WHAT causes people to risk their lives to help strangers?

Recently, three young American men and a British businessman thwarted a gunman’s attack on a French passenger train, acting within seconds and at enormous personal risk. When interviewed afterward, they stressed the unthinking nature of their actions. “It was just gut instinct,” said one, in a characteristic remark. “It wasn’t really a conscious decision.”

This turns out to be typical of heroes. Last year, one of us, Professor Rand, together with his colleague Ziv Epstein, conducted an analysis of recipients of the Carnegie Medal for heroism, which is awarded to those who risk their lives for others. After collecting interviews given by 51 recipients and evaluating the transcripts, we found that the heroes overwhelming described their actions as fast and intuitive, and virtually never as carefully reasoned.

This was true even in cases where the heroes had sufficient time to stop and think. Christine Marty, a college student who rescued a 69-year-old woman trapped in a car during a flash flood, said she was grateful that she didn’t take the time to reflect: “I’m thankful I was able to act and not think about it.” We found almost no examples of heroes whose first impulse was for self-preservation but who overcame that impulse with a conscious, rational decision to help.

It is striking that our brute instincts, rather than our celebrated higher cognitive faculties, are what lead to such moral acts. But why would anyone ever develop such potentially fatal instincts?