This book shows that rational thinking, like intelligence, is a measurable cognitive competence. Drawing on theoretical work and empirical research from the last two decades, The Rationality Quotient presents the first prototype for an assessment of rational thinking analogous to an IQ test: the CART (Comprehensive Assessment of Rational Thinking). The book describes the theoretical underpinnings of the CART, distinguishing the algorithmic mind from the reflective mind. It discusses the logic of the tasks used to measure cognitive biases. The book presents a unique typology of thinking errors. The Rationality Quotient explains the components of rational thought assessed by the CART, including probabilistic and scientific reasoning; the avoidance of “miserly” information processing; and the knowledge structures needed for rational thinking. The book discusses studies of the CART and the social and practical implications of such a test. An appendix offers sample items from the test.