Keith Sawyer, a psychologist at Washington University, puts it succinctly saying, “Decades of research have consistently shown that brainstorming groups think of far fewer ideas than the same number of people who work alone and later pool their ideas.” Yet the myth of brainstorming success persists.
The problem seems to lie in one of the basic precepts of traditional brainstorming: no one can criticize anyone else’s idea. That prevents constructive debate and is the reason the technique does not live up to its promise.
For more on brainstorming, go to the March-April 2014 issues of Research-Technology Management to read the roundup by MaryAnne M. Gobble entitled "Beyond Brainstorming," pp. 60-61.