In Long Beach, Calif., last June, Richard Kemp jumped an armed robber from behind and wrestled him to the ground after the robber held up his bank.
Wesley Autry was waiting at a New York subway station in 2007 when a young man having a seizure fell onto the tracks. Wesley jumped down to help and covered the man with his body as the train rolled over them with inches to spare.
Autry told ABC News' David Muir at the time, "It's a split-second decision. I don't look at myself as a hero. I gotta be humble about this. I don't want people to blow this out of proportion."
Deshpande said it's impossible to know for sure if someone may be a hero in the making. Possible signs are charitable giving, public service, and some belief in a higher moral order, though heroes do not necessarily belong to any organized religion.
Interestingly, those who become heroes often don't have any formal training in hostile situations.
"Perhaps they had life experiences that made them outward directed, that made them more concerned about other people," Deshpande said.
Of course, not all of us risk our lives without hesitation. A video caught dozens of passengers on the D.C. Metro last week seemingly standing idle as a man was savagely beaten.
Historically, studies of human nature have tended toward the negative. In a famous 1950s Milgram study, ordinary people gave what they believed were real electric shocks to participants in a staged experiment. Last March, a similar experiment was duplicated on a French reality TV show.
"The research indicates the majority of people will just stand by and not help," Deshpande said. "They don't want to get involved to the point where they will not only walk by, they also don't call for help."
But research also shows that some of us have a selfless reflex to sacrifice ourselves for others, as Badger did when he took down Loughner.
"I don't think anybody thought about getting hurt doing it," Badger said. "They just wanted to put a stop to him shooting people."