There is an article in the September 2010 issue of Discover Magazine that points out that the average size of the human brain is shrinking. In the last 20,000 years human brains have shrunk 150 cubic centimeters, from 1500 to 1350 ccs. That's about the size of a tennis ball.
Why? I'd provide you with a link to the article, but as I write this no link appears to exist, or my little brain can't find it. If you can, I'd suggest you try to find your way to a news stand to find a copy.
Before you start blaming television, this is a worldwide phenomenon. The Cro-Magnons who carried spears and wore furs and huddled around fires in caves had lots more gray matter than even your college professors.