Last year, during the mammoth floods in Pakistan, one teenage boy, managed to save the life of ten people stranded on the rooftop of a house environed by tidal waves. He could not, however, survive his eleventh lap and was swallowed by the waves into its salty bowels. Again, that is exception.

What I was talking about it general behavior. Also it does not mean situation where somone's own life would be in danger. Usually, when in a situation, where help can be extended to a person without incurring a risk yourself, you would act usually when there are no other people around. This has been proven by research. :)