“When people gather and discuss in a group, independence of thought and expression can be lost. Maybe one person is a loudmouth who dominates the discussion, or a bully, or a superficially impressive talker, or someone with credentials that cow others into line. In so many ways, a group can get people to abandon independent judgment and buy into errors. When that happens, the mistakes will pile up, not cancel out. This is the root of collective folly, whether it’s Dutch investors in the seventeenth century, who became collectively convinced that a tulip bulb was worth more than a laborer’s annual salary, or American home buyers in 2005, talking themselves into believing that real estate prices could only go up. But loss of independence isn’t inevitable in a group, as JFK’s team showed during the Cuban missile crisis. If forecasters can keep questioning themselves and their teammates, and welcome vigorous debate, the group can become more than the sum of its parts.”