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This article is about the psychological phenomenon. In an unrelated use, the term bystander effect is also used in radiobiology to describe the effects of radiation on cells that are adjacent to those directly affected by radiation.

The bystander effect (also known as bystander apathy, Genovese syndrome, or bystander intervention) is a psychological phenomenon in which someone is less likely to intervene in an emergency situation when other people are present and able to help than when he or she is alone.

Solitary individuals will typically intervene if another person is in need of help: this is known as bystander intervention. However, researchers were surprised to find that help is less likely to be given if more people are present. In some situations, a large group of bystanders may fail to help a person who obviously needs help. An example which shocked many people is the Kitty Genovese case. Kitty Genovese was stabbed to death in 1964 by a serial rapist and murderer. The murder took place over a period of about 1 hour, after which it was reported that dozens of alleged "witnesses" failed to help the victim. For this reason, the name Genovese syndrome or Genovese effect was used to describe the phenomenon at the time. In 1972, Dr. Wolfgang Friedmann, professor of law at Columbia University, was murdered in broad daylight and bled to death on the sidewalk. The death of Deletha Word near Detroit in 1995 after witnesses failed to thwart her attackers, as well as the James Bulger murder case, may have been other well-publicized cases of the effect.

A 1968 study by John Darley and Bibb Latane first demonstrated the bystander effect in the laboratory. They ran some simple studies such as the following: A participant is placed alone in a room and is told he can communicate with other participants through an intercom. In reality, he is just listening to an audio recording and is told his microphone will be off until it is his turn to speak. During the recording, one participant suddenly pretends he is having a seizure. The study found that how long the participant waits before alerting the experimenter varies directly with the perceived number of other participants. In some cases, the participant never told the experimenter.

A common explanation of this phenomenon is that, with others present, observers all assume that someone else is going to intervene and so they each individually refrain from doing so and feel less responsible. This is an example of how diffusion of responsibility leads to social loafing. People may also assume that other bystanders may be more qualified to help, such as being a doctor or police officer, and their intervention would thus be unneeded. People may also fear "losing face" in front of the other bystanders, being superseded by a "superior" helper, or offering unwanted assistance. Another explanation is that bystanders monitor the reactions of other people in an emergency situation to see if others think that it is necessary to intervene. Since others are doing exactly the same, everyone concludes from the inaction of others that other people do not think that help is needed. This is an example of pluralistic ignorance and social proof. An alternative to explanations of rational motivation is that emotional cues to action can be as powerful as irrational ones, and the presence of a group of inactive others is a pre-rational emotional cue to inaction that must be overcome.

To counter the bystander effect when you are the victim, a studied recommendation is to pick a specific person in the crowd to appeal to for help rather than appealing to the larger group generally. If you are the only person reacting to an emergency, point directly to a specific bystander and give them a specific task such as, "You. Call the police." These steps place all responsibility on a specific person instead of allowing it to diffuse. Furthermore, pluralistic ignorance is countered by the implication that all bystanders are indeed interested in helping, and social proof kicks in when one or more of the crowd steps in to assist.

• Conformity
• Diffusion of responsibility
• Milgram experiment
• Moral hazard
• NIMBY
• Somebody Else's Problem - used in the Hitchhiker's Guide to the Galaxy series
• Empathy-altruism hypothesis

A September 2007 article in American Psychologist that reviewed coverage of the Kitty Genovese murder in social psychology textbooks concluded the story of 38 witnesses is not supported by fact, and is more like a parable. See 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.

• Darley, J. M. & Latané, B. (1968). Bystander intervention in emergencies: Diffusion of responsibility. Journal of Personality and Social Psychology 8, 377-383.

• Piliavin, Rodin and Piliavin (1969). Good Samaritanism: An Underground Phenomenon? Journal of Personality and Social Psychology Vol. 13, No. 4., 289-299.

• Gladwell, Malcolm. (2000). The Tipping Point: How Little Things Can Make a Big Difference. ISBN 0-316-31696-2.

• "Don't Just Stand There - Do Something", Safety Canada, January 2004.
• The Bystander's Dilemma, Greater Good 3(2), Fall/Winter 2006-2007.
• "I kept saying, 'Help me, help me.' But no one did." by Tara McCartney, The Guardian, August 4, 2005.
• "Act now to defy 'bystander effect'" by Craig Kielburger and Marc Kielburger, Toronto Star, March 29, 2007.
• "Respect for life missing in a video of death", Wichita Eagle, July 2, 2007. (archived at Topix.net)
• "Police: Shoppers Stepped Over Victim" by Roxana Hegeman, Associated Press, July 4, 2007. (at SFGate.com)
• "A rape witnessed, a rape ignored" by Mara H. Gottfried, Pioneer Press, August 23, 2007.