Democrat and Republican. Meat Eaters and Vegetarians. Black and White. As human beings we sort ourselves into groups. 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?             In this award-winning book, David Berreby describes how twenty-first-century science is addressing these age-old questions. Ably linking neuroscience, social psychology, anthropology, and other fields, Us and Them investigates humanity's "tribal mind" and how this alters our thoughts, affects our health, and is manipulated for good and ill. From the medical effects of stress to the rhetoric of politics, our perceptions of group identity affect every part of our lives. Science, Berreby argues, shows how this part of human nature is both unexpectedly important and surprisingly misunderstood.             Humans need our tribal sense--it tells us who we are, how we should behave, and links us to others as well as the past and future. Some condemn this instinct, while others celebrate it. Berreby offers in Us and Them a third alternative: how we can accept and understand our inescapable tribal mind. "[A] brave book. . . . Berreby's quest is to understand what he sees as a fundamental human urge to classify and identify with 'human kinds.'"--Henry Gee, Scientific American