Why is it some people put themselves in peril to help others?

Tonight I was watching the show Nightline and their coverage of the theater shooting in Colorado. They had a story of two black men who were heroes, helping women with their children escape from the theater to safety. Then, while still talking about the tragedy, Diane Sawyer talked about the  7 year-old autistic girl who, earlier this week, was out on top of an air conditioner who fell  into the waiting arms of a bus driver below, another black man, named Steven St Bernard.

Diane Sawyer spoke about a man named Phil Zimbardo, head of the Heroic Imagination Project. He did  a study that,  though unpublished, yielded  some preliminary findings that gave me a feeling of pride. For example, people in cities were more likely to be reported taking heroic action then those living in rural areas. But what made me feel good was hearing that Black people were 8 times more likely to have done heroic deeds than caucasians, and Hispanic men- not women , were 2 times more likely than caucasians.

How come?
That segment of the show ended right there, with no explanation. 

This requires further review, for a future blog entry.