The Nobel Prize in Economics was awarded in 2002 for work on judgment and decision-making tasks that are the operational measures of rational thought in cognitive science. Because assessments of intelligence (and similar tests of cognitive ability) are taken to be the quintessence of good thinking, it might be thought that such measures would serve as proxies for the assessment of rational thought. It is important to understand why such an assumption would be misplaced. It is often not recognized that rationality and intelligence (as traditionally defined) are two different things conceptually and empirically. Distinguishing between rationality and intelligence helps explain how people can be, at the same time, intelligent "and" irrational. Thus, individual differences in the cognitive skills that underlie rational thinking must be studied in their own right because intelligence tests do not explicitly assess rational thinking. In this article, I describe how my research group has worked to develop the first prototype of a comprehensive test of rational thought (the Comprehensive Assessment of Rational Thinking).