[dawgr]

I know it's a joke and you might be right, but I don't think the need to reduce brain size would be such a huge evolutionary pressure to see a decrease of 10% in just 20,000 years, that's relatively little.

Granted that the bigger the brain, the more energy it needs, but if the brain uses 25% of our energy, 0.25*0.1= 2.5%. Even if that's wrong by a few percent, it doesn't sound like such a big advantage to cause that decrease in so little time and as it's been pointed out, it actually started happening when more food/energy was available from agriculture or whatever.