It is a fact that humans destroy the lives of other humans - strangers, friends, lovers, and kin - and have been doing so for a long time. These cases are unsurprising and easily explained: We harm others when it benefits us directly, fighting to win resources or wipe out the competition. In this sense we are no different from any other social animal. The mystery is why seemingly normal people torture, mutilate, and kill others for the fun of it - or for no apparent benefit at all. Why did we, alone among the social animals, develop an appetite for gratuitous cruelty? This is the core problem of evil. It is a problem that has engaged scholars for centuries and is the central topic of this book. Drawing on the latest scientific discoveries, Hauser provides a novel and elegant explanation for why some individuals engage in evil and why we uniquely evolved this capacity: Evildoers emerge when unsatisfied desires combine with the denial of reality, enabling individuals to engage in gratuitous cruelty toward innocent victims. This simple recipe is part of human nature, and part of our brain's uniquely evolved capacity to combine different thoughts and emotions. The implications of Hauser's theory of evil are unsettling: due to individual differences that begin with our biology, and can be enhanced by certain environments, seemingly normal people are capable of causing horrific harms, feeling rewarded and justified or nothing at all.