Is violent conflict declining, or isn’t it? I’ve written here and elsewhere about evidence that warfare and mass atrocities have waned significantly in recent decades, at least when measured by the number of people killed in those episodes. Not everyone sees the world the same way, though. Bear Braumoeller asserts that, to understand how war prone the world is, we should look at how likely countries are to use force against politically relevant rivals, and by this measure the rate of warfare has held pretty steady over the past two centuries. Tanisha Fazal argues that wars have become less lethal without becoming less frequent because of medical advances that help keep more people in war zones alive. Where I have emphasized war’s lethal consequences, these two authors emphasize war’s likelihood, but their arguments suggest that violent conflict hasn’t really waned the way I’ve alleged it has.

This week, we got another important contribution to the wider debate in which my shallow contributions are situated. In an updated working paper, Pasquale Cirillo and Nassim Nicholas Taleb claim to show that

Violence is much more severe than it seems from conventional analyses and the prevailing “long peace” theory which claims that violence has declined… Contrary to current discussions…1) the risk of violent conflict has not been decreasing, but is rather underestimated by techniques relying on naive year-on-year changes in the mean, or using sample mean as an estimator of the true mean of an extremely fat-tailed phenomenon; 2) armed conflicts have memoryless inter-arrival times, thus incompatible with the idea of a time trend.

Let me say up front that I only have a weak understanding of the extreme value theory (EVT) models used in Cirillo and Taleb’s paper. I’m a political scientist who uses statistical methods, not a statistician, and I have neither studied nor tried to use the specific techniques they employ.

Bearing that in mind, I think the paper successfully undercuts the most optimistic view about the future of violent conflict—that violent conflict has inexorably and permanently declined—but then I don’t know many people who actually hold that view. Most of the work on this topic distinguishes between the observed fact of a substantial decline in the rate of deaths from political violence and the underlying risk of those deaths and the conflicts that produce them. We can (partly) see the former, but we can’t see the latter; instead, we have to try to infer it from the conflicts that occur. Observed history is, in a sense, a single sample drawn from a distribution of many possible histories, and, like all samples, this one is only a jittery snapshot of the deeper data-generating process in which we’re really interested. What Cirillo and Taleb purport to show is that long sequences of relative peace like the one we have seen in recent history are wholly consistent with a data-generating process in which the risk of war and death from it have not really changed at all.

Of course, the fact that a decades-long decline in violent conflict like the one we’ve seen since World War II could happen by chance doesn’t necessarily mean that it is happening by chance. The situation is not dissimilar to one we see in sports when a batter or shooter seems to go cold for a while. Oftentimes that cold streak will turn out to be part of the normal variation in performance, and the athlete will eventually regress to the mean—but not every time. Sometimes, athletes really do get and stay worse, maybe because of aging or an injury or some other life change, and the cold streak we see is the leading edge of that sustained decline. The hard part is telling in real time which process is happening. To try to do that, we might look for evidence of those plausible causes, but humans are notoriously good at spotting patterns where there are none, and at telling ourselves stories about why those patterns are occurring that turn out to be bunk.

The same logic applies to thinking about trends in violent conflict. Maybe the downward trend in observed death rates is just a chance occurrence in an unchanged system, but maybe it isn’t. And, as Andrew Gelman told Zach Beauchamp, the statistics alone can’t answer this question. Cirillo and Taleb’s analysis, and Braumoeller’s before it, imply that the history we’ve seen in the recent past  is about as likely as any other, but that fact isn’t proof of its randomness. Just as rare events sometimes happen, so do systemic changes.

Claims that “This time really is different” are usually wrong, so I think the onus is on people who believe the underlying risk of war is declining to make a compelling argument about why that’s true. When I say “compelling,” I mean an argument that a) identifies specific causal mechanisms and b) musters evidence of change over time in the presence or prevalence of those mechanisms. That’s what Steven Pinker tries at great length to do in The Better Angels of Our Nature, and what Joshua Goldstein did in Winning the War on War.

My own thinking about this issue connects the observed decline in the the intensity of violent conflict to the rapid increase in the past 100+ years in the size and complexity of the global economy and the changes in political and social institutions that have co-occurred with it. No, globalization is not new, and it certainly didn’t stop the last two world wars. Still, I wonder if the profound changes of the past two centuries are accumulating into a global systemic transformation akin to the one that occurred locally in now-wealthy societies in which organized violent conflict has become exceptionally rare. Proponents of democratic peace theory see a similar pattern in the recent evidence, but I think they are too quick to give credit for that pattern to one particular stream of change that may be as much consequence as cause of the deeper systemic transformation. I also realize that this systemic transformation is producing negative externalities—climate change and heightened risks of global pandemics, to name two—that could offset the positive externalities or even lead to sharp breaks in other directions.

It’s impossible to say which, if any, of these versions is “true,” but the key point is that we can find real-world evidence of mechanisms that could be driving down the underlying risk of violent conflict. That evidence, in turn, might strengthen our confidence in the belief that the observed pattern has meaning, even if it doesn’t and can’t prove that meaning or any of the specific explanations for it.

Finally, without deeply understanding the models Cirillo and Taleb used, I also wondered when I first read their new paper if their findings weren’t partly an artifact of those models, or maybe some assumptions the authors made when specifying them. The next day, David Roodman wrote something that strengthened this source of uncertainty. According to Roodman, the extreme value theory (EVT) models employed by Cirillo and Taleb can be used to test for time trends, but the ones described in this new paper don’t. Instead, Cirillo and Taleb specify their models in a way that assumes there is no time trend and then use them to confirm that there isn’t. “It seems to me,” Roodman writes, “that if Cirillo and Taleb want to rule out a time trend according to their own standard of evidence, then they should introduce one in their EVT models and test whether it is statistically distinguishable from zero.”

If Roodman is correct on this point, and if Cirillo and Taleb were to do what he recommends and still find no evidence of a time trend, I would update my beliefs accordingly. In other words, I would worry a little more than I do now about the risk of much larger and deadlier wars occurring again in my expected lifetime.