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Evolutionary theory makes three major contributions to the offensive realist theory of international politics: (1) a novel ultimate cause of the primary traits of offensive realist behavior (self-help, power maximization, and fear); (2) an extension of offensive realism to any domain in which human actors compete for power (e.g., civil war, ethnic conflict, or domestic politics); and (3) an explanation for why individual leaders themselves, not just states, behave as they do. Novel ultimate cause and theoretical foundation Our theory advances offensive realist arguments without seeking an ultimate cause in the anarchic international state system. Offensive realism based on evolutionary theory makes the same predictions for state behavior, but the ultimate causal mechanism is different: human evolution in the anarchic, dangerous, and competitive conditions of the late-Pliocene and Pleistocene eras. Evolutionary theory explains why humans are egoistic, strive to dominate others, and make ingroup/outgroup distinctions. These adaptations in turn serve as a foundation for offensive realism—what Mearsheimer independently identified as self-help, power maximization, and fear. The central issue raised by our theory is what causes states to behave as offensive realists predict. Mearsheimer argues that anarchy is the fundamental cause of such behavior. The fact that there is no world government compels the leaders of states to take steps to ensure their security, such as striving to have a powerful military, forging and maintaining alliances, and acting aggressively when necessary. This is what neorealists call a self-help system: Leaders of states are forced to take these steps because nothing else can guarantee their security in the anarchic world of international politics. We argue that evolutionary theory also offers a fundamental cause for offensive realist behavior (see Table 1). Humans evolved in a state of nature where competition for resources and dangers from other humans and the environment were great. Humans survived (and note that several other Hominin species did not) by virtue of evolved behavioral traits—among them egoism, dominance, and the ingroup/outgroup bias—which were adaptations to competitive ecological conditions. These traits help to explain why humans (including political leaders) will behave, in the proper circumstances, as offensive realists expect them to behave. In sum, evolutionary theory offers realist scholars a natural-scientific behavioral foundation for offensive realism. Human evolution explains why people seek control over resources, why some of us (particularly males) will seek to dominate others in order to achieve and maintain a privileged position in a dominance hierarchy, and why we are suspicious and wary of other groups.150 Since the leaders of states are human, they too will be influenced—perhaps especially influenced—by this evolutionary legacy as they react to the actions of other states and decide how to respond.151,Reference McDermott152,Reference McDermott153 Extensions of offensive realism to other domains The second contribution of our theory is that it offers an explanation of the behavior of humans in a wide variety of contexts extending beyond international politics. Our evolutionary theory of offensive realism is unlimited in time, explaining behavior from the ancestral environment to the present day, whereas offensive realism is conventionally inapplicable prior to 1648, when the Treaty of Westphalia established the European state system. Our theory is also unlimited in domain, explaining behavior wherever there are human actors and weak external constraints on their actions, from ancestral human groups, ethnic conflict, and civil wars to domestic politics, free markets, and international relations. The theory might thus be extended to explain the behavior and actions of many phenomena: the Roman Empire, warfare among Papua New Guinean or Native American tribes, the European conquest of South America, the race for the American west and the failed Mormon and Confederate secessions, the imperialist scrambles over African colonies, institutions like the medieval Catholic Church, commercial organizations from the East India Company to Coca-Cola, the struggles of rival ethnic groups the world over, and the ruthlessness of electoral campaigns. Table 2 illustrates the range of domains to which an evolutionary theory of offensive realism applies. Note that the table captures key patterns, not universal behavior. Clearly, not all individuals or businesses or states act the same way all the time or in all circumstances. Behavior varies considerably, just as standard offensive realism predicts for states, and countervailing forces would sometimes mitigate power-maximization strategies—although the very need for and difficulties of those countervailing forces help to demonstrate the fact that offensive realist behavior remains an underlying problem. Indeed, part of the beauty of evolutionary approaches is their ability to predict sources of variation—the socio-ecological conditions under which we should expect to see humans acting (in this case) more fearful and more self-interested, and pursuing more power maximization, rather than less. Older versions of evolutionary theory sometimes presented strategies and behaviors as fixed or “hard wired.” Modern biology stresses the contingent, context-dependent nature of behavioral adaptations, which generates finer predictions for when we should expect to see different types of behavior.Reference Davies, Krebs and West155 This is an important point to which we will return. Wherever actors are left to compete with each other by relying on their own devices (whether in the human evolutionary past or today), we predict that actors within those systems will exhibit similar behavior, not least self-help behavior to maximize power. Offensive realists can thus explain more than the behavior of states or great powers. If anchored on evolutionary theory, offensive realism allows new insights to elucidate why individuals and substate groups are self-interested, vie for power, and fear each other, and it can explain political behavior and war that occurred long before the creation of the modern state system in 1648. Explaining leaders’ lust for power The third contribution of our theory is that it identifies a more explicit role for leaders (see Table 3). Mearsheimer’s offensive realism argues that states gain power to ensure security. That certainly may be, as he attempts to demonstrate. However, an overtone of this argument is that power or domination is distasteful for leaders—that they tolerate it only for the sake of their state’s security. Leaders are forced to maximize power when perhaps they would rather cooperate or share power with others. By contrast, our theory posits that a tendency toward offensive realist behavior, however modulated by other tendencies, would have conferred a fitness advantage in the environment in which humans evolved and should thus have led to dispositions to seek and like power. Indeed, a wide range of empirical evidence from psychology and neuroscience suggests instead that humans, especially men, not only want to be leaders but also enjoy the pursuit of power (as well as its material fruits).156,Reference Robertson157,158 The force of this motivation is frequently revealed in victors’ expressions of the satisfaction of conquest. As Chinggis Khan is purported to have said: “The greatest happiness is to vanquish your enemy, to drive him before you, to see his cities reduced to ashes.”159 Although not usually expressed in such stark terms, the pleasure of competition and victory has been widely recognized as a feature of human nature from classical times to the present day, and success in competitive interactions and the domination of others are known to increase testosterone and dopamine responses in men—the so-called victory effect.160 Such dominance behavior is, we suggest, exaggerated among leaders because they are generally ambitious and competitive, and usually male. Moreover, the very acquisition and exercise of power itself is known to inflate dominance behavior further.161