Amateur Economics

I am not an economist, so I'm sure the issue is ten times more complicated than this. But investment, stocks, and the like are built on credit and trust, right? Investors know the money isn't all there in hard specie. The "fake money" that the Jeffersonians thundered against in the early 19th century is common currency now. Why, then, do investors make a run on banks and firms when something shakes their confidence? Doesn't a run benefit almost no one? They can't all be so reflexively greedy that each one is so desperate to be the lucky one who gets a real return on his investment before the bank can't pony up. I sometimes get the feeling that if all these investors just kept their heads and had patience, things would stabilize. But they never do, so there must a lot more to it I'm completely unaware of. Any economics whizzes out there who can shed some light?