Two Is Not Always Better Than One: A Critical Evaluation of Two-System Theories.

Over the past two decades, there has been an upsurge in theoretical frameworks alluding to the existence of two different processing systems that supposedly operate according to different rules. This article critically examines the scientific advance offered by these theories (in particular advances in the domains of reasoning, decision making, and social cognition) and questions their theoretical coherence as well as the evidence for their existence. We scrutinize the conceptual underpinnings of two-system models and explicate the assumptions underlying these models to see whether they are reasonable. We also evaluate the empirical paradigms used to validate two-system models and ponder about their explanatory strength and predictive power. Given the popularity of these models, we discuss the appeal of two-system theories and suggest potential reasons for their prevalence. We comment on the potential costs associated with these models and allude to the desired nature of potential alternatives. We conclude that two-system models currently provide little scientific advance, and we encourage researchers to adopt more rigorous conceptual definitions and employ more stringent criteria for testing the empirical evidence in support for two-system theories.