Deindividuation is a concept in social psychology that is generally thought of as the loss of self-awareness in groups, although this is a matter of contention (see below). For the social psychologist, the level of analysis is the individual in the context of a social situation. As such, social psychologists emphasize the role of internal psychological processes. Other social scientists, such as sociologists, are more concerned with broad social, economic, political, and historical factors that influence events in a given society.

Theories of deindividuation propose that it is a psychological state of decreased self-evaluation and decreased evaluation apprehension causing antinormative and disinhibited behavior. Deindividuation theory seeks to provide an explanation for a variety of antinormative collective behavior, such as violent crowds, lynch mobs, etc. Deindividuation theory has also been applied to genocide and been posited as an explanation for antinormative behavior online and in computer-mediated communications.

Although generally analyzed in the context of negative behaviors, such as mob violence and genocide, deindividuation has also been found to play a role in positive behaviors and experiences. There still exists some variation as to understanding the role of deindividuation in producing anti-normative behaviors, as well as understanding how contextual cues affect the rules of the deindividuation construct. Deindividuation is losing the sense of self in a group.^{[clarification needed]}

In contemporary social psychology, deindividuation refers to a diminishing of one's sense of individuality that occurs with behavior disjointed from personal or social standards of conduct. For example, someone who is an anonymous member of a mob will be more likely to act violently toward a police officer than a known individual. In one sense, a deindividuated state may be considered appealing if someone is affected such that he or she feels free to behave impulsively without mind to potential consequences.^{[clarification needed]} However, deindividuation has also been linked to "violent and anti-social behavior."

Gustave Le Bon was an early explorer of this phenomenon as a function of crowds. Le Bon introduced his crowd psychology theory in his 1895 publication The Crowd: A study of the Popular Mind. The French psychologist characterized his posited effect of crowd mentality, whereby individual personalities become dominated by the collective mindset of the crowd. Le Bon viewed crowd behavior as "unanimous, emotional, and intellectually weak." He theorized that a loss of personal responsibility in crowds leads to an inclination to behave primitively and hedonistically by the entire group. This resulting mentality, according to Le Bon, belongs more to the collective than any individual, so that individual traits are submerged. Already, Le Bon was tending toward the conception of deindividuation as a state brought on by a lowering of accountability, resulting from a degree of anonymity due to membership within a crowd, where attention is shifted from the self to the more stimulating, external qualities of the group's action (which may be extreme).

Essentially, individuals of Le Bon's crowd are enslaved to the group's mindset and are capable of conducting the most violent and heroic acts. Le Bon's group-level explanation of behavioral phenomena in crowds inspired further theories regarding collective psychology from Freud, McDougall, Blumer, and Allport. Festinger, Pepitone, and Newcomb revisited Le Bon's ideas in 1952, coining the term deindividuation to describe what happens when persons within a group are not treated as individuals. According to these theorists, whatever attracts each member to a particular group causes them to put more emphasis on the group than on individuals. This unaccountability inside a group has the effect of "reducing inner restraints and increasing behavior that is usually inhibited." Festinger et al. agreed with Le Bon's perception of behavior in a crowd in the sense that they believed individuals do become submerged into the crowd leading to their reduced accountability. However, these relatively modern theorists distinguished deindividuation from crowd theory by reforming the idea that the loss of individuality within a crowd is replaced by the group's mindset. Instead, Festinger et al. argued that the loss of individuality leads to loss of control over internal or moral constraints.

Alternatively, R. C. Ziller (1964) argued that individuals are subject to deindividuation under more specific situational conditions. For instance, he suggested that under rewarding conditions, individuals have the learned incentive to exhibit individualized qualities in order to absorb credit for themselves; whereas, under punishing conditions, individuals have the learned tendency to become deindividuated through submergence into the group as a means of diffusing responsibility.

P. G. Zimbardo (1969) suggested "the expression of normally inhibited behavior" may have both positive and negative consequences. He expanded the proposed realm of factors that contribute to deindividuation, beyond anonymity and loss of personal responsibility, to include: "arousal, sensory overload, a lack of contextual structure or predictability, and altered consciousness due to drugs or alcohol", as well as "altered time perspectives... and degree of involvement in group functioning." Zimbardo postulated that these factors lead to "loss of identity or loss of self-consciousness", which result in unresponsiveness to external stimuli by the individual and the loss of "cognitive control over motivations and emotions." Consequently, individuals reduce their compliance with good and bad sanctions held by influences outside the group.