Many contemporary accounts of human reasoning assume that the mind is equipped with multiple heuristics that could be deployed to perform a given task. This raises the question of how the mind determines when to use which heuristic. To answer this question, we developed a rational model of strategy selection, based on the theory of rational metareasoning developed in the artificial intelligence literature. According to our model people learn to efficiently choose the strategy with the best cost-benefit tradeoff by learning a predictive model of each strategy's performance. We found that our model can provide a unifying explanation for classic findings from domains ranging from decisionmaking to arithmetic by capturing the variability of people's strategy choices, their dependence on task and context, and their development over time. Systematic model comparisons supported our theory, and four new experiments confirmed its distinctive predictions. Our findings suggest that people gradually learn to make increasingly more rational use of fallible heuristics. This perspective reconciles the two poles of the debate about human rationality by integrating heuristics and biases with learning and rationality.Keywords: bounded rationality; strategy selection; heuristics; meta-decisionmaking; metacognitive reinforcement learningTo succeed in life we have to solve a wide range of problems that place very different demands on us: sometimes we have to think fast and sometimes we have to think slow (cf. Kahneman, 2011). For instance, avoiding a car accident requires a split-second decision, whereas founding a successful start-up requires investing a lot of time into anticipating the future and weighting potential outcomes appropriately. No single decision mechanism works well across all situations. To meet the wide range of demands posed by different decision problems, it has been proposed that the human brain is equipped with multiple decision systems (Dolan & Dayan, 2013) and decision strategies (Payne, Bettman, & Johnson, 1988). Dual-process theories are a prominent example of this perspective (Evans & Stanovich, 2013;Evans, 2003;Kahneman, 2011). The coexistence of multiple alternative strategies is not specific to decision making. People also appear to possess multiple strategies for inference (Gigerenzer & Selten, 2002), memory (Bjorklund & Douglas, 1997), self-control (Braver, 2012), problem solving (Fum & Del Missier, 2001), and mental arithmetic (Siegler, 1999) to name just a few.The availability of multiple strategies that are applicable to the same problems raises the question how people decide when to use which strategy. The fact that so many different strategies have been observed under different circumstances shows that people's strategy choices are highly variable and contingent on the situation and the task (Beach & Mitchell, 1978;Fum & Del Missier, 2001;Payne, 1982;Payne et al., 1988). Overall, the contingency of people's strategy choices appears to be adaptive. Even though under certain circumstances p...