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The bystander effect or Genovese syndrome is a social psychological phenomenon that refers to cases where individuals do not offer any means of help in an emergency situation to the victim when other people are present. The probability of help has often appeared to be inversely related to the number of bystanders; in other words, the greater the number of bystanders, the less likely it is that any one of them will help. The mere presence of other bystanders greatly decreases intervention. In general, this is believed to happen because as the number of bystanders increases, any given bystander is less likely to notice the situation, interpret the incident as a problem, and less likely to assume responsibility for taking action.
The bystander effect was first demonstrated in the laboratory by John Darley and Bibb Latané in 1968 after they became interested in the topic following the murder of Kitty Genovese in 1964. These researchers launched a series of experiments that resulted in one of the strongest and most replicable effects in social psychology. In a typical experiment, the participant is either alone or among a group of other participants or confederates. An emergency situation is then staged. The researchers then measure how long it takes the participants to act, and whether or not they intervene at all. These experiments have often found that the presence of others inhibits helping, often by a large margin. For example, Bibb Latané and Judith Rodin staged an experiment around a woman in distress in 1969. 70 percent of the people alone called out or went to help the woman after they believed she had fallen and gotten hurt, but when there were other people in the room only 40 percent offered help.
There are, in fact, many reasons why bystanders in groups fail to act in emergency situations, but social psychologists have focused most of their attention on three main processes. In order for a person to help, they must first notice the situation, interpret it as an emergency, and take responsibility for helping. Based on early research, subsequent research looked at the ways in which each of these three decisions might be influenced by the presence of bystanders.
The first step required for a bystander to intervene is that they notice the situation at all. In a 1968 experiment, Latane and Darley had men from Columbia University complete a questionnaire in a room either alone or with two strangers. While they were completing the questionnaire smoke was pumped into the room through a wall vent to simulate an emergency. When students were working alone they often looked around the room and as a result, noticed the smoke almost immediately (within 5 seconds). However, the students that were working in groups were less likely to be looking around the room and took longer (around 20 seconds) to notice the smoke.  This occurs because in groups people are simply less likely to be looking around. In most western cultures, politeness dictates that it is inappropriate to be staring at people in public. As a result, in large groups, passers-by are more likely to be keeping their attention to themselves, than when alone, where people are more likely to be conscious of their surroundings and therefore more likely to notice a person in need of assistance.
Once a situation has been noticed, in order for a bystander to intervene they must interpret the incident as an emergency. According to a basic principle of social influence, bystanders monitor the reactions of other people in an emergency situation to see if others think that it is necessary to intervene. Since everyone is doing exactly the same thing (nothing), they all conclude from the inaction of others that help is not needed. This is an example of pluralistic ignorance or social proof.
The importance of interpretations in determining the likelihood of bystander intervention is also illustrated in Latane and Darley’s smoke-filled room experiment. Even once those students in groups had clearly noticed the smoke, or it had become so thick that it was obscuring their vision, irritating their eyes or causing them to cough, they were still unlikely to report it. Only one participant in the group condition reported the smoke within the first four minutes, and by the end of the experiment, in five of the eight groups no-one had reported the smoke at all. In the groups that did not report the smoke, the interpretations of its cause, and the likelihood that it was genuinely threatening was also less serious, with no-one suggesting fire as a possible cause, preferring less serious explanations such as air-conditioning leaks. 
Similarly, interpretations of the context played an important role in people's reactions to a man and woman fighting in the street. When the woman yelled, "Get away from me; I don't know you," bystanders intervened 65 percent of the time, but only 19 percent of the time when the woman yelled "Get away from me; I don't know why I ever married you".
General bystander effect research was mainly conducted in the context of non-dangerous, non-violent emergencies. A 2006 study tested bystander effect in emergency situations to see if they would get the same results from other studies testing non-emergenices. It turns out that they did not. In situations with low potential danger, more help was given in the solitary condition than in the bystander condition. However, in situations with high potential danger, participants confronted with an emergency alone or in the presence of another bystander were similarly likely to help the victim.  On the basis of Darley and Latane’s initial research this suggests that in situations of greater seriousness it is more likely that people will interpret the situation as one in which help is needed so that they will be more likely to intervene.
The other major obstacle to intervention is known as diffusion of responsibility. This occurs when observers all assume that someone else is going to intervene and so each individual feels less responsible and refrains from doing anything. People may also fail to take responsibility for a situation and therefore intervene for more complex reasons depending on the context. They may assume that other bystanders are more qualified to help, such as doctors or police officers, and that their intervention would be unneeded. They may also be afraid of being superseded by a superior helper, offering unwanted assistance, or facing the legal consequences of offering inferior and possibly dangerous assistance. An example is the limitation of California's Good Samaritan Law, limiting liability for those attempting to provide medical services as opposed to non-medical (extraction from automobile) services. People may also experience evaluation apprehension and fear losing face in front of the other bystanders.
In one study the effects of masculinity and the bystander effect were studied. Subjects participated in a simulated group discussion via headphones. One member of the group apparently had a choking fit and called for help. "Highly masculine" subjects were less likely to take action to help the victim than were other subjects. Femininity and actual gender had no effect on likelihood of helping. Results are interpreted according to past research evidence that "highly masculine" subjects fear potential embarrassment and loss of poise, so they may be reluctant to intervene in emergencies.
In one study done by Abraham S. Ross, the effects of increased responsibility on bystander intervention were studied by increasing the presence of children. This study was based on the reaction of 36 male undergraduates presented with emergency situations. The prediction was that the intervention would be at its peak due to presence of children around those 36 male undergraduates participants. This was experimented and showed that the prediction was not supported and was concluded as "the type of study did not result in significant differences in intervention." 
The importance of taking responsibility explains why behaviours consistent with the bystander effect can be observed in situations even outside a typical crowd situation, such as a series of passers-by in a public place. Even though they may be alone at the time, the knowledge of others who are likely to pass by also can negate responsibility.
A 2011 meta-analysis of studies of the bystander effect  reported that "The bystander effect was attenuated when situations were perceived as dangerous (compared with non-dangerous), perpetrators were present (compared with non-present), and the costs of intervention were physical (compared with non-physical). This pattern of findings is consistent with the arousal-cost-reward model, which proposes that dangerous emergencies are recognized faster and more clearly as real emergencies, thereby inducing higher levels of arousal and hence more helping." They also "identified situations where bystanders provide welcome physical support for the potentially intervening individual and thus reduce the bystander effect, such as when the bystanders were exclusively male, when they were naive rather than passive confederates or only virtually present persons, and when the bystanders were not strangers."
An alternative explanation has been proposed by Stanley Milgram, who hypothesized that the bystanders′ callous behavior was caused by the strategies they had adopted in daily life to cope with information overload. This idea has been supported to varying degrees by empirical research.
Timothy Hart and Ternace Miethe used data from the National Crime Victimization Survey (NCVS) and found that a bystander was present in 65 percent of the violent victimizations in the data. Their presence was most common in cases of physical assaults (68%), which accounted for the majority of these violent victimizations and less likely in robberies (49%) and sexual assaults (28%). The actions of bystanders were most frequently judged by victims as "neither helping nor hurting" (48%), followed by "helping" (37%), "hurting" (10%), and "both helping and hurting" (3%). Half of the attacks that a bystander was present at occurred in the evening where the victim and bystander were strangers.
Based on general altruism research which suggests that helping behaviour is more likely where there is similarity between the helper and the person being helped, recent research has considered the role of similarity, and more specifically, shared group membership, in encouraging bystander intervention. In a 2005 experiment, researchers found that passers-by were more likely to help an apparently injured person if that person was wearing a football jersey which supported the same team as them than an opposing team. However, when their shared identity as football fans was made salient instead, supporters of both teams were likely to be helped, significantly more so than a person wearing a plain shirt. 
In 2008 a study by Mark Levine and Simon Crowther found that increasing group size inhibited intervention in a street violence scenario when bystanders were strangers but encouraged intervention when bystanders were friends. They also found that when gender identity is salient group size encouraged intervention when bystanders and victim shared social category membership. In addition, group size interacted with context-specific norms that both inhibit and encourage helping. The bystander effect is not a generic consequence of increasing group size. When bystanders share group-level psychological relationships, group size can encourage as well as inhibit helping.
These, and many other findings, which suggest that helping behaviour in general, and specifically bystander intervention is more likely in the context of shared group membership can be explained in terms of self-categorization and empathy. From the perspective of self-categorization theory, a person’s own social identity, well-being is tied to their group membership so that when a group based identity is salient, the suffering of one group member can be considered to directly affect another group member. This shared identity, referred to as self-other merging, is able to form the basis of empathy, which has been found to predict helping behaviour. For example, in a study relating to helping after eviction where both empathy, social identification and helping behaviour were measured, both social identification and empathy were found to predict helping, however, when social identification was controlled, empathy no longer predicted helping behaviour.
A 2009 study published by International Ombudsman Association in the Journal of the International Ombudsman Association suggests that—in reality—there are dozens of reasons why people do not act on the spot or come forward in the workplace when they see behavior they consider unacceptable.
The most important reasons cited for not acting were: the fear of loss of important relationships in and out of the workplace, and a fear of "bad consequences." There also were many reasons given by people who did act on the spot or come forward to authorities.
This practitioners' study suggests that the "bystander effect" can be studied and analyzed in a much broader fashion. The broader view includes not just a) what bystanders do in singular emergencies, b) helping strangers in need, when c) there are (or are not) other people around. The reactions of bystanders can also be analyzed a) when the bystanders perceive any of a wide variety of unacceptable behavior over time, b) they are within an organizational context, and c) with people whom they know. The practitioners' study reported many reasons why some bystanders within organizations do not act or report unacceptable behavior. The study also suggests that bystander behavior is, in fact, often helpful, in terms of acting on the spot to help,and reporting unacceptable behavior (and emergencies and people in need.) The ombuds practitioners' study suggests that what bystanders will do in real situations is actually very complex, reflecting views of the context and their managers (and relevant organizational structures if any) and also many personal reasons.
In support of the idea that some bystanders do indeed act responsibly, there is an important 2010 article about a survey of investigators funded by the US National Institutes of Health that shows that informal intervention by peers and bystanders can interrupt or remedy unacceptable scientific behavior. See Gerald Koocher and Patricia Keith-Spiegel's "Peers Nip Misconduct in the Bud" (July 22, 2010) Nature 466, 438-440 How Do Researchers Respond to Perceived Scientific Wrongdoing? Overview, Method and Survey Results.
Many institutions have worked to provide options for bystanders who see behavior they find unacceptable. These options are usually provided through complaint systems - so bystanders have choices about where to go. One option that is particularly helpful is that of an organizational ombudsman, who keeps no records for the employer and is near-absolutely confidential.
Research suggests that good training may be very helpful. (See for example, Gerald Koocher and Patricia Keith Speigel in NATURE Vol 466 22 July 2010: Peers Nip Misconduct in the Bud, and (with Joan Sieber) Responding to Research Wrongdoing: A User Friendly Guide, July 2010.)
Organizations do many kinds of bystander training. The United States Department of the Army is doing bystander training with respect to sexual assault. Some organizations routinely do bystander training with respect to safety issues. Many organizations have been doing bystander training with respect to diversity issues.
The Charter of human rights and freedoms of Quebec makes it mandatory to "come to the aid of anyone whose life is in peril, either personally or calling for aid, unless it involves danger to himself or a third person, or he has another valid reason". It is therefore a legal obligation to assist people in Quebec.
Likewise, the Brazilian Penal Code makes it a crime not to rescue (or call emergency services when appropriate) injured or disabled people and those found under grave and imminent danger, including abandoned children, as long as it is safe to do so. 
Although most research has been conducted on adults, children can be bystanders too. A study conducted by Robert Thornberg in 2007 came up with seven reasons why children do not help when another classmate is in distress. These include: trivialisation, dissociation, embarrassment association, busy working priority, compliance with a competitive norm, audience modelling, and responsibility transfer. In a further study, Thornberg concluded that there are seven stages of moral deliberation as a bystander in bystander situations among the Swedish schoolchildren he observed and interviewed: (a) noticing that something is wrong, i.e., children pay selective attention to their environment, and sometimes they don't tune in on a distressed peer if they're in a hurry or their view is obstructed, (b) interpreting a need for help - sometimes children think others are just playing rather than actually in distress or they display pluralistic ignorance, (c) feeling empathy, i.e., having tuned in on a situation and concluded that help is needed, children might feel sorry for an injured peer, or angry about unwarranted aggression (empathic anger), (d) processing the school's moral frames - Thornberg identified five contextual ingredients influencing children's behavior in bystander situations (the definition of a good students, tribe caring, gender stereotypes, and social-hierarchy-dependent morality), (e) scanning for social status and relations, i.e., students were less likely to intervene if they didn't define themselves as friends of the victim or belonging to the same significant social category as the victim, or if there were high-status students present or involved as aggressors - conversely, lower-status children were more likely to intervene if only a few other low-status children were around, (f) condensing motives for action, such as considering a number of factors such as possible benefits and costs, and (g) acting, i.e., all of the above coalesced into a decision to intervene or not. It is striking how this was less an individual decision than the product of a set of interpersonal and institutional processes. 
The case of Kitty Genovese is often cited as an example of the "bystander effect". It is also the case that originally stimulated social psychological research in this area. On March 13 1964 Genovese, 28 years old, was on her way back to her Queens, New York, apartment from work at 3am when she was stabbed to death by a serial rapist and murderer. According to newspaper accounts, the attack lasted for at least a half an hour during which time Genovese screamed and pleaded for help. The murderer attacked Genovese and stabbed her, then fled the scene after attracting the attention of a neighbor. The killer then returned ten minutes later and finished the assault. Newspaper reports after Genovese's death claimed that 38 witnesses watched the stabbings and failed to intervene or even contact the police until after the attacker fled and Genovese had died. This led to widespread public attention, and many editorials.
According to an article published in American Psychologist in 2007, the original story of Genovese's murder was exaggerated by the media. Specifically, there were not 38 eyewitnesses, the police were contacted at least once during the attack, and many of the bystanders who overheard the attack could not actually see the event. The authors of the article suggest that the story continues to be misrepresented in social psychology textbooks because it functions as a parable and serves as a dramatic example for students.
On June 16, 2008, on a country road outside Turlock, California, friends, family and strangers, including a volunteer fire chief, stood by as Sergio Aguiar methodically stomped his two-year-old son Axel Casian to death, explaining in a calm voice that he "had to get the demons out" of the boy. He stopped at one point to turn on the hazard lights on his truck. No one moved to take the child or attack Aguiar. Witnesses said they were all afraid to intervene because Aguiar "might have something in his pocket", although some people looked for rocks or boards hoping to find something to subdue him. The fire chief's fiancee called 911. Police officer Jerry Ramar arrived by helicopter and told Aguiar to stop. Aguiar gave Ramar the finger and Ramar shot him in the head. Police officers and psychologists later explained that the inaction of the crowd was justified in that "ordinary people aren't going to tackle a psychotic," that they were not "psychologically prepared" to intervene, and that being frozen in indecision and fear is a normal reaction.
In June 2008 49-year-old Esmin Green collapsed in the waiting room of Kings County Hospital Center in Brooklyn after waiting nearly 24 hours for treatment. She was ignored by other people present in the room and two security guards. People tried to help her only after an hour had passed. Ms. Green died. The hospital staff falsified their records to minimize the time she had lain on the floor unaided. Kings County Hospital had been previously cited for unsanitary conditions and patient neglect.
In April 2010 Hugo Alfredo Tale-Yax was stabbed to death in New York City after coming to the aid of a woman who was being attacked by a robber. Yax was on the sidewalk for more than an hour before firefighters arrived. Almost twenty-five people walked by while he lay dying on a sidewalk in Queens, several stared at Yax; one of them took pictures, however none of them helped or called emergency services.
On Memorial Day, 2011, Raymond Zack, of Alameda, California, walked into the waters off of Robert Crown Memorial Beach and stood neck deep in water roughly 150 yards offshore for almost an hour. His foster mother, Dolores Berry, called 9-1-1 and said that he was trying to drown himself. (There are conflicting reports about Zack's intentions.) Firefighters and police responded but did not enter the water. The firefighters called for a United States Coast Guard boat to respond to the scene. According to police reports, Alameda police expected the firefighters to enter the water. Firefighters later said that they did not have current training and certifications to perform land-based water rescue. Dozens of civilians on the beach, and watching from their homes across from the beach, did not enter the water, apparently expecting public safety officers to conduct a rescue. Eventually, Zack collapsed in the water, apparently from hypothermia. Even then, nobody entered the water for several minutes. Finally, a good samaritan entered the water and pulled Zack to shore, after which he died.
In October 2011, a two-year-old girl, Wang Yue, was hit by a small, white van in the city of Foshan, China, then run over by a large truck when she was not moved by bystanders. A total of 18 people ignored her, some going so far as to walk around the blood, and the girl was left for 7 minutes before a recycler, Chen Xianmei, picked up the toddler and called for help. The child died eight days later.