In his book 59 Seconds, psychologist Richard Wiseman assembles a number of scientific tests on human behavior. A few of them in his chapter on creativity show that people come up with more and better ideas on their own than in groups.
That has been my experience. But it's nice to see some data behind it. Incidentally, I highly recommend ANY book by Wiseman — especially Quirkology, my favorite from him.
—Steve Cuno