This paper discusses social psychological processes in computer-mediated communication (CMC) and group decision-making, in relation to findings that groups communicating via computer produce more polarized decisions than face-to-face groups. A wide range of possible explanations for such differences have been advanced, in which a lack of social cues, disinhibition, "de-individuation" and a consequent tendency to antinormative behaviour are central themes. In these explanations, both disinhibition and greater equality of participation are thought to facilitate the exchange of extreme persuasive arguments, resulting in polarization. These accounts are briefly reviewed and attention is drawn to various problematic issues. We provide an alternative model and explanation based on social identity (SI) theory and a re-conceptualization of de-individuation, which takes into account the social and normative factors associated with group polarization. Predictions from both sets of explanations are explored empirically by means of an experiment manipulating the salience of the discussion group, and de-individuation operationalized as the isolation and anonymity of the participants. In this experiment we were able to partial out the effects of the CMC technology which have confounded comparisons with face-to-face interaction in previous research. The results challenge the explanations based on persuasive arguments, while being consistent with our SI model. We discuss our approach in relation to other very recent research in group computer-mediated communication and offer a reinterpretation of previous findings. © 1991.