Human brain size has been shrinking for last 20,000 years

From the WSJ:

First, it's clear, from glancing around society, that clever people—who on average have slightly bigger brains—aren't having more babies than less-clever people. Second, the fossil record strongly suggests that our brain size peaked at 1,500 cubic centimeters around 20,000 years ago and has since shrunk to 1,350 cc. . . .