The “Great Rationality Debate” is based on whether or not humans are rational. We have two systems of thinking at our disposal: Deliberation and intuition. These systems sometimes agree, but often demand different things. This is reflected in negotiation: Because of the paradoxical nature of the task, the two systems regularly recommend different approaches. This chapter explains the benefits and limitations of both intuition and deliberation, illustrated with case studies from all walks of life.

The trap is the illusion of accumen, which makes us believe that our thinking about the challenge at hand is already accurate, even though we might only follow our intuition or deliberation. This is aggravated by the difficulty of obtaining good feedback in a “wicked” learning environment, where there are no incentives to disclose valuable information when the transaction is concluded. Together, the three illusions can block our advance in learning to negotiate. Because we have correctly learned something (such as one side of the Yin & Yang, some of the tools from the toolkit, one method of thinking), realizing that its opposite can also be true is very difficult.

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.