When animals are directly selected for reduced reactive aggression (domestication), either naturally or artificially, they are indirectly selected for other traits too, like depigmentation, floppy ears, shorter muzzles, smaller teeth, docility, smaller brains, more frequent estrous cycles, juvenile behavior and curly tails.

Some scientists believe that the decrease in human brain size that occurred over the last 10,000 years may have been an indirect effect of domestication, but there are two problems with this theory:

1. Head size has rapidly rebounded over the 20th century (as has height), suggesting the brain size reduction during the Holocene was perhaps not an evolutionary change, but merely suboptimum nutrition caused by disruption of healthy hunter-gatherer life style in aspiring agriculturalists and the peoples they colonized.
2. If humans did self-domesticate ourselves, the evidence suggests it began hundreds of   thousands of years ago, not merely in the Holocene, and yet brain size reduction only occurred in the latter.

How might domestication have occurred?  One theory is that capital punishment, in which about 15% of the population (usually hyper-aggressive males who were bullying the rest of the tribe) were killed off in a “Revenge of the Nerds” scenario.

The fact that alpha males were such evolutionary losers is very humiliating and painful to commenters like “philosopher” who probably come from a long line of big husky rednecks, so they must convince themselves that nerds were selected for by masters looking for slaves, when in reality, nerds were the authors of their own evolutionary success, and simply murdered the bullies.

Because these alpha-male bullies tend to be very manly men, when their genes are removed, the tribe starts looking less like men and more like little boys.

This may help explain why early humans looked more like manly chimps while later humans look more like baby chimps.  It may also explain why a lot of nerds act more like little boys than grown men, preferring to play video games or play chess, and watch Star Wars or Star Trek instead of pursuing money and sex.

But in the rare cases where nerds do pursue money (i.e. Bill Gates) they often slaughter the alpha male competition in record time because they are so much smarter, particularly if they’re self-aware enough to start their own business instead of trying to climb the corporate ladder which they often lack the charisma to do.

But this leads to a paradox.  If domestication reduces brain size and makes people more nerdy, why are nerds smart, and why is there no evidence of brain size reduction until the Holocene (and even that may simply be malnutrition) when other signs of domestication (facial size reduction) occurred hundreds of thousands of years earlier?

One possibility is that modern humans in general and nerds in particular, were shaped by two evolutionary forces:  One selecting for less reactive aggression (domestication) and the other selecting for intelligence, and the latter prevented brain size from shrinking.

For more information about this topic, please see the following video: