"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."
Could this be related to [Nobel laureate] Daniel Kanheman's work on the Anchoring bias, where [apparently individual] contributions during a brainstorming session overly rely (anchor) on each others' ideas?
Tversky, A. & Kahneman, D. (1974). Judgment under uncertainty: Heuristics and biases
The Medici Effect