David Berreby's book on the Science of Identity: Democrat and Republican. Meat Eaters and Vegetarians. Black and White. As human beings we sort ourselves into groups or foundations. And once we identify ourselves as a member of a particular group-say, Red Sox fans-we tend to feel more comfortable with others of our own kind, rather than, say, Yankees fans. Yet we all belong to multiple groups at the same time-one might be a woman, a mother, an American, a violinist. How do we decide which identities matter and why they matter so much? And what makes us willing to die for, or to kill for, a religion, a nation, or a race?

David Berreby's book on the Science of Identity: Democrat and Republican. Meat Eaters and Vegetarians. Black and White. As human beings we sort ourselves into groups or foundations. And once we identify ourselves as a member of a particular group-say, Red Sox fans-we tend to feel more comfortable with others of our own kind, rather than, say, Yankees fans. Yet we all belong to multiple groups at the same time-one might be a woman, a mother, an American, a violinist. How do we decide which identities matter and why they matter so much? And what makes us willing to die for, or to kill for, a religion, a nation, or a race?