Crowd psychology, also known as mob psychology, is a branch of social psychology. Social psychologists have developed several theories for explaining the ways in which the psychology of a crowd differs from and interacts with that of the individuals within it. Major theorists in crowd psychology include Gustave Le Bon, Gabriel Tarde, Sigmund Freud, and Steve Reicher. This field relates to the behaviors and thought processes of both the individual crowd members and the crowd as an entity. Crowd behavior is heavily influenced by the loss of responsibility of the individual and the impression of universality of behavior, both of which increase with the size of the crowd.
The psychological study of crowd phenomena was documented decades prior to 1900 as European culture was imbued with thoughts of the fin de siècle. This "modern" urban culture perceived that they were living in a new and different age. They witnessed marvelous new inventions and experienced life in new ways. The population, now living in densely packed, industrialized cities, such as Milan and Paris, witnessed the development of the light bulb, radio, photography, moving-picture shows, the telegraph, the bicycle, the telephone, and the railroad system. They experienced a faster pace of life and viewed human life as segmented, so they designated each of these phases of life with a new name. They created new concepts like "the Adolescent," "Kindergarten," "the Vacation," "camping in Nature," "the 5-minute segment," and "Travel for the sake of pleasure" as a leisure class to describe these new ways of life.
Likewise, the abstract concept of "the Crowd" grew as a new phenomenon simultaneously in Paris, France, and Milan, the largest city in the Kingdom of Italy. Legal reformers motivated by Darwin's evolutionary theory, particularly in the Kingdom of Italy, argued that the social and legal systems of Europe had been founded on antiquated notions of natural reason, or Christian morality, and ignored the irrevocable biology laws of human nature. Their goal was to bring social laws into harmony with biological laws. In pursuit of this goal, they developed the social science of criminal anthropology, which is tasked with the mission of changing the emphasis from one of the study of legal procedures to one of studying the criminal.
"Criminal anthropology," writes Signor Sergi, "studies the delinquent in his natural place, that is to say, in the field of biology and pathology". The Italian Cesare Lombroso, professor of forensic medicine and hygiene in Turin, greatly advanced their agenda in 1878, when he published L'uomo delinquente, a highly influential book which went through five editions. The book, published in English in 1900 under the title "Criminal Man," solidified the links between social evolutionary theories and the fear of crowds with its concept of the "born" criminal as the savage in the midst of civilized society. The book influenced both European and American legal experts interested in assigning responsibility to individuals engaged in dubious behavior while engaged within a crowd.
The first debate in crowd psychology began in Rome at the first International Congress of Criminal Anthropology on 16 November 1885. The meeting was dominated by Cesare Lombroso and his fellow Italians who emphasized the biological determinates.
- "Lombroso detailed before the first congress his theories of the physical anomalies of criminals and his classification of criminals as 'born criminals', or criminals by occasion and mattoids. Ferri expressed his view of crime as degeneration more profound than insanity, for in most insane persons the primitive moral sense has survived the wreck of their intelligence. Along similar lines were the remarks of Benedickt, Sergi and Marro."
A weak response was offered by the French, who put forward an environmental theory of human psychology.
- "M. Anguilli called attention to the importance of the influence of the social environment upon crime. Professor Alexandre Lacassagne thought that the atavistic and degenerative theories as held by the Italian school were exaggerations and false interpretations of the facts, and that the important factor was the social environment."
In Paris during 10–17 August 1889, the Italian school received a stronger rebuke of their biological theories during the 2nd International Congress of Criminal Anthropology. A radical divergence in the views between the Italian and the French schools was reflected in the proceedings.
- "Professor Lombroso laid stress upon epilepsy in connection with his theory of the 'born criminal.' Professor Léonce Pierre Manouvrier characterized Lombroso's theory as nothing but the exploded science of phrenology. The anomalies observed by Lombroso were met with in honest men as well as criminals, Manouvrier claimed, and there is no physical difference between them. Baron Raffaele Garofalo, Drill, Alexandre Lacassagne and Benedikt opposed Lombroso's theories in whole or in part. Pugliese found the cause of crime in the failure of the criminal to adapt himself to his social surroundings, and Benedikt, with whom Tarde agreed, held that physical defects were not marks of the criminal qua criminal." It is in this context that you have a debate between Scipio Sighele, an Italian lawyer and Gabriel Tarde, a French magistrate on how to determine criminal responsibility in the crowd and hence who to arrest. (Sighele, 1892; Tarde, 1890, 1892, 1901) 
Literature on crowds and crowd behavior appeared as early as 1841, with the publication of Charles Mackay's book Extraordinary Popular Delusions and the Madness of Crowds. The attitude towards crowds underwent an adjustment with the publication of Hippolyte Taine's six-volume tome The Origins of Contemporary France (1875). In particular Taine's work helped to change the opinions of his contemporaries on the actions taken by the crowds during the 1789 Revolution. Many Europeans held him in great esteem. While it is difficult to directly link his works to crowd behavior, it may be said that his thoughts stimulated further study of crowd behavior. However, it was not until the latter half of the 19th century that scientific interest in the field gained momentum. French physician and anthropologist Gustave Le Bon became its most-influential theorist.
Types of crowds
There is limited research into the types of crowd and crowd membership and there is no consensus as to the classification of types of crowds. Two recent scholars, Momboisse (1967) and Berlonghi (1995) focused upon purpose of existence to differentiate among crowds. Momboisse developed a system of four types: casual, conventional, expressive, and aggressive. Berlonghi classified crowds as spectator, demonstrator, or escaping, to correlate to the purpose for gathering.
Another approach to classifying crowds is sociologist Herbert Blumer's system of emotional intensity. He distinguishes four types of crowds: casual, conventional, expressive, and acting. His system is dynamic in nature. That is, a crowd changes its level of emotional intensity over time, and therefore, can be classed in any one of the four types.
Generally, researchers in crowd psychology have focused on the negative aspects of crowds, but not all crowds are volatile or negative in nature. For example, in the beginning of the socialist movement crowds were asked to put on their Sunday dress and march silently down the street. A more-modern example involves the sit-ins during the Civil Rights Movement. Crowds can reflect and challenge the held ideologies of their sociocultural environment. They can also serve integrative social functions, creating temporary communities.
Crowds can be active (mobs) or passive (audiences). Active crowds can be further divided into aggressive, escapist, acquisitive, or expressive mobs. Aggressive mobs are often violent and outwardly focused. Examples are football riots and the L.A. Riots of 1992. Escapist mobs are characterized by a large number of panicked people trying to get out of a dangerous situation. These types of mobs are why it is illegal to yell, "Fire!" in a crowded theater. Acquisitive mobs occur when large numbers of people are fighting for limited resources, such as the crowds who looted after Hurricane Katrina in 2005. An expressive mob is any other large group of people gathering for an active purpose. Civil disobedience, rock concerts, and religious revivals all fall under this category.
Gustave Le Bon
Le Bon held that crowds existed in three stages: submergence, contagion, and suggestion. During submergence, the individuals in the crowd lose their sense of individual self and personal responsibility. This is quite heavily induced by the anonymity of the crowd. Contagion refers to the propensity for individuals in a crowd to unquestioningly follow the predominant ideas and emotions of the crowd. In Le Bon's view, this effect is capable of spreading between "submerged" individuals much like a disease. Suggestion refers to the period in which the ideas and emotions of the crowd are primarily drawn from a shared racial unconscious. This behavior comes from an archaic shared unconscious and is therefore uncivilized in nature. It is limited by the moral and cognitive abilities of the least capable members. Le Bon believed that crowds could be a powerful force only for destruction. Additionally, Le Bon and others have indicated that crowd members feel a lessened sense of legal culpability, due to the difficulty in prosecuting individual members of a mob.
Le Bon's idea that crowds foster anonymity and generate emotion has been contested by some critics. Clark McPhail points out studies which show that "the madding crowd" does not take on a life of its own, apart from the thoughts and intentions of members. Norris Johnson, after investigating a panic at a 1979 The Who concert concluded that the crowd was composed of many small groups of people mostly trying to help each other. Additionally, Le Bon's theory ignores the socio-cultural context of the crowd, which some theorists argue can disempower social change. R. Brown disputes the assumption that crowds are homogenous, suggesting instead that participants exist on a continuum, differing in their ability to deviate from social norms.
Sigmund Freud's crowd behavior theory primarily consists of the idea that becoming a member of a crowd serves to unlock the unconscious mind. This occurs because the super-ego, or moral center of consciousness, is displaced by the larger crowd, to be replaced by a charismatic crowd leader. McDougall argues similarly to Freud, saying that simplistic emotions are widespread, and complex emotions are rarer. In a crowd, the overall shared emotional experience reverts to the least common denominator (LCD), leading to primitive levels of emotional expression. This organizational structure is that of the "primal horde" – pre-civilized society - and Freud states that one must rebel against the leader (re-instate the individual morality) in order to escape from it. Moscovici expanded on this idea, discussing how dictators such as Mao Zedong and Joseph Stalin have used mass psychology to place themselves in this "horde leader" position 
Theodor Adorno criticized the belief in a spontaneity of the masses: according to him, the masses were an artificial product of "administrated" modern life. The Ego of the bourgeois subject dissolved itself, giving way to the Id and the "de-psychologized" subject. Furthermore, Adorno stated the bond linking the masses to the leader through the spectacle is feigned:
"When the leaders become conscious of mass psychology and take it into their own hands, it ceases to exist in a certain sense. ... Just as little as people believe in the depth of their hearts that the Jews are the devil, do they completely believe in their leader. They do not really identify themselves with him but act this identification, perform their own enthusiasm, and thus participate in their leader's performance. ... It is probably the suspicion of this fictitiousness of their own 'group psychology' which makes fascist crowds so merciless and unapproachable. If they would stop to reason for a second, the whole performance would go to pieces, and they would be left to panic."
Deindividuation theory argues that in typical crowd situations, factors such as anonymity, group unity, and arousal can weaken personal controls (e.g. guilt, shame, self-evaluating behavior) by distancing people from their personal identities and reducing their concern for social evaluation. This lack of restraint increases individual sensitivity to the environment and lessens rational forethought, which can lead to antisocial behavior. More recent theories have stated that deindividuation hinges upon a person being unable, due to situation, to have strong awareness of their self as an object of attention. This lack of attention frees the individual from the necessity of normal social behavior.
American social psychologist Leon Festinger and colleagues first elaborated the concept of deindividuation in 1952. It was further refined by American psychologist Philip Zimbardo, who detailed why mental input and output became blurred by such factors as anonymity, lack of social constraints, and sensory overload. Zimbardo's famous Stanford Prison Experiment is a strong argument for the power of deindividuation. Further experimentation has had mixed results when it comes to aggressive behaviors, and has instead shown that the normative expectations surrounding the situations of deindividuation influence behavior (i.e. if one is deindividuated as a KKK member, aggression increases, but if it is as a nurse, aggression does not increase).
A further distinction has been proposed between public and private deindividuation. When private aspects of self are weakened, one becomes more subject to crowd impulses, but not necessarily in a negative way. It is when one no longer attends to the public reaction and judgement of individual behavior that antisocial behavior is elicited.
Convergence theory holds that crowd behavior is not a product of the crowd, but rather the crowd is a product of the coming together of like-minded individuals. Floyd Allport argued that "An individual in a crowd behaves just as he would behave alone, only more so." Convergence theory holds that crowds form from people of similar dispositions, whose actions are then reinforced and intensified by the crowd.
Convergence theory claims that crowd behavior is not irrational; rather, people in crowds express existing beliefs and values so that the mob reaction is the rational product of widespread popular feeling. However, this theory is questioned by certain research which found that people involved in the 1970s riots were less likely than nonparticipant peers to have previous convictions.
Critics of this theory report that it still excludes the social determination of self and action, in that it argues that all actions of the crowd are born from the individuals' intents.
Emergent norm theory
Ralph Turner and Lewis Killian put forth the idea that norms emerge from within the crowd. Emergent norm theory states that crowds have little unity at their outset, but during a period of milling about, key members suggest appropriate actions, and following members fall in line, forming the basis for the crowd's norms.
Key members are identified through distinctive personalities or behaviors. These garner attention, and the lack of negative response elicited from the crowd as a whole stands as tacit agreement to their legitimacy. The followers form the majority of the mob, as people tend to be creatures of conformity who are heavily influenced by the opinions of others. This has been shown in the conformity studies conducted by Sherif and Asch. Crowd members are further convinced by the universality phenomenon, described by Allport as the persuasive tendency of the idea that if everyone in the mob is acting in such-and-such a way, then it cannot be wrong.
Emergent norm theory allows for both positive and negative mob types, as the distinctive characteristics and behaviors of key figures can be positive or negative in nature. An antisocial leader can incite violent action, but an influential voice of non-violence in a crowd can lead to a mass sit-in.
A major criticism of this theory is that the formation and following of new norms indicates a level of self-awareness that is often missing in the individuals in crowds (as evidenced by the study of deindividuation). Another criticism is that the idea of emergent norms fails to take into account the presence of existent sociocultural norms. Additionally, the theory fails to explain why certain suggestions or individuals rise to normative status while others do not.
Social identity theory
Social identity theory posits that the self is a complex system made up primarily of the concept of membership or non-membership in various social groups. These groups have various moral and behavioral values and norms, and the individual's actions depend on which group membership (or non-membership) is most personally salient at the time of action. This influence is evidenced by findings that when the stated purpose and values of a group changes, the values and motives of its members are shown to also change. Crowds are an amalgam of individuals, all of whom belong to various conflicting groups. However, if the crowd is primarily related to some identifiable group (such as Christians or civil-rights activists), then the values of that group will dictate crowd action. In crowds which are more ambiguous, individuals will assume a new social identity as a member of the crowd. This group membership is made more salient by confrontation with other groups, a relatively common occurrence for crowds.
The group identity serves to create a set of standards for behavior; for certain groups violence is legitimate, for others it is unacceptable. This standard is formed from stated values, but also from the actions of others in the crowd, and sometimes from a few in leadership-type positions.
A concern with this theory is that while it explains how crowds reflect social ideas and prevailing attitudes, it does not explain the mechanisms by which crowds enact social change.
- Bystander effect
- Class consciousness
- Collective behavior
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- Conformity (psychology)
- Crowd manipulation
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- The Wisdom of Crowds
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- Understanding crowd behaviours, gov.uk