This dissertation seeks to expand our understanding of variation in foreign policy. Although we have a series of large, extant literatures dealing with the sources of foreign policy, there has been less attention paid over the last decade to understanding why states change their behavior. At the same time, the thesis argues that foreign policy change is best understood as a result of the role of individual decision-makers and the role that emotion plays in their foreign policy calculations.Foreign policy depends on the decisions made by individual leaders. The type of individual thus determines the specific policy. Here individuals are categorized as ideological or adaptable. Ideological individuals are more rigid in their belief structures, are more likely to select policies that fit with their extant understandings of the world and the position of their state in it, and more likely to rely on the emotional or affective appeal an object or issue holds for them. Adaptable leaders are more flexible, not tied to specific ideologies or reliant on emotion to guide their thinking, and thus more likely to choose or learn ideas that best respond to changing environmental conditions. At the same time, how a state's decision-making institutions are structured tells us how likely it is that an individual's own predilections matter. In polities where decision-making is centralized (e.g., in the office of the prime minister), individuals have greater leeway to put their ideas (whether based on their ideological outlooks or shifting environmental circumstances) into practice, while in de-centralized polities other actors constrain the leader from autonomous decision-making. In such cases, it is likely that an individual's ideas will conform to those of the constraining actors. Finally, the role of ideas is taken into consideration, as the dominant national ideas about foreign policy regarding a specific issue-area help us better understand the context in which individuals make (or change) foreign policy.This model is tested against alternate explanations---systemic imperatives, Constructivism, public opinion, poliheuristic theory, and prospect theory---in two case studies: the Israeli decision to pursue and sign the 1993 Oslo Accords, and the 2002 decision by the Islamist government in Turkey to actively lobby for membership in the European Union. Both foreign policies represent significant variation, and both provide important theoretical and empirical puzzles for scholars
