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When people are not accountable and cannot evaluate their own efforts, responsibility is diffused across all group members (Harkins & Jackson, 1985; Kerr & Bruun, 1981). By contrast, the social facilitation experiments increased exposure to evaluation. When made the center of attention, people self-consciously monitor their behavior (Mullen & Baumeister, 1987). So, when being observed increases evaluation concerns, social facilitation occurs; when being lost in a crowd decreases evaluation concerns, social loafing occurs.
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David G. Myers (Social Psychology)