William J Bernstein: Delusional crowds

“What separates delusional crowds from wise ones is the extent of their members’ interactions with each other…. The more a group interacts, the more it behaves like a real crowd, and the less accurate its assessments become. Occasionally, crowd interaction becomes so intense that madness results. As put most succinctly by Friedrich Nietzsche, ‘Madness is rare in the individual—but with groups, parties, peoples, and ages it is the rule.’ Mackay also recognized this; perhaps the most famous line in Extraordinary Popular Delusions is ‘Men, it is said, think in herds; it will be seen that they go mad in herds, while they only recover their senses more slowly, and one by one.’” —William J. Bernstein (“The Delusions Of Crowds”).