With everyone, there is always a level of uncertainty when posed with this type of situation. I always give an indecisive answer, but I'm opposed to these types of hypothetical dilemmas. Even with the most kind-hearted people, the course of action is entirely conditional. A person can assume all they want about what they'd do, but there is always the probability that when one finds themself in the situation, they'll at least consider other option.
I'd say that there is a 50/50 chance that I'll even consider returning the money. I'm a somewhat good person and I like to do the right thing and obviously returning it is the right thing to do, but I also like money. There are a number of things I can do with $20, but the very fact that she may be in a position where she needs it for something more important than what I would use it for, would make me doubt keeping it. Then there's the part of me that would say, "Fuck her, I'm having this. If she really needed it, she would have been more inclined to put it somewhere safe so she wouldn't drop it." So, I really don't know.
Hell, I'm not above going through the pockets of corpses, so picking up a free twenty dropped by a walking semi-conscious cadaver doesn't even spark a "Maybe..." in me.
Well, that, and my mother somehow happened to lose a case containing $20,000 when I was fourteen. I'm still sore over that.
As much as I like to say I would keep it, christ I would want to, somewhere deep down inside me there is this incredibly loud voice that goes off when I do something bad. I think it might be my conscience, which is bizarre as it doesn't always go off when I'm doing something illegal or immoral.
If I saw the person drop it, I'd at first consider keeping the money for myself. However, my guilt would gnaw at me until I convince myself to follow the person who dropped their money and hand the money back to them.