We studied this type of thing in my super-basic-elementary psych class, and it's been shown that in a large crowd, everyone assumes that someone else is taking care of the situation. People are much more likely to react when it's one on one, and they realize that they're the ones that have to act.
The example I've always heard is Kitty Genovese:
http://www.crimelibrary.com/serial_k...itty_genovese/