Adrian Furnham, 50 psychology ideas your really need to know (Quercus 2008 at 118, discusses brainstorming and concludes that “people working alone on a creative project produce better and more answers than a brainstorming group.” Any group, in fact, could suffer from these debilitations. He offers three explanations, and I quote his terms.
(1) Because of “evaluation bias,” people become self-conscious in groups and clam up even if they have good ideas.
(2) “Social loafing” allows some people in a group to ride along on the coat-tails of the more energetic members of the group.
(3) “Production blocking’ allows people to say that they cannot think clearly with all the hubbub, or perhaps it is true.
Furnham also cites research that shows “group polarization.” We may believe that group decisions lead to moderate positions and less extreme decisions, but he explains two forces that militate against such outcomes. Group members compare themselves to others in the group and harden in their attitudes. Secondly, a very persuasive or confident member can sway the others toward a more extreme position.