The term bystander effect refers to the phenomenon in which the greater the number of people present, the less likely people are to help a person in distress. People are more likely to take action in a crisis when there are few or no other witnesses present.
Social psychologists Bibb Latané and John Darley popularized the concept of the bystander effect following the infamous murder of Kitty Genovese in New York City in 1964.
In a series of classic studies, they found that the amount of time it takes the participant to take action and seek help varies depending on how many other observers are in the room. In one experiment, subjects were placed in one of three treatment conditions: alone in a room, with two other participants, or with two confederates who pretended to be normal participants. As the participants sat filling out questionnaires, smoke began to fill the room.
When participants were alone, 75% reported the smoke to the experimenters. In contrast, just 38% of participants in a room with two other people reported the smoke. In the final group, the two confederates in the experiment noted the smoke and then ignored it, which resulted in only 10% of the participants reporting the smoke.
Latané and Darley attributed the bystander effect to two factors: diffusion of responsibility and social influence. The perceived diffusion of responsibility means that the more onlookers there are, the less personal responsibility individuals will feel to take action. Social influence means that individuals monitor the behavior of those around them to determine how to act.