Productivity loss in brainstorming groups: Toward the solution of a riddle.

We conducted four experiments to investigate free riding, evaluation apprehension, and production blocking as explanations of the difference in brainstorming productivity typically observed between real and nominal groups. In Experiment 1, we manipulated assessment expectations in group and individual brainstorming. Although productivity was higher when subjects worked under personal rather than collective assessment instructions, type of session still had a major impact on brainstorm-ing productivity under conditions that eliminated the temptation to free ride. Experiment 2 demon-strated that inducing evaluation apprehension reduced productivity in individual brainstorming. However, the failure to find an interaction between evaluation apprehension and type of session in Experiment 3 raises doubts about evaluation apprehension as a major explanation of the productiv-ity loss in brainstorming groups. Finally, by manipulating blocking directly, we determined in Exper-iment 4 that production blocking accounted for most of the productivity loss of real brainstorming groups. The processes underlying production blocking are discussed, and a motivational interpreta-tion of blocking is offered. In his influential book, Osborn (1957) suggested brainstorm-ing as a method of group problem solving that considerably in-