"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,...