Last October, in Foshan, China, a 2-year-old girl was run over by a van. The driver did not stop. Over the next seven minutes, more than a dozen people walked or bicycled past the injured child. A second truck ran over her. Eventually, a woman pulled her to the side, and her mother arrived. The child died in a hospital. The entire scene was captured on video and caused an uproar when it was shown by a television station and posted online. A similar event occurred in London in 2004, as have others, far from the lens of a video camera.

Yet people can, and often do, behave in very different ways.

A news search for the words “hero saves” will routinely turn up stories of bystanders braving oncoming trains, swift currents and raging fires to save strangers from harm. Acts of extreme kindness, responsibility and compassion are, like their opposites, nearly universal.

Why are some people prepared to risk their lives to help a stranger when others won’t even stop to dial an emergency number?

Scientists have been exploring questions like this for decades. In the 1960s and early ’70s, famous experiments by Stanley Milgram and Philip Zimbardo suggested that most of us would, under specific circumstances, voluntarily do great harm to innocent people. During the same period, John Darley and C. Daniel Batson showed that even some seminary students on their way to give a lecture about the parable of the Good Samaritan would, if told that they were running late, walk past a stranger lying moaning beside the path. More recent research has told us a lot about what happens in the brain when people make moral decisions. But are we getting any closer to understanding what drives our moral behavior?