Most research experiments on brainstorming involved searching for solutions to ridiculously simple or very complex problems — and those are just the types of situations in which brainstorming loses out to solo thinking. In dealing with highly complex problems, brainstorming can have a stifling effect, dangerously limiting the number of proposals that get serious consideration. You've probably experienced this yourself: Your glimmer of an idea quickly fades as other group members talk about their ideas.
But by breaking down the ideation process into its factors and modeling it mathematically, Kavadias and Sommer found that when the target problem is a "moderately" complex issue that involves multiple organizational functions, brainstorming is more effective than solitary musing. In those situations, "brainstorming groups can exploit the competence diversity of their members," and participants can build on one another's suggestions — just as Mike and Bill point out.