In the last three decades, mainstream economics has been influenced by authors associated with new institutional economics and new behavioral economics. The dispute over rationality as an assumption of economic theories is becoming particularly evident and is taking new forms. The aim of this article is to examine the connections between the institutional and behavioral approaches as well as between researchers’ ideas as to what rationality is and their beliefs regarding an optimal economic system. It will demonstrate that so-called behavioral and institutional economists have more in common than not. Institutions play a key role in the arguments of behavioural economists, whereas the argument of institutional economists is almost always based on the issue of human cognitive abilities and emotions. What directly links the two trends is the attention given to the rationality of actions that an individual takes as a premise of economic choices and as an assumption of economic theories. Differences in views relate to the understanding of rationality and exist within the framework of behavioral economics itself. At the core of the dispute is the distinction between two concepts of rationality: constructivist and ecological. This distinction serves as a starting point for the second matter discussed in the article. The author argues that the concept of constructivist rationality is related to the vision of the top-down creation of social order, while the proponents of the ecological approach to rationality stress the importance of market institutions. Interestingly, from the perspective of cognitive psychology and the heuristics of Daniel Kahneman, it can be presumed that the convictions of a scholar about the “ideal system” can influence his or her arguments on the essence of human rationality.