Does your dog get sad when you leave for the day? Does your cat purr because she loves you? Do bears attack when they're angry? You can't very well ask them. In fact, scientists haven't been able to reach a consensus on whether animals even have emotions like humans do, let alone how to study them. Yet studies of animal emotion are critical for understanding human emotion and mental illness. In The Nature of the Beast, pioneering neuroscientist David J. Anderson describes a new approach to solving this problem. He and his colleagues have figured out how to study the brain activity of animals as they navigate real-life scenarios, like fleeing a predator or competing for a mate. His research has revolutionized what we know about animal fear and aggression. Here, he explains what studying emotions and related internal brain states in animals can teach us about human behavior, offering new insights into why isolation makes us more aggressive, how sex and violence connect, and whether there's a link between aggression and mental illness. Full of fascinating stories, The Nature of the Beast reconceptualizes how the brain regulates emotions—and explains why we have them at all.