Found on smartex, and believed by jibbajabba to occasionally be true of group brainstorming...
Wikipedia on "Brainstorming".
Although brainstorming has become a popular group technique, researchers have generally failed to find evidence of its effectiveness for enhancing either quantity or quality of ideas generated. Because of such problems as distraction, social loafing, evaluation apprehension, and production blocking, brainstorming groups are little more effective than other types of groups, and they are actually less effective than individuals working independently. In the Encyclopedia of creativity, Tudor Rickards provides the article on brainstorming, summarizing the controversies. He also indicates the dangers of conflating productivity in group work with quantity of ideas.
2. Nijstad, B. A., Stroebe, W., Lodewijkx, H. F. M. (2003). Production blocking and idea generation: Does blocking interfere with cognitive processes? Journal of Experimental Social Psychology, 39, 531-548.
3. Diehl, M., & Stroebe, W. (1991). Productivity loss in idea-generating groups: tracking down the blocking effect. Journal of Personality and Social Psychology, 61, 392-403.
4. Mullen, B., Johnson, C., & Salas, E. (1991). Productivity loss in brainstorming groups: a meta-analytic integration. Basic and Applied Social Psychology. 12, 3-23.
5. Rickards, T., (1999) Brainstorming, M Runco & S Pritzker, Eds, Encylopedia of creativity, San Diego: Academic Press Vol 1 219-228.