From Frans Johansson’s book The Medici Effect:
Brainstorming [is] used in nearly all of the world’s largeset companies, nonprofits, and government organizations. And the reasons seem obvious… “The average person can think of twice as many ideas when working with a group than when working alone.”… But is it true?
In 1958… psychologists let groups of four people brainstorm about the practical benefits or difficulties that would arise if everyone had an extra thumb on each hand after next year. These people were called “real groups” since they actually brainstormed together. Next, the researchers let “virtual groups” of four people generate ideas around the “thumb problem”, but they had to brainstorm individually, in separate rooms. The researchers combined the answers they received from each [virtual group] individual and eliminated redundancies… They then compared the performance between real groups and virtual groups…
To their surprise, the researchers found that virtual groups, where people brainstormed individually, generated nearly twice as many ideas as the real groups.
The result, it turned out, is not an anomaly. In a [1987 study, researchers] concluded that brainstorming groups have never outperformed virtual groups. Of the 25 reported experiments by psychologists all over the world, real groups have never once been shown to be more productive than virtual groups. In fact, real groups that engage in brainstorming consistently generate about half the number of ideas they would have produced if the group’s individuals had [worked] alone.
In addition, in the studies where the quality of ideas was measured, researchers found that the total number of good ideas was much higher in virtual groups than in real groups.