Another example: study groups in grad school. We all believed they helped, by providing “collective tutoring” — but we know what we already know, and when someone explains what they already know to someone else, that may help the learner but it is a “waste” of time for the teacher. In other words, putting our keen mental efforts into solving problems, and not wasting time rehashing already-known concepts, or listening to things we already know, or going into mental auto-pilot as the group slowly moves forward — all hinder the path to quick, optimal, best solutions to problems.

In this vein, I think small groups probably arrive at better answers than large groups.