theories of neorealism are balance of power theory, balance of threat theory, security dilemma theory, offense-defense theory, hegemonic stability theory and power transition theory. Liberalism Liberalism has a shorter history than realism but has been a prominent theory since World War I. It is a concept with a variety of meanings. Liberal thinking dates back to philosophers such as Thomas Paine and Immanuel Kant, who argued that republican constitutions produce peace. Kant's concept of Perpetual Peace is arguably seen as the starting point of contemporary liberal thought. Economic liberalism Economic liberalism assumes that economic openness and interdependence between countries makes them more peaceful than countries who are isolated. Eric Gartzke has written that economic freedom is 50 times more effective than democracy in creating peace. Globalization has been important to economic liberalism. Liberal institutionalism Liberal institutionalism views international institutions as the main factor to avoid conflicts between nations. Liberal institutionalists argue that; although the anarchic system presupposed by realists cannot be made to disappear by institutions; the international environment that is constructed can influence the behavior of states within the system. Varieties of international governmental organizations (IGOs) and international non-governmental organizations (INGOs) are seen as contributors to world peace. Some believe that these international institutions lead to neotrusteeship, or postmodern imperialism. International institutions lead to an interconnectedness between strong and weak or post-conflict nations. In a situation such as a collapsed, weak-nation without the means of autonomous recovery, international institutions often lead to involvement by a stronger nation to aid in recovery. Because there is no definite international security policy to address weak or post-conflict nations, stronger nations sometimes face “mission-creep,” a shift from supplying and aiding nations to an escalation of mission goals, when aiding weaker nations. In addition, there is some debate due to lack of testing that international intervention is not the best institution to aid weak or post-war nations. Possible mission-creep, as well as inefficiencies in international intervention, creates debate as to the effectiveness of international institutions in peacekeeping. Comparison between realism and liberalism Constructivism Since its founding in the 1980s, constructivism has become an influential approach in international security studies. "It is less a theory of international relations or security, however, than a broader social theory which then informs how we might approach the study of security." Constructivists argue that security is a social construction. They emphasize the importance of social, cultural and historical factors,