From Tyler Cowen's book: Discover Your Inner Economist: Use Incentives to Fall in Love, Survive Your Next Meeting, and Motivate Your Dentist:
Brainstorming sessions are a counterproductive way of spending time. People usually think of more new ideas on their own than they do in a group. Psychologists refer to “the illusion of group productivity.” One meta-study found that people were less productive in groups in 18 of 22 comparisons. Furthermore, the larger the group, the greater the loss of productivity. We all know that many people rely too much on the work of others and become “free riders.” Yet surprisingly, we still tend to think highly of group discussions. More than 80 percent of people polled think that group brainstorming produces better results than does individual brainstorming.
Self-deception is one culprit for this failure of perception. When we are in groups, someone else is usually talking. We feel less pressure. We don't feel stupid just because we are silent or devoid of new ideas. Rather the sense of continuous activity gives us the feeling of being engaged in collective discovery. If I am experiencing no revelation, well, maybe someone else is. After all, something good must be happening; why else would we all be gathered in this room? We do like being on teams, especially winning teams.
Many people, after working in groups, mistake other people's ideas for their own. After the meeting they feel better. Furthermore, if the problem is hard, everyone can see that everyone else found it hard too; this makes us all feel better.
In contrast, when working alone, it is painfully obvious when no progress is forthcoming. The voice of your Inner Economist is not drowned out. You can't deny you aren't being productive, and you have no excuse.
When do brainstorming groups work the worst?
… when group members are asked to wait their turn before speaking up. When people are waiting they tend to rehearse their prepared speech in their minds. They oscillate between nervousness and imagining how they will stun the group with their brilliance. Most people will not start a search for new ideas. Many people, due to cognitive limitations, are unable to search for new ideas when they are waiting. The discussion becomes static.
Still Curious? Read Why Brainstorming Doesn't Work