Collective irrationality is shaped by psychological processes and plays an enormously important role in human societies, but is almost completely neglected by psychologists. The objective of this brief volume is to explore the nature of collective irrationality from a psychological perspective and also to put forward a new theory of collective irrationality, which will help stimulate further research, as well as work on practical solutions. The discussion takes place through a “societies to cells” rather than a reductionist “cells to societies” perspective. Collective irrationality is different from the sum of individual irrationalities. We are in an age of rising collective irrationality, influenced by authoritarian strongmen in both Western and non-Western societies.