more equal distribution of power is more stable, and "movements toward unipolarity are destabilizing." However, evidence has shown power polarity is not actually a major factor in the occurrence of wars. Power transition theory: Hegemons impose stabilizing conditions on the world order, but they eventually decline, and war occurs when a declining hegemon is challenged by another rising power or aims to preemptively suppress them. On this view, unlike for balance-of-power theory, wars become more probable when power is more equally distributed. This "power preponderance" hypothesis has empirical support. The two theories are not mutually exclusive and may be used to explain disparate events according to the circumstance. Liberalism as it relates to international relations emphasizes factors such as trade, and its role in disincentivizing conflict which will damage economic relations. Realists respond that military force may sometimes be at least as effective as trade at achieving economic benefits, especially historically if not as much today. Furthermore, trade relations which result in a high level of dependency may escalate tensions and lead to conflict. Empirical data on the relationship of trade to peace are mixed, and moreover, some evidence suggests countries at war don't necessarily trade less with each other. Societal-level Diversionary theory, also known as the "scapegoat hypothesis", suggests the politically powerful may use war to as a diversion or to rally domestic popular support. This is supported by literature showing out-group hostility enhances in-group bonding, and a significant domestic "rally effect" has been demonstrated when conflicts begin. However, studies examining the increased use of force as a function of need for internal political support are more mixed. U.S. war-time presidential popularity surveys taken during the presidencies of several recent U.S. leaders have supported diversionary theory. Individual-level These theories suggest differences in people's personalities, decision-making, emotions, belief systems, and biases are important in determining whether conflicts get out of hand. For instance, it has been proposed that conflict is modulated by bounded rationality and various cognitive biases, such as prospect theory. Ethics The morality of war has been the subject of debate for thousands of years. The two principal aspects of ethics in war, according to the just war theory, are jus ad bellum and jus in bello. Jus ad bellum (right to war), dictates which unfriendly acts and circumstances justify a proper authority in declaring war on another nation. There are six main criteria for the declaration of