The president of France awarded three Americans and a Brit with his nation's highest honor on Monday. 

The honor comes after the men subdued a heavily armed attacker on a high speed train carrying 500 passengers.

Throughout the world, these four are being revered as heroes - but what exactly makes a hero? David Rand is a professor of psychology, economics and management at Yale University. He is also one of the authors of a recent study of why people take extreme risks to save lives.

Heroes just do it

Rand says what separates a hero from everybody else is the impulse to not think before jumping into action.

"There's a lot of evidence that many people do have an impulse to help others and to be cooperative," he said. "But the issue is that if you're the kind of person that stops and thinks, then even if you have the impulse to help, you'll take a minute and you'll say, 'Well, should I do it?' And you'll say, 'Well, maybe I should go ahead and not do it.'"

Rand said that was the common thread among those in his study who performed heroic acts.

"Almost everyone said something along the lines of, 'I didn't think, I just acted,'" he said. "One of the things that happens when you stop and think and you deliberate, is you start coming up with rationalizations for why it would not be a good idea to act ... Once you stop and think, it's psychologically too late."

To listen to the full interview, click on the blue audio player above