According to a new report in Discover Magazine, the human brain, which has expanded for most of our biological history, has begun to shrink. Kathleen McAuliffe writes that, according to new research, "Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimeters to 1,350 cc, losing a chunk the size of a tennis ball. The female brain has shrunk by about the same proportion." And that shrinking appears to still be happening on an evolutionary scale.

So why haven't you heard about this yet? Discover Magazine's McAuliffe thinks it may have something to do with the fact that scientists--at least the few who have stumbled onto this trend--don't really have an explanation yet. "As I soon discover, only a tight-knit circle of paleontologists seem to be in on the secret, and even they seem a bit muddled about the matter. Their theories as to why the human brain is shrinking are all over the map," she writes.

At the Discover Magazine blog, Razib Khan lays out the two biggest theories:
Roughly, some think we’re getting less intelligent, while others believe that the brain is reorganizing its structure and development. Remember that the brain uses about ~20% of our caloric intake. It’s a metabolically expensive organ.
So it's possible that human brains are merely becoming more efficient. But it's also possible, according to paleoneurologists, that we really are evolving to be dumber. As studious observers of the U.S. Congress, we at the Atlantic Wire certainly find that theory plausible.