When Abigail Marsh was 19, a dog ran in front of her car on the highway. She swerved, spun out, and found herself facing an oncoming lane of traffic with an unresponsive engine. Across the highway, another driver saw her emergency lights flashing, pulled over, and ran to her aid. He pushed her into the passenger seat, kicked her engine into gear, and drove her to safety. Then he disappeared into the night. She never saw him again. [Find out more about the science of why some people are good and others really are not.]

What makes someone put his life in danger to help a person he’s never met, and why do other people intentionally cause harm and