Bruce Shapiro, executive director of the Dart Center for Journalism and Trauma, said there are two faces to courage: those who respond and those who cope.
"The first are the people who, in the moment of the bombing, run toward danger -- the first responders, the photographers and the reporters and others who simply keep a calm head," he said. "Equally important, in the aftermath, are those who seem to stay resilient in times of crisis."
Doctors at Massachusetts General Hospital, where 12 marathon victims were treated, showed a "calm and collected response."
Most victims were admitted with metal pieces from the bomb -- pellets and nails -- inside of them and required surgery.
"The ones that could talk when they were arrived, they were amazingly responsive and resilient," said Dr. George Velmahos, division chief of trauma, emergency surgery and surgical critical care. "We were able to provide better care because of it."
Melanie Greenberg, a psychologist who specializing in stress and overcoming trauma, said cultural and personal ethics play a role in courage.
"One of the key things about heroism is that it's not the absence of fear, but feeling fear and choosing to act anyway," she said. "Something is more important than fear and survival. It's certain values you have that drive your life and make it meaningful."
She was hesitant to describe cowardice: "Sometimes, you freeze like a deer in the headlights. That's the primitive lizard in us, when the social compassion switches off."
Greenberg, who blogged about the qualities of courage for Psychology Today, cited "empathy, kindness and compassion" as the motivators for heroism.
Training for emergencies gives responders courage, according to Greenberg.
Others draw it from "a sense community and compassion," she said. "The culture of being together, that you are brothers and doing it for your buddy -- that inspires heroism. The support group means more than the individual life."
Standing up for what is right, "following your heart, doing what the situation calls for," is another motivator for heroism.
"It brings out the best in people to stand up to terrorism and not let the bad guys win," said Greenberg. "You are part of something larger than yourself."