I would say something along the lines of what jess1ca is saying—it is a case by case basis. I would be more likely to jump in and save a family member or a friend, than a complete stranger. We can all sit here an act like we would be heroes, that we would put our lives on the line to save another person, but how many people will actually go through with it? If you believe in evolution, like I do, and natural selection, it is all about protecting your own genes—but humans seem to have taken a different path than this simple definition.
Another question that is really famous is lets say you are on a train and you know everyone on the train is going to die in a minute. You have a decision: you can either do some action (i cant remember what) which will result in the death of one person, or you could do nothing and everyone dies. Believe it or not, a lot of people would choose to do nothing, because they couldnt stand themselves if they knew they were responsible for someone else’s death.