This is generally false unless the group has a clue what's going on.
You got me. I misrepresented the premise. I should have said "usually," and I did not explain, as the author did, that the groups had to have some understanding of the problem or question at hand. He did not claim that any group will always make better decisions than any single expert. I think I posted before I'd gotten my second cup. It's pretty obvious that market analysis would find polling data useful, but many of the examples cited had to do with predictions regarding concrete, verifiable facts: locating a sunken submarine, or guessing the dressed weight of a steer in a county fair contest. By averaging the best guesses of a number of experts, a point was chosen on a map that proved to be within 450 yards of where the sub was finally located. In the second example, the average of the attendees' guesses was more accurate than the best estimate of any individual expert.
In other words, I guess, more heads are better than one, given a minimal degree of expertise.
He also points out that professional groups often inhibit decisions and undermine innovation. What he termed "group think" or conformity to professional standards is behind this. I suspect that this is true, that education is only partly about knowledge; it also involves indoctrination of conventions and values. I agree with shared values and conventions if they're good and sensible. But it strikes me that a particular kind of values has taken hold.
My experience (in the mathematical and physical sciences) is that what the author terms "group think" doesn't exist. The problem isn't so much inhibiting ideas as in culling out all the bad ones. I have a pretty good reputation for making good mathematical guesses mostly because I don't often mention the 99.44% failures that I have already screened out. On the other hand, in the sciences, one always has the opportuinty of demonstrating that the ideas are good (by submitting them to peer challenges.)
I do find that politicians do seem to get caught in a stereotyped way of thinking. Perhaps it's becaust they have to please the crowd.