The bystander effect is also called the Genovese Effect, Genovese Syndrome or diffusion of responsibility. The theory behind this phenomenon is that an individual’s likelihood of helping a person in need is directly tied to the number of people witnessing the person’s need at the same time. According to the psychological literature on the bystander effect, a person is far less likely to help someone else in need if he or she is not the sole witness to the person in need.

The opposite of the bystander effect is bystander intervention. If a person requires help, another person, unaccompanied by friends or witnesses, is much more likely to offer help. When many people together witness a person in need of help, confusion begins to arise about who should help, and most people assume someone else will intervene.

The Genovese Effect is named for such an instance that shocked the nation in 1964. Kitty Genovese, native New Yorker, was sexually assaulted and killed by Winston Moseley, in front of her very large apartment building. It was early in the morning, and many dismissed Genovese's calls for help as a domestic fight between a couple. One person even shouted out the window at Moseley. Moseley initially left the scene after stabbing Genovese twice. Ten minutes later, Moseley returned, and Genovese remained desperately attempting to reach her apartment. Since no one had gone out to offer help or assistance to Genovese, Moseley brutally raped and killed her.

Evidence does suggest that several calls were placed to police, but no one left their apartment to assist Ms. Genovese. Even when she had succeeded in getting into the hall of her apartment, no one checked on her. Though evidence on how many people witnessed the crime was exaggerated, it was clear that response to the crime by a number of people was to do nothing. Too many people dismissed the incident or thought someone else would help.

This incident enraged Americans, prompting psychologists to study whether there truly existed a bystander effect. A study conducted in 1968 examined how a group of people might react to a fake “seizure” happening to a member of the group. In some cases, people failed to even alert the conductors of the experiment that someone had had a seizure. From these studies, and subsequent other murders that have occurred before several witnesses, psychologists have concluded that the bystander effect is a verifiable phenomenon.

What seems to occur to witnesses of a crime is the sense that “someone else” is more qualified to help. For example, calls to police by people who witnessed the Genovese murder were seen as “ending” a person’s social responsibility to another person. Some psychologists suggest that the bystander effect may be due in part to the reactions of witnesses. They may look to other witnesses to guide their own course of action. When no one appears to be reacting, then no single person is likely to act.

While it would seem that a person in a public place would be relatively safe from an attack, research on the bystander effect proves this not to be the case. In self defense classes, teachers often cover the bystander effect and tell people that if they are attacked in front of a group of people, they should appeal to one person in the crowd only, as specifically as possible, identify things about them. Eye contact with that person may also be helpful in gaining help and countering the bystander effect.