Steven Pinker observes that a decline in war does not require a romantic theory of human nature. In fact, it is compatible with a hardheaded view of human violent inclinations, firmly rooted in evolutionary biology. Homo sapiens evolved with violent tendencies, but they are triggered by particular circumstances rather than a hydraulic urge that must periodically be discharged. And, although our species evolved with motives that can erupt in violence, it also evolved motives that can inhibit violence, including self-control, empathy, a sense of fairness, and open-ended cognitive mechanisms that can devise technologies for reducing violence. In the same forum, Bradley Thayer argues that the decline of war thesis is flawed because the positive forces identified by these authors do not rule outside of the West or even fully inside of it. Their analysis also neglects the systemic causes of conflict and its insights for increasingly intense security competition between China and the United States. Jack Levy & William Thompson question some of the theoretical arguments advanced to explain the historical pattern of declining violence. They argue that cultural and ideational explanations for the decline in interstate war underestimate the extent to which those factors are endogenous to material and institutional variables. Arguments about the pacifying effects of the rise of the state and of commerce fail to recognize that in some historical contexts, those factors have contributed to the escalation of warfare. The introduction by Nils Petter Gleditsch outlines briefly some of the major issues in the debate: nature versus nurture, the reliability of the data, how broadly violence should be defined, whether there is more agreement on the phenomenon than on its causes, and finally whether the future will be like the past.