The Bystander Effect

In this clip the narrator explains the bystander effect and the diffusion of responsibility. Experimenters use the Liverpool Street Station in London to conduct their experiment.

A male actor lies on the floor, crumpled in pain. Numerous people walk by as he is on the floor for more than 20 minutes without aid. A female actor lies on the floor and is helped after five minutes; once one man helps another woman, who was watching from afar, comes to help as well. What happens in the third experiment? Why? How does similarity with the actor (in appearance) prompt others to help?