The attention we give to (or withhold from) tragedies has little to do with numbers: many hundreds can die in a cholera outbreak in Zimbabwe or hundreds of thousands in an earthquake in China and it receives nowhere near the press and public outrage as nearly 200 killed in terrorist attacks in Mumbai. (This isn't meant to diminish the tragedy of Mumbai, only to act as comparison.)

Still, we can imagine the evolutionary logic of this: our mind quickly jumps to the cause of a disaster, evaluating it for relevance to our own survival. Once we've "solved the crime" of culpability, it's as if the case is closed-we know what to be wary of and our brains can move on. Thus, because we understand the causes and risks of natural disasters (and may take a similar point of view about car bombings in areas in which they've become commonplace), the tragedies that grab our attention are those that are unexpected, and that have causes and motivations we struggle to explain. The Mumbai gun attacks make everyone nervous because, well, we wonder if it could happen here, to us.

Intention matters, too. Shankar Vedentam points out in an article for the Washington Post that the person who tries to kill a child and fails is seen as worse than the drunk driver who kills a child in an accident.