes and produced a much different animal. After about 12 generations the foxes had developed a curly tail, floppy ears and a different coat pattern. The skeleton was smaller, more delicate, and the cranium had changed shape. They became less noticably sexually dimorphic - and they wagged their tail, something the wild form never did.
3. Although our brains have become smaller, the ratio of brain volume to body mass has always remained the same. We have not become dumber because our brains have rewired to become more efficient. But the reduction in brain size was not uniform throughout this period. During times of low population density, brain size increased; during times of high population density, brain size decreased.
If we continue "losing our minds" at this rate, in 20,000 years we'll be back pre-human size.