To follow up on what cesperanza said, there's also the idea of diffusion of responsibility, in that people are more likely to take action when fewer people are involved because there are fewer people who could potentially be held responsible. It's one of the reasons you're more likely to get help if you're stranded on the side of the road in a backwoods town than you are in the middle of the city. I don't think there's any hard evidence on how that interacts with altruism, but these were really interesting questions.