When it comes to guessing answers to simple factual questions (e.g. “What is the temperature of this room?” or “How many jellybeans are in that jar?”), crowds do better than individuals. The averaged answer of a sizable crowd always is nearer to the truth than the typical individual answer. It is a statistical phenomenon. Many of the individual misses are wild, but the extreme ones largely cancel out. Social scientists have been demonstrating this in experiments for the past century.
In 2004 James Surowiecki, in a popular book, The Wisdom of Crowds, went a bit further and argued that crowds produce better decisions than experts on broader questions if there are (1) diverse opinions, (2) independence of participants (so social pressures can’t impose groupthink), (3) decentralization, and (4) a proper method for averaging opinions.
It is tempting to perceive the virtues of democracy in this, and some folks do just that. Yet, even if Surowiecki’s proposal has merit, his crowd is a very artificial one. In real world situations, crowds are seldom wise and his four conditions are rarely met. Anyone who ever has witnessed a first class riot (as I have) has seen a crowd do things far dumber and scarier than the individuals in it ever would do alone. Anyone who ever has bought into an asset bubble (as I have) is feeling a little dumb himself.
Political democracy in particular is not analogous to counting jellybeans. For one thing, elections do not average opinions. One candidate wins and another loses. A ballot proposal succeeds or it doesn’t. Winner takes all. No averaging takes place. There is no statistical nudge toward correctness. Groupthink (or two competing groupthinks) is the norm. The riot rather than the jellybean-count is a more apt model for crowds in politics. Shakespeare isn’t misguided when he depicts the Roman crowd swinging impetuously from support of Brutus to support of Antony.
I'm not arguing against democracy. It has its virtues. Primary among them (if I may crib from Churchill) is simply that it isn't one of the other types of government. Wisdom may be too much to expect from it though.