The goal of this book is to provide, first, a critical discussion of collective irrationality from the perspectives of different research traditions in psychological science and, second, introduce a forward- looking theory of collective irrationality, one that will help stimulate greater attention to this topic on the part of researching and practicing psychologists. Collective irrationality is a neglected topic in mainstream psychology. Individual irrationality has been extensively examined from different research perspectives, most recently through the lens of cognitive neuroscience, but collective irrationality continues to be ignored. This is in large part due to the reductionism of mainstream psychology, dominated as it is by the individualistic culture of the United States. However, the most serious challenges confronting humankind, including global warming and nuclear proliferation, arise out of collective irrationality, which is in a number of important ways different from individual irrationality. Thus, the neglect of collective irrationality represents an important gap that this work will help fill.