Organized around several major theoretical traditions in international relations, this essay suggests which literature in psychology should be of greatest interest to different kinds of international relations scholars. New work in cognitive social psychology and behavioral decision theory simultaneously expands on and qualifies earlier error-and-bias portraits of the foreign policy maker, thereby enriching our understanding of internal divisions within the realist camp. Work on bounded rationality in competitive markets and mixed-motive games, as well as the literature on the power of human emotions to shape judgments of what represents an equitable allocation of scarce resources or a just resolution of conflicts of interest, can inform neo-institutionalist and constructivist theories. Developments in cross-cultural social psychology shed light on constructivist arguments about the creation and maintenance of international social order that typically rest on assumptions about decision making that are qualitatively different from realist and institutionalist approaches to world politics.

Contrary to conventional wisdom, nationalism remains alive and well across an increasingly integrated Europe. While most nationalisms are not violent, the desire for greater national voice by both states and groups continues to exist in both the East and the West. As the European Union deepens and widens, states and groups are choosing among four nationalist strategies: traditional, substate, transsovereign, and protectionist. The interplay among these nationalisms is a core part of Europe's dynamic present and future.

As the world moves away from the familiar bipolar cold war era, many international relations theorists have renewed an old debate about which is more stable: a world with two great powers or a world with many great powers. Based on the chief assumptions of structural realism—namely, that the international system is characterized by anarchy and that states are unitary actors seeking to survive in this anarchic system—some security analysts are predicting that a world of several great powers will lead to a return to the shifting alliances and instabilities of the multipolar era that existed prior to World War II. For instance, John Mearsheimer argues that “prediction[s] of peace in a multipolar Europe [are] flawed.” Thomas Christensen and Jack Snyder argue that states in a multipolar world can follow either the pre-World War I or the pre-World War II alliance pattern, thus implying that a third course is improbable. They further assert that “the fundamental, invariant structural feature, international anarchy, generally selects and socializes states to form balancing alignments in order to survive in the face of threats from aggressive competitors.” The realist argument predicts that great powers in a self-help international system will balance one another through arms races and alliance formations.

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