Richard Wiseman says that group brainstorming is a myth. It’s a bold thing to say since every other company uses it for almost anything, but he has got his statistics and research from the psychologists which say otherwise.
He says (and I found it true for at least myself) that when we are in a group most of us stay in a dormant state. We only speak (most of us) when we no longer could stay silent. So if the motive of the group brainstorming is to collect ideas without criticizing anybody else, we end up accepting ideas from only a few people in the group, people who are outspoken(who may not be as creative as a shy guy in the room who is the most creative of them all).
Dr. Richard suggests that we should not brainstorm with too many people in the room, 3-4 people are enough and by reducing the number to the minimum we can get maximum participation from all the candidates which lead to a greater probability of coming up with creative ideas.