Myside bias occurs when people evaluate evidence, generate evidence, and test hypotheses in a manner biased toward their own prior opinions and attitudes. Myside bias is displayed by people in all demographic groups, and it is exhibited even by expert reasoners, the highly educated, and the highly intelligent. Surprisingly, however, the degree of myside bias shown is not predictable from individual difference variables that we would expect to be associated with it. For example, it is not attenuated by cognitive sophistication, as measured by cognitive ability or thinking dispositions. Another way in which myside bias is an outlier bias is that, in most circumstances, it shows very little domain generality and appears to be very content dependent. Individuals who display high myside bias on one issue do not necessarily show high myside bias on another, unrelated issue. Because of these unusual characteristics, myside bias needs a different type of model – a content-based model, such as those deriving from memetic theory.

There are individual differences in rational thinking that are less than perfectly correlated with individual differences in intelligence because intelligence and rationality occupy different conceptual locations in models of cognition. A tripartite extension of currently popular dual-process theories is presented in this chapter that illustrates how intelligence and rationality are theoretically separate concepts. 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. We close the chapter by describing our attempt to develop the first prototype of a comprehensive test of rational thought, the Comprehensive Assessment of Rational Thinking (CART). With the CART, we aim to draw more attention to the skills of rational thought by measuring them systematically and by examining the correlates of individual differences in these cognitive skills.