Darley, J. M., & Latané, B. (1968). Bystander intervention in emergencies: Diffusion of responsibility. Journal of Personality and Social Psychology, 8, 377-383.
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In Darley and Latané (1968), the authors conducted an experiment to show that the more observant (bystanders) present in an emergency situation, the less likely it became that anyone would help. In the experiment, all participants overheard an epileptic seizure. Some participants believed they were the only observant of the emergency, others believed they were one of many (two or four). Participants who knew that other observant could report the emergency felt less responsible and reported the incident more slowly than participants who were the only observant to the scene.
Darley and Latané argued that this can be attributed to what they refer to as the "diffusion of responsibility". They argue that individuals feel less personally responsible when they share the responsibility with many bystanders. A single witness bears the whole responsiblity for action (and is more likely to act), whereas a group of witnesses shares the responsibility among its members (and hence each individual member of the group of observant is less likely to act). Yet, participants who were members of a group of observant reported that they felt that the presence of others had no effect on their own behavior.
This seminal article started a vast body of literature on bystander intervention.
Manning, R., Levine, M., & Collins, A. (2007). The Kitty Genovese murder and the social psychology of helping: The parable of the 38 witnesses. American Psychologist, 62, 555-562.