International relations theory attempts to provide a conceptual model upon which international relations can be analyzed. Each theory is reductive and essentialist to different degrees, relying on different sets of assumptions respectively. As Ole Holsti describes them, international relations theories act as a pair of coloured sunglasses, allowing the wearer to see only the salient events relevant to the theory. An adherent of realism may completely disregard an event that a constructivist might pounce upon as crucial, and vice versa.
The number and character of the assumptions made by an international relations theory also determine its usefulness. Realism, a parsimonious and very essentialist theory is useful in accounting for historical actions (for instance why did X invade Y) but limited in both explaining systemic change (such as the end of the Cold War) and predicting future events. Liberalism, which examines a very wide number of conditions, is less useful in making predictions, but can be very insightful in analyzing past events. Traditional theories may have little to say about the behavior of former colonies, but post-colonial theory may have greater insight into that specific area, where it fails in other situations.
International relations theories can be divided into "positivist/rationalist" theories which focus on a principally state-level analysis, and "post-positivist/reflectivist" ones which incorporate expanded meanings of security, ranging from class, to gender, to postcolonial security. Many often conflicting ways of thinking exist in IR theory, including Constructivism, Institutionalism, Marxism, Neo-Gramscianism, and others. However, two positivist schools of thought are most prevalent: Realism and Liberalism; though increasingly, Constructivism is becoming mainstream and postpositivist theories are increasingly popular, particularly outside the United States.
Realism makes several key assumptions. It assumes that nation-states are unitary, geographically-based actors in an anarchic international system with no authority above capable of regulating interactions between states as no true authoritative world government exists. Secondly, it assumes that sovereign states, rather than IGOs, NGOs or MNCs, are the primary actors in international affairs. Thus, states, as the highest order, are in competition with one another. As such, a state acts as a rational autonomous actor in pursuit of its own self-interest with a primary goal to maintain and ensure its own security—and thus its sovereignty and survival. Realism holds that in pursuit of their interests, states will attempt to amass resources, and that relations between states are determined by their relative levels of power. That level of power is in turn determined by the state's military and economic capabilities.
Some realists (offensive realists) believe that states are inherently aggressive, that territorial expansion is constrained only by opposing powers, while others (defensive realists) believe that states are obsessed with the security and continuation of the state's existence. The offensive view can lead to a security dilemma where increasing one's own security can bring along greater instability as the opponent(s) builds up its own arms, making security a zero-sum game where only relative gains can be made.
The precursor to liberal IR theory was "idealism"; however, this term was applied in a critical manner by those who saw themselves as 'realists', for instance E. H. Carr. Idealism in international relations usually refers to the school of thought personified in American diplomatic history by Woodrow Wilson, such that it is sometimes referred to as "Wilsonianism." Idealism holds that a state should make its internal political philosophy the goal of its foreign policy. For example, an idealist might believe that ending poverty at home should be coupled with tackling poverty abroad. Wilson's idealism was a precursor to liberal international relations theory, which would arise amongst the "institution-builders" after World War II.
Liberalism holds that state preferences, rather than state capabilities, are the primary determinant of state behavior. Unlike realism where the state is seen as a unitary actor, liberalism allows for plurality in state actions. Thus, preferences will vary from state to state, depending on factors such as culture, economic system or government type. Liberalism also holds that interaction between states is not limited to the political/security ("high politics"), but also economic/cultural ("low politics") whether through commercial firms, organizations or individuals. Thus, instead of an anarchic international system, there are plenty of opportunities for cooperation and broader notions of power, such as cultural capital (for example, the influence of films leading to the popularity of the country's culture and creating a market for its exports worldwide). Another assumption is that absolute gains can be made through co-operation and interdependence - thus peace can be achieved.
The democratic peace theory argues that liberal democracies have never (or almost never) made war on one another and have few lesser conflicts between each other. This is seen as contradicting especially the realist theories and this empirical claim is now one of the great disputes in political science. Numerous explanations have been proposed for the democratic peace. It has also been argued, as in the book Never at War, that democracies conduct diplomacy in general very differently from nondemocracies. (Neo)realists disagree with Liberals over the theory, often citing structural reasons for the peace, as opposed to the state's government.
Institutionalism in international relations holds that the international system is not—in practice—anarchic, but that it has an implicit or explicit structure which determines how states will act within the system.
Institutions are rules that determine the decision-making process. In the international arena, institution has been used interchangeably with 'regime', which has been defined by Krasner as a set of explicit or implicit "principles, norms, rules, and decision-making procedures around which actors expectations converge in a given issue-area."
Institutionalist scholars hold a wide array of beliefs stemming from the central proposition that institutions "matter" in answering the question, what explains a particular outcome? There are four reasons for this:
They structure choices, they provide incentives, they distribute power and they define identities and roles.
The 'English School' of international relations theory, also known as International Society, Liberal Realism, Rationalism or the British institutionalists, maintains that there is a 'society of states' at the international level, despite the condition of 'anarchy' (literally the lack of a ruler or world state).
A great deal of the work of the English School concerns the examination of traditions of past international theory, casting it, as Martin Wight did in his 1950s-era lectures at the London School of Economics, into three divisions: 1. Realist or Hobbesian (after Thomas Hobbes), 2. Rationalist (or Grotian, after Hugo Grotius), 3. Revolutionist (or Kantian, after Immanuel Kant).
In broad terms, the English School itself has supported the rationalist or Grotian tradition, seeking a middle way (or via media) between the 'power politics' of realism and the 'utopianism' of revolutionism.
Many schools of thought in international relations have criticized the status-quo - both from other positivist positions as well as postpositivist positions. The former include Marxist and Neo-Marxist approaches and Neo-Gramscianism. The latter include postmodernist, postcolonial and feminist approaches, which differ from both realism and liberalism in their epistemological and ontological premise.
Marxist and Neo-Marxist international relations theories are positivist paradigms which reject the realist/liberal view of state conflict or cooperation; instead focusing on the economic and material aspects. It makes the assumption that the economic concerns transcend others; allowing for the elevation of class as the focus of study. Marxists view the international system as an integrated capitalist system in pursuit of capital accumulation.
Whereas realism deals mainly with security and material power, and liberalism looks primarily at economic interdependence and domestic-level factors, constructivism most concerns itself with the role of ideas in shaping the international system (Indeed it is possible there is some overlap between constructivism and realism or liberalism, but they remain separate schools of thought). By "ideas" constructivists refer to the goals, threats, fears, identities, and other elements of perceived reality that influence states and non-state actors within the international system. Constructivists believe that these ideational factors can often have far-reaching effects, and that they can trump materialistic power concerns. For example, constructivists note that an increase in the size of the US military is likely to be viewed with much greater concern in Cuba, a traditional antagonist of the US, than in Canada, a close US ally. Therefore, there must be perceptions at work in shaping international outcomes. As such, constructivists do not see anarchy as the invariable foundation of the international system, but rather argue, in the words of Alexander Wendt, that "anarchy is what states make of it." Constructivists also believe that social norms shape and change foreign policy over time rather than security which realists cite.
Functionalism is a theory of international relations that arose principally from the experience of European integration. Rather than the self-interest that realists see as a motivating factor, functionalists focus on common interests shared by states. Integration develops its own internal dynamic: as states integrate in limited functional or technical areas, they increasingly find that momentum for further rounds of integration in related areas. This "invisible hand" of integration phenomenon is termed "spill-over." Although integration can be resisted, it becomes harder to stop integration's reach as it progresses. This usage, and the usage in functionalist in international relations, is the less commonly used meaning of the term functionalism.
More commonly, however, functionalism is a term used to describe an argument which explains phenomena as functions of a system rather than an actor or actors. Immanuel Wallerstein employed a functionalist theory when he argued that the Westphalian international political system arose to secure and protect the developing international capitalist system. His theory is called "functionalist" because it says that an event was a function of the preferences of a system and not the preferences of an agent. Functionalism is different from structural or realist arguments in that while both look to broader, structural causes, realists (and structuralists more broadly) say that the structure gives incentives to agents, while functionalists attribute causal power to the system itself, bypassing agents entirely.