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 colored 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.