Deindividuation theory is a social psychological account of the individual in the crowd. Deindividuation is a psychological state of decreased self-evaluation, causing anti-normative and disinhibited behavior. The impact of deindividuation theory in science and society (especially 20th century politics) make it one of social science’s more influential contributions. Deindividuation theory is rooted in some of the earliest social psychological theorizing, more than a century ago. It seeks to explain the apparent transformation of rational individuals into an unruly group or crowd. It posits that the group provides an environment in which the individual —submerged and anonymous — suffers from a loss of self-awareness (Zimbardo, 1969). Deindividuation hinders reflection about the consequences of actions, rendering social norms impotent while increasing suggestibility to random outside influences. The theory has been invoked to account for a range of phenomena such as collective behavior, behavior in online groups and in CMC, and the results of the classic Stanford Prison Experience. Despite its status and impact, empirical support for deindividuation theory is minimal. Recently, this lack of support has been attributed to the faulty assumption that crowds cause a loss of self. Instead, it has been proposed that deindividuation marks a transition from individual identity to social dimensions of the self. This transition to a social identity may increase responsiveness to social norms particular to the crowd, instead of decreasing responsiveness to generic social norms, as suggested by deindividuation theory.