Personally, as an introvert, I tend to be more productive in group projects than individual projects, not because group brainstorming provided me with useful ideas (I don't recall this ever happening,) but because being productive in the group project would improve my status among my peers in the group, and being unproductive would lower it.

Pair me up with an unproductive person, and I can produce more than the two of us put together would have produced separately.

On the other hand, if other members of the group are productive enough that they deliberately shift the work burden to themselves, I will tend to become less productive (and accept the consequent status loss,) because I'm not forceful enough to demand "Hey, give me a bigger share of work."