What is a "rational" decision? Economists traditionally viewed rationality as maximizing expected satisfaction. This view has been useful in modeling basic microeconomic concepts, but falls short in accounting for many everyday human decisions. It leaves unanswered why some things reliably make people more satisfied than others, and why people frequently act to make others happy at a cost to themselves. Drawing on an evolutionary perspective, we propose that people make decisions according to a set of principles that may not appear to make sense at the superficial level, but that demonstrate rationality at a deeper evolutionary level. By this, we mean that people use adaptive domain-specific decision-rules that, on average, would have resulted in fitness benefits. Using this framework, we re-examine several economic principles. We suggest that traditional psychological functions governing risk aversion, discounting of future benefits, and budget allocations to multiple goods, for example, vary in predictable ways as a function of the underlying motive of the decision-maker and individual differences linked to evolved life-history strategies. A deep rationality framework not only helps explain why people make the decisions they do, but also inspires multiple directions for future research.Consider the array of decisions facing a 30-year old MBA graduate just beginning her first full-time management position. In her first days on the job, she will be asked to choose between several investment packages for retirement, with different mixtures of risky versus safe investments. Not long thereafter, she will need to make equally complex decisions about how to invest her scarce time and effort at work (e.g., choosing between various projects, forming alliances with coworkers, impressing superiors, managing subordinates), and in her personal life (e.g., finding a romantic partner, spending time with friends and family, starting her own family) as well as making decisions about how to make trade-offs between work and personal life. Such decisions are fundamentally microeconomic, in that they involve an individual's allocation of limited resources. Traditionally, microeconomic decisions have been modeled on broad notions of rational choice whereby entities attempt to maximize their utility, or expected satisfaction (e.g. Bronfenbrenner, Sichel, & Gardner, 1990;Mas-Collel, Whinston, & Green, 1995 In what follows, we suggest an expanded, evolutionarily-informed view of rationality that utilizes classic economic tools and also takes into account recent theory and findings at the intersection of evolutionary biology and cognitive science. Evolutionary approaches are inherently economic in nature, focusing on individuals' allocation of scarce resources to various fitness-relevant activities. A key assumption of the evolutionary perspective is that the human brain contains not one monolithic "rational decision-making device," but rather a number of different decision-systems, each operating according to different rules. ...