Why and under what circumstances do people support aggressive action in the international system? And can political psychology actually give us insights into state behavior? This dissertation argues against conventional accounts that hold that the public is rational and strategic with the regards to the use of power. Relying on the concepts of the cognitive miser and rational ignorance among the voting public, the author uses experimental methods to show that with regards to foreign policy individuals are motivated by the same prejudices and moral intuitions that guide domestic political behavior. The first chapter argues against folk realist theories and shows that constructivist theories based on the need to maintain a positive self-image do a better job of predicting when Americans support the use of force abroad. Another chapter shows that when Americans consider altruistic policies, hearing that the policy in question can financially benefit the United States makes them less likely to support it. Furthermore, the implications of differences between conservatives and liberals are explored. When conservatives are considering whether to support humanitarian intervention, they show a bias towards helping Christians over Muslims, but no racial prejudice. Liberals, in contrast, show little to no religious prejudice but are more likely to want to intervene in the scenario where whites are oppressing blacks rather than the other way around. Prejudice can even influence more abstract moral values, as when conservatives heard about Christians being killed by Muslims, they were not only more likely to support humanitarian intervention, but also to say that the United States had a general moral obligation to help foreign populations facing government persecution. The final chapter explores whether psychological differences between conservatives and liberals matter with regards to the making of foreign policy. Relying on measures of affinity, or S-scores, the author uses United Nations General Assembly voting data from six Anglophone democracies to show that in each of these countries conservative governments vote less in line with the rest of the world. This work hopes to inspire future research that can continue to establish a link between political psychology and research on state behavior.