Like the other contributors to this conversation, I agree with the statement attributed to Yogi Berra that predictions are hard, especially about the future. No responsible person can predict with certainty whether the Long Peace among great powers and developed states will persist. And because we can witness the unfolding of only one universe one time, any statement of probability can be no more than a statement the theorist’s level of subjective confidence.
Still, that level of confidence can be justified to varying degrees, and it seems to me the quantitative trends underlying The Long Peace (nicely superimposed into a single graph by Allan Dafoe in his introduction) reflect genuine changes in the international system. That is, they are not just a gambler’s lucky streak that is sure to run out, an artifact of the way that wars and their human costs are counted, or a temporary lull in an inexorable cycle. As such they support a reasonable degree of confidence that The Long Peace will persist (subject to a class of exceptions I will present at the end of this essay).
None of the reasons to dismiss the trends underlying The Long Peace strike me as sound. The wisecracks about the man plummeting off the skyscraper shouting “So far so good!” and the turkey on the eve of Thanksgiving celebrating the 364-day period of coexistence between humans and turkeys respectively assume that history is driven by an inexorable directional force or by a strict cycle. Neither theory of history is supported by data on long-term trends in wars involving great powers or developed countries. In Better Angels I summarized these data (from Jack Levy, Lewis Richardson, Peter Brecke, and others) as a superposition of four patterns (p. 192): (1) No cycles; (2) A big dose of randomness; (3) A long-term escalation in the destructiveness of war, which made a substantial U-turn after 1945; and (4) Long-term declines in the frequency and duration of war. Multiplying the trends in (3) and (4) yields the overall decline in war that we call The Long Peace, and Factor (2) should keep us humble and cautious. But nothing supports the systematic pessimism of the fables about plummeting men or complacent turkeys.
Nor do the cautionary tales about pre-World-War I optimism tell us much, except that we should always be cautious. First, the infamous Norman Angell did not predict that war was impossible, only that it was economically irrational. He feared that ideology and fear might lead the leaders of great powers to blunder into a disastrous war, and he was right. Second, though the world of a century ago had seen unprecedented levels of trade and economic integration, Bruce Russett and John Oneal have shown that when they are measured quantitatively (as a proportion of GDP) they are a tiny fraction of the levels the world has seen since 1945 (Better Angels, p. 286). Russett and Oneal’s two other statistical predictors of peace (democracy and membership in intergovernmental organizations) are also far higher today, and other indicators of war-readiness such as the prevalence and length of conscription, the proportion of the population in uniform, overall prosperity, and the political participation of women are also more favorable today. In the realm of ideas, romantic militarism and nationalism have ceded ground to war aversion and liberal humanism, and of course nowadays we have knowledge of two destructive world wars and an awareness of the possibility of a nuclear catastrophe. In no other realm of human experience could one credibly say that we have learned nothing in the past century and that the assessments of today are no more trustworthy than those of a hundred years ago.
There is, of course, a tragic-poetic vision of the human condition in which we are condemned to repeat history, to be felled by our own hubris, to regress to our nature red in tooth and claw, and so on, but it is not grounded in historical or biological fact. If The Long Peace endures, it would not be the first time in history that a longstanding barbaric institution has been abolished or at least decimated. Wars involving great powers and developed states could very well join human sacrifice, chattel slavery, public torture-executions, auto-da-fés, debtors’ prisons, bear-baiting, foot-binding, gentlemanly dueling, witch hunts, and trephination on the ash heap of history.
Nor does a realistic, nonromantic view of human nature require perpetual war (and I can speak with some authority on this, having championed a thoroughly unsentimental understanding of the crooked timber of humanity in How the Mind Works and The Blank Slate). Though Homo sapiens undoubtedly evolved with violent instincts, those instincts are triggered by particular circumstances; they are not a hydraulic urge that must periodically be discharged. And though we evolved motives that can erupt in violence, we 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.
None of this is to say that The Long Peace must endure. On top of the many systematic trends that militate against the resumption of great-power and developed-state war, there are the black swans, long tails, and unknown unknowns that could poke big spikes up into the declining gradient. Perhaps there is some charismatic politician working his way up through the Chinese nomenklatura who dreams of rectifying the intolerable insult of Taiwan once and for all, provoking an American or international response. Perhaps an aging Putin will seek historical immortality and restore Russian greatness by swallowing a former Soviet republic or two. Perhaps terrorists from some liberation movement no one has heard of are plotting an attack of unprecedented destruction, or a utopian ideology is fermenting in the mind of a cunning fanatic somewhere who will take over a major country and try to impose it everywhere.
Certainly no one could rule out these low-probability/high-impact events, or even begin to estimate their cumulative probability. But acknowledging our ignorance about improbable, trend-defying events is different from denying the existence of the trends. It seems to me that the Long Peace is a genuine trend that clusters with other, more-often-than not, more-or-less successful attempts over the course of human history to contain our violent impulses.
Steven Pinker is Harvard College Professor of Psychology at Harvard University and the author of The Better Angels of Our Nature: Why Violence Has Declined.