“SM” is a bit of an emotional anomaly. The 44-year-old mother isn’t scared of snakes. She doesn’t shriek when she sees a scary movie. Even haunted houses don’t give her chills. SM is pretty much fearless — and now scientists think they’ve figured out why. She lacks amygdala, clusters of neurons in the brain that play a role in fear and anxiety. Research on her might lead to new therapies for post-traumatic stress disorder, such as new forms of psychotherapy that hinder the amygdala’s activity. Still, fear is an important emotion, notes neurobiologist David Anderson.