The Nobel Prize winning psychologist and economist Daniel Kahneman recounts the story behind a famous experiment conducted at New York University:

Participants in that experiment were led to individual booths and invited to speak over the intercom about their personal lives and problems. They were to talk in turn for about two minutes. Only one microphone was active at any one time. There were six participants in each group, one of whom was a stooge. The stooge spoke first, following a script prepared by the experimenters. He described his problems adjusting to New York and admitted with obvious embarrassment that he was prone to seizures, especially when stressed. All the participants had a turn.

When the microphone was again turned over to the stooge, he became agitated and incoherent, said he felt a seizure coming on, and asked for someone to help him. The last words heard were, "C-could somebody-er-er-help-er-uh-uh-uh [choking sounds]. I … I'm gonna die-er-er-er-er … I'm gonna die-er-er-I seizure I-er [chokes, then quiet]." At this point the microphone of the next participant automatically became active, and nothing more was heard from the possibly dying individual.

How do you think the participants responded to the person they thought was having a seizure? Kahneman reports that only four of the fifteen participants responded immediately to the appeal for help; six stayed in the booth; five came out only after the "seizure victim" had nearly choked to death.

Kahneman concludes:

The experiment shows that individuals feel relieved of responsibility when they know that others have heard the same request for help. Did the results surprise you? Very probably. Most of us think of ourselves as decent people who would rush to help in such a situation, and we expect other decent people to do the same. The point of the experiment, of course, was to show that this expectation is wrong. Even normal, decent people do not rush to help when they expect others to take on the unpleasantness of dealing with a seizure. And that means you, too.