The root cause of fear, and how to treat it, has been one of modern psychology’s central questions. In the early twentieth century, Sigmund Freud argued phobias were “protective structures” springing from a patient’s “repressed longing” for his mother. In 1920, however, the American psychologist John B. Watson put forward a simpler theory: People develop fears through negative experiences. To test his hypothesis, he sought to condition an infant, whom he called “Little Albert,” to fear a white rat by presenting the rat to the child and simultaneously striking a steel bar. ... Different types of memories consolidate in different parts of the brain. Explicit memories of life events, for instance, consolidate in the hippocampus, the long, podlike structures near the center of the brain. Emotional memories, including fear, consolidate nearby in the amygdala, which activates the fight-or-flight response when it senses danger. The subjective experience of fear often involves both of these memory systems—a person will consciously remember past experiences while also undergoing several automatic physiological responses, such as increased heart rate—but they operate independently of each other.