Smokey room social experiment
If you saw someone in trouble, would you try to help them? Psychologists have found that the answer to this question largely depends on the number of other people there. If we are the only witness, we are much more likely to help, but if we are part of a crowd, we are much less likely to lend a hand.
This phenomenon came to public attention after the gruesome murder of a young woman named Kitty Genovese. According to the classic story, multiple people may have witnessed her attack, but no one called for help until it was too late.
This behavior was identified as an example of the bystander effect, or not taking action when others are present. (Actually, the actual Genovese incident was not a perfect example of the bystander effect, as several witnesses immediately called 911.) 
In one classic experiment, researchers had participants sit in a room and fill out a questionnaire. Suddenly the room began to fill with smoke. Sometimes there was only one participant, sometimes there were three unsuspecting participants in the room, and in the final state there was one participant and two accomplices.
In a situation involving two Confederate military personnel who were participating in the experiment, these personnel ignored the smoke and continued filling out the questionnaire. When participants were alone, about three-quarters of them quietly left the room and reported the smoke to researchers.
In the situation with three actual participants, only 38% reported smoke. In the final situation, where both Confederates ignored the smoke, only 10% of participants reported smoke and left.This experiment is a good example of how much people rely on the reactions of others to guide their actions.
When something is happening and no one seems to be reacting, people tend to take their cues from the group and think that no response is necessary.