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Abstract'I gratefully acknowledge Willem Dose's comments and Denise Jodelet's insightful discussion of this paper as a whole and detailed points of it.

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Comparisons of opinion and judgment ratings of experimental subjects in individual and collective situations were made. A total of 140 male secondary school students comprised the sample. Group discussions to consensus resulted in statistically significant shifts toward the extremes of the scales. This polarization effect also characterized subjects' postconsensus individual ratings. These results challenged two widely held assumptions: (a) that group judgments are less extreme than individual judgments, and (b) that the "risky shift" phenomenon is a content-bound exception to the averaging tendency of the group. A reinterpretation of available data suggests that a normative commitment may be the underlying variable responsible for polarization effects.

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Research has shown that people prefer supporting to conflicting information when making decisions. Whether this biased information search also occurs in group decision making was examined in three experiments. Experiment 1 indicated that groups as well as individuals prefer supporting information and that the strength of this bias depends on the distribution of the group members' initial decision preferences. The more group members had chosen the same alternative prior to the group discussion (group homogeneity), the more strongly the group preferred information supporting that alternative. Experiment 2 replicated these results with managers. Experiment 3 showed that the differences between homogeneous and heterogeneous groups reflect group-level processes. Higher commitment and confidence in homogeneous groups mediated this effect. Functional and dysfunctional aspects of biased information seeking in group decision making are discussed.

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This study presents the outline of a model for collective phenomena. A symmetrybreaking model combines a number of well-established social psychology hypotheses PRELIMINARY REMARKSThe finest and most distinctive aspect of our sciences concerns the study of collective phenomena. Yet, in both social and natural sciences, this aspect is most difficult and certainly rather recent. When facing this difficulty, the tendency is either to go back to the individual or to treat groups as collective individuals. Our purpose here is to carry further the study of these phenomena to fill the gap between individual and social descriptions. We will proceed combining established psychological concepts with those from contemporary statistical physics (Haken, 1977; Reif, 1965;