Book excerpt: "Philosophy, economics, and biology have rarely been so entertaining."—Matt Ridley, author of Genome Paul J. Zak's proclivity for taking blood samples has earned him a nickname as the "vampire economist." But his sanguinary habit is backed by his scientifi­c quest: What if there was a master switch for human behavior? On, and people are loving and generous. Off, and they revert to violence and greed. By studying thousands of blood samples, Zak has pinpointed just such a switch: a brain chemical called oxytocin. Sprinting around the globe and into the human brain, ­The Moral Molecule is a dazzling narrative as erudite and entertaining as bestsellers like Flow, Drive, and Why We Love.