The Parallel Evolution of Humanity and Savagery

Human male violence is paradoxical. On the one hand within social groups there is a strong tendency for avoidance of direct conflicts such that confrontations between angry individuals or groups normally end without serious harm. On the other hand our species has a consistent history of intense deliberate violence, ranging from planned homicides and low-level and long-lasting warfare among nomadic hunter-gatherers to massive intermittent conflicts among states. A common interpretation of this two-fold system holds that while low-level confrontational aggression is an understandable consequence of our biology, deliberate violence comes from non-evolutionary sources such as evil individuals or ideologies, novel contexts or cognitive failures. Here, by contrast, I show that the combination of aggression styles is better understood as being due to two neurobiologically distinct patterns that have been subject to contrasting selective regimes. By comparison with chimpanzees reactive (or ‘hot’, impulsive) aggression is down-regulated in humans, whereas proactive (or ‘cold’, premeditated) aggression has been subject to positive selection. I suggest specific explanations for each trend. The combination of these two styles of aggression makes humans well adapted for both war and avoidance of war.