The study of international relations takes a wide range of theoretical approaches. Some
emerge from within the discipline itself; others have been imported, in whole or in part,
from disciplines such as economics or sociology. Indeed, few social scientific theories
have not been applied to the study of relations amongst nations. Many theories of
international relations are internally and externally contested, and few scholars believe
only in one or another. In spite of this diversity, several major schools of thought are
discernable, differentiated principally by the variables they emphasize—eg military
power, material interests, or ideological beliefs.