It's a belief that unites the left and right, psychologists and philosophers, writers and historians. It drives the headlines that surround us and the laws that touch our lives. From Machiavelli to Hobbes, Freud to Dawkins, the roots of this belief have sunk deep into Western thought. Human beings, we're taught, are by nature selfish and governed by self-interest. Humankind makes a new argument: that it is realistic, as well as revolutionary, to assume...