Bystander Effect can be defined as "When there is an emergency, the more bystanders there are, the less likely it is that any of them will actually help. Pluralistic ignorance is where they assume nothing is wrong because nobody else looks concerned." [Changing Minds (2008 1)].
Authors Darley and Latene  define Bystander Effect as “a psychological phenomenon where person are less likely to intervene in emergencies situations when they are in a group. They tend to assume that others in the group are more qualified then they are, thus their intervention would be unneeded.
The bystander effect is a psychosocial behavior whereas bystanders are less likely to intervene in a situation the greater the number of bystanders around. The bystander effect is more dramatic in emergency situations, and these situations (fights, abductions, etc) are the ones that have been the most studied. In such situations, not only the number of people present but also the ambiguity oh the need for help (i.e. whether the bystander perceives both a real victim or not and whether help is really needed), the empathy and personal responsibility that a bystander believes he can play in helping, the potential harm to the bystander if he intervenes, and other variables, also contributes to the effect.
The bystander effect is also called the Genovese Effect, Genovese Syndrome or diffusion of responsibility [Christensen (2008 2)]. 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 [Christensen (2008 2)].
This phenomenon is cause when there is a diffusion of responsibility (The more people in a group, the less responsible each individual feels), Pluralistic-Ignorance (people tend to look to others for instruction) and Audience-Inhibition (The more people that are in a group, the more fear people have of making a mistake).
Bystanders go through a five-step process, during each of which they can decide to do nothing.
•Notice the event (or in a hurry and not notice).
•Realize the emergency (or assume that as others are not acting, it is not an emergency).
•Assume responsibility (or assume that others will do this).
•Know what to do (or not)
•Act (or worry about danger, legislation, embarrassment, etc.)
A curious phenomenon linked to the bystander effect is that as soon as a someone intervenes, other bystanders will tend to join as well. Apparently, as soon as someone intervenes, the situation becomes less ambiguous (i.e. bystanders assume help is really needed and join in) or, else, bystanders find comfort or safety in numbers and join to help.
Case studies and examples
|In this experiment, a seven-year old girl is being abducted. Notice that the situation should have been rather unambiguous, especially as the girl struggles to escape and keeps shouting "Someone, help me! He is not my dad!" Notice also that there seems to be rather few people around, so intervention by bystanders should have been more likely. But it does not happen so. Many bystanders were aware something odd, probably a real abduction, was taking place, but did nothing to help, probably "passing" the responsibility to help to other bystanders. A couple of adults walked by, each other reinforcing a "no intervention" behavior in the other (notice how the woman pushes her partner away, and how the guy starts walking, "dragging" her partner away with him, as soon as the pedestrian lights go green).
After hours of "experimenting", two friends did intervene, a third bystander jumped from his car and joined the group, and an old lady was also phoning the police. All these behaviors reinforced each other, as well. The friends certainly had the comfort of numbers and reinforced each other to intervene once they started to do so. Their behavior most probably triggered the help of they guy in the car (he seems to jump from the car when the two friends were already walking towards the "assailant"), and may have also prompted the old lady to phone the police -she may have done so independently, though; however, there is no information regarding anybody else having done so earlier.
|In this series of experiments, there probably are more than the two conflicting rules stated, i.e. to help or to conform to what others are doing -which is not helping. For example, some people may not want to get involved one way or another, some others may fear to be burgled or harmed by an impostor.
In any case, we continue observing variables that play a role in the bystander effect: people's behavior tend to reinforce each others', gender and attractiveness -as per the way of dressing, for example- may play a role in receiving help o not: the woman received help quicker than the first man.
Although the way of dressing may explain the third situation, such conclusion is dubious: the first woman on offering help called him "Sir" and asked very "sharp" questions. And who usually interact like that? Police, for example. So, it may be that the woman who first helped was a policewoman, a security guard, even a doctor or a nurse. Thus, the way the man was dressing may not be the reason why he received such quick help! Also, he received help in about 4 seconds, which means the woman probably observed his behavior and had assumed he was in trouble (i.e. the situation was rather unambiguous).
|This last video explains more clearly what underlies the bystander effect: in social situations, people take clues from others regarding both how to perceive (or re-interpret) a given situation, and, therefore, how to act. When people are by themselves, they have not such clues and, thus, have to react according to how they interpret the situation. In many of these situations the majority of people tend to get involved.||
The Kitty Genovese Story Written by Christensen 2008 
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 the 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. 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 exist a bystander effect phenomenon. 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.
Want to know more?
- Wikipedia - Bystander effect
- This Wikipedia page offers some more information about this phenomenon.