The aim of this multi-paper dissertation (i.e. Scandinavian model PhD thesis) was to address several of the gaps identified in the literature related to the assessment of individual differences in reasoning and decision-making quality, sometimes also referred to as rationality. In first two studies, we tested the construct validity of the cognitive reflection test (CRT) and its nonnumerical counterpart belief-bias syllogisms (BBS; Study 1) and investigated different ways and approaches to solving these tasks (Study 2). We concluded that CRT and another numerical test that does not contain lures are factorially indistinguishable and that lures do not play role in the success on or predictive validity of the CRT or BBS (Study 1). Additionally, we showed that in many cases when CRT and BBS tasks are solved correctly, they are solved relying solely on intuition and not analytical thinking. We conclude that these two tests are therefore not particularly good measure of reflection and analytical thinking engagement as thought before. In Study 3, across two studies, we examined a factorial structure of different cognitive biases tasks in a bid to understand the structure of rationality captured by these tasks and to validate identified rationality factor(s). In both studies, one-factor solution was the most appropriate and, to a large degree, explained by numerical abilities and thinking disposition called actively open-minded thinking (AOT; 61% of the rationality factor variance in Study 1 and 75% in Study 2). We conclude that cognitive biases tasks are highly heterogeneous and not particularly good solution for measuring rationality. Instead, we argue that individual differences in rational judgments and decisions matter are better captured by AOT. In Study 4, across three samples (undergraduates, employed participants and entrepreneurs), we showed that decision-making styles matter for various real-life and work-related outcomes, often adding explanatory power over cognitive abilities and personality traits. Finally, in Study 5, we examined the relevance of managerial AOT for positive organizational and employee-level outcomes. Over two studies, managers’ AOT correlated positively with range of positive personal and organizational outcomes. We concluded that concluded that it would be worthwhile to focus on AOT in selection for leadership positions as well as teach current leaders about the benefits and implementation of this way of thinking. In sum, results of our five studies point to the conclusion that how we think is as important, if not more, for consequential real-life outcomes, than how smart we are. What seem to be the most important is not to avoid decision-making and making them after the process of thinking in an actively open-minded way.