Any time you see something tragic, someone that needs help, chances are that your actions will mostly be dictated by those around you. If there is no one else around, chances are you'll go and help if possible. If there is a large crowd that has seen the same tragedy, chances are, you'll do nothing. Alone, all the responsibility falls on you. In a large group, the same sense of responsibility is spread very thin through the crowd, and no one feels like they should do anything.