"RATIONALITY focuses on perhaps the key fault-line in the social and cognitive sciences. Many theorists, in philosophy, economics, artificial intelligence and cognitive science explain mind, behaviour, and their consequences for social phenomena, by rational explanation. Others, in judgment and decision making, social psychology, behavioural economics and the neurosciences, argue that people systematically violate rational principles, typically focusing on the influence of mechanistic, not rational, constraints on thought. Having worked extensively in both traditions over the last two decades, I aim to establish how, and to what extent, these viewpoints interact and how far they can be reconciled, by mathematical, computational, and experimental methods. This research programme will have fundamental implications both for scientific and normative questions. It will clarify the interplay of rational and mechanistic explanation of inference, learning, decision making, communication, and social phenomena; and will explore the cognitive underpinnings of our conflicting normative intuitions, helping to inform normative questions in ethics and political philosophy."