Our brains were designed for tribal life, for getting along with a select group of others (Us), and for fighting off everyone else (Them). But modern life has thrust the world's tribes into a shared space, creating conflicts of interest and clashes of values, along with unprecedented opportunities. As the world shrinks, the moral lines that divide us become more salient and more puzzling. We fight over everything from tax codes to gay marriage to global warming, and we wonder where, if at all, we can find our common ground.

A grand synthesis of neuroscience, psychology, and philosophy, Moral Tribes reveals the underlying causes of modern conflict and lights a way forward. Our emotions make us social animals, turning Me into Us. But they also make us tribal animals, turning Us against Them. Our tribal emotions make us fight, sometimes with bombs, sometimes with words, and often with life-and-death stakes. Drawing inspiration from moral philosophy and cutting-edge science, Moral Tribes shows when we should trust our instincts, when we should reason, and how the right kind of reasoning can move us forward.

Joshua Greene is the director of Harvard University's Moral Cognition Lab, a pioneering scientist, a philosopher, and an acclaimed teacher. The great challenge of Moral Tribes is this: How can we get along with Them when what they want feels so wrong? Finally, Greene offers a surprisingly simple set of maxims for navigating the modern moral terrain, a practical road map for solving problems and living better lives.

Joshua Greene is the John and Ruth Hazel Associate Professor of the Social Sciences and the director of the Moral Cognition Lab in Harvard University's Department of Psychology.