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.

When a layperson thinks of individual differences in reasoning they think of IQ tests. It is quite natural that this is their primary associate, because IQ tests are among the most publicized products of psychological research. This association is not entirely inaccurate either, because intelligence – as measured using IQ-like instruments – is correlated with performance on a host of reasoning tasks (Ackerman, Kyllonen, and Roberts 1999; Carroll 1993; Hunt 1999; Lohman 2000; Lubinski 2004; Rips and Conrad 1983; Sternberg 1977, 1985). Nonetheless, a major theme of this chapter will be that certain very important classes of individual differences in thinking are ignored if only intelligence-related variance is the primary focus. A number of these ignored classes of individual differences are those relating to rational thought.

In this chapter, I will argue that intelligence-related individual differences in thinking are largely the result of differences at the algorithmic level of cognitive control. Intelligence tests thus largely fail to tap processes at the intentional level of control. Because understanding rational behavior necessitates understanding processes operating at both levels, an exclusive focus on intelligence-related individual differences will tend to obscure important differences in human thinking.

This argument obviously depends strongly on differentiating the algorithmic from the intentional level of analysis. Therefore, in the next section I will outline the sources that I rely on for this conceptual distinction and how I will utilize the distinction to provide a framework for thinking about individual differences in reasoning.