Images of inhumanity and atrocity are burned into our memories. Jewish men, women and children being herded into gas chambers. Entire villages destroyed by rampaging gangs in Rwanda. The systematic use of rape and the destruction of communities as part of ethnic cleansing in the Balkans. The massacre at My Lai in South Vietnam, the abuse of Iraqi prisoners in Abu Ghraib, and most recently, the carnage wrought by suicide bombers in Baghdad, Jerusalem, London and Madrid. Reflecting on these events, we inevitably ask: What makes people so brutal? Are they mentally ill? Are they the products of dysfunctional families or cultures? Or, more disturbingly, is anyone capable of taking part in collective ruthlessness given the right--or rather, the wrong--circumstances? Now the latest research, including possibly the largest social-psychology experiment in three decades, is providing a new window on these conundrums.