Who will help?

I learned about this in a sociology class I took a couple years back. I can't remember what it is called but I DO remember it striking my interest. I will explain it in an example. That is about the only way I can describe it and still make sense.
Imagine a woman being mugged in the middle of the street at night. It is well lit, but the street is empty. The only witnesses are those in their houses or apartments. The example from sociology says that most people in most situations wont do a darn thing about it. They will assume, in their head, that "somebody else with do something." The circle goes around and around until it is all over. Nobody called the cops and the woman is on her own. Perhaps somebody will say something later. Maybe if the cops starts asking around... they may speak up. The point is... NOBODY helps her while it is happening.
Why is this? I have caught myself doing it before. I see somebody pulled over along the highway with their hood up. I assume they have either already called somebody, or somebody behind me will pull over to help them. I think to myself "I am a 5 foot 4 girl... I can't really defend myself in ALL situations" (I like to act tougher than I really am!). It is terrible that one has to think of that when deciding whether or not to help. The point is I am guilty, too.
It just frustrates me that this happens! What would I do in the mugging situation? Would I just assume somebody else will help? I guess I don't know unless I am forced into making that choice. What would you do? Think about how many times you passed somebody by. Think about it next time so see somebody pulled over on the side of the road. The least you can do it pull over, crack your window just to see if everything is okay... maybe check to see if they need to use your cell.
HMMMMMmmmm.