It is now widely accepted that psychological biases affect decision-making in economics and politics, with the strong implication that individuals should train themselves to compensate for these biases and behave more like rational actors. In this rich book, full of insights, Johnson challenges the view that such corrections are either necessary or desirable. Ingrained biases, he argues, are really evolutionary adaptations to fast-moving, uncertain situations. They served a purpose, and perhaps they still do. In war, leaders who are more confident than situations warrant may inspire their followers to an improbable victory. A strong bias in favor of one’s own group may reinforce solidarity in the face of an opponent trying to divide and rule. Johnson draws on examples from the American War of Independence, World War I, the Cuban missile crisis, and other conflicts to identify the circumstances in which biases helped produce positive outcomes.