Put another way, the two scientists demonstrated that we often assume something happening more often than it is or explain an event not it terms of the frequency or the facts but by what is most easily called to mind. Hence, after the evil massacre of this past week the first reaction many of us had (the killer was crazy, gun control laws are too weak, media is too violent and too real in its depiction of violence) was based on the beliefs, events and images that most easily come to mind.
There is another element of this horrible event: The way in which people acted to save the lives of others -- perfect strangers and family members alike -- without regard to their own well-being or survival. Our moral sense is shaped by another aspect of availability: that life is precious and we must do what we can to preserve it when it is threatened. None of us can know how we would have acted with certainty. But it is equally certain that most would seek to protect and help.