20 years ago, a complete stranger risked his own life to save the life of psychology researcher Abigail Marsh.
Ever since then, she has pondered the difference between people who go out of their way and those who look the other way when someone needs help.
Marsh looks at the biology of altruism in her new book The Fear Factor: How One Emotion Connects Altruists, Psychopaths, and Everyone In-Between
Photo: Ted.com
