The positive case for diversity, as we’ve seen, is that it expands a group’s set of possible solutions and allows the group to conceptualize problems in novel ways. The negative case for diversity is that diversity makes it easier for a group to make decisions based on facts, rather than on influence, authority, or group allegiance. Homogenous groups, particularly small ones, are often victims of what the psychologist Irving Janis called “groupthink.” After a detailed study of a series of American foreign-policy fiascos, including the Bay of Pigs invasion and the failure to anticipate Pearl Harbour, Janis argued that when decision makers are too much alike – in worldview and mind-set – they easily fall prey to groupthink. Homogenous groups become cohesive more easily than diverse groups, and as they become more cohesive they also become more dependent on the group, more insulated from outside opinions, and therefore more convinced that the group’s judgement on important issues must be right. These kinds of groups, Janis suggested, share an illusion of invulnerability, a willingness to rationalize away possible counterarguments to the group’s position, and a conviction that dissent is not useful.
James Surowiecki, The Wisdom of Crowds, 36-37.