(This video is no longer available for streaming.) As Americans grapple with ever-increasing waistlines, science is looking at the biology behind the compulsion to eat. What exactly makes some people weigh 350 pounds and others 150? Researchers have discovered a chemical in your brain proven to regulate body weight, but can pinpointing a "fat gene" signal the road to a remedy? Learn the latest in weight research in this NOVA scienceNOW video.