Cognitive stimulation and interference in groups: Exposure effects in an idea generation task Bernard A. Nijstad, a, * Wolfgang Stroebe, b and Hein F.M. Lodewijkx b a Department of Work and Organizational Psychology, University of Amsterdam, Roetersstraat 15, 1018 WB, Amsterdam, The Netherlands b Utrecht University, Utrecht, The Netherlands Received 5 December 2000; revised 21 January 2002 Abstract The effects of idea sharing on cognitive processes and performance were assessed in an idea exposure paradigm. Participants generated ideas while being exposed to stimulus ideas that were semantically homogeneous or diverse, and were offered in an or- ganized or a random sequence. As compared to a control condition, participants generated more diverse ideas when exposed to ideas from a wide range of semantic categories, and they generated more ideas per category when exposed to many ideas from only a few categories. The semantic organization of ideas was higher when participants were exposed to ideas that were organized in semantic clusters than when participants were exposed to unorganized ideas. Idea exposure had positive effects in general, because it reduced response latencies for category changes. Implications for information processing in groups are discussed. Ó 2002 Elsevier Science (USA). All rights reserved. Groups increasingly perform cognitive tasks, such as problem solving, decision making, inference, and idea generation. Recently, it has been argued that groups, much like individuals, can be conceptualized as infor- mation processors (Hinsz, Tindale, & Vollrath, 1997). Information processing in groups involves activities that occur at the individual as well as the group level. At the individual level, group members must process informa- tion, which leads to individual solutions, preferences, and ideas. At the group level, information processing involves the sharing of these solutions, preferences, and ideas during discussion (Tindale & Kameda, 2000). A key question regarding groups as information processors is how information or ideas suggested by one group member affect the mental processes of other members, and how this relates to group effectiveness. Hinsz et al. (1997) argued that the utterances of others may both stimulate and interfere with the mental pro- cesses of group members, and that this provides ‘‘a new area to address the relative impact of process gains (e.g., stimulated cognitive processes...) and process losses (e.g., interference...) in small-group performance’’ (p. 49). When stimulation is stronger than interference, the result is an assembly bonus effect (Collins & Guetzkow, 1964), in which groups outperform their separate members. In this paper, we will focus on idea generation by groups and consider the question of how ideas sug- gested by one group member can affect the cognitive processes of the other members. Research on group idea generation has a long history in social and organizational psychology (for reviews see Lamm & Trommsdorff, 1973; Stroebe & Diehl, 1994). A very consistent finding in this literature is that a number of people working individually (nominal groups) can produce more ideas and more good ideas than can an equal number of people working in a group (Mullen, Johnson, & Salas, 1991). One important cause for this productivity loss in groups is mutual production blocking (Diehl & Stroebe, 1987). Usually, only one group member speaks at a given moment, so group members must often wait for their turn before they can express their ideas. It has been shown that group members cannot think effectively while waiting for their turns, and that the blocking effect is thus due to cog- nitive interference (Diehl & Stroebe, 1991; Nijstad, 2000). Journal of Experimental Social Psychology 38 (2002) 535–544 www.academicpress.com Journal of Experimental Social Psychology * Corresponding author. Fax: +31-20-639-0531. E-mail address: nijstad@psy.uva.nl (B.A. Nijstad). 0022-1031/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII:S0022-1031(02)00500-0