Little Greek letters become weapons in war of words over trend in violence 

Two intellectual titans are arguing over whether humanity has become less violent. In his 2011 book, Steven Pinker contends that violence is way down since the stone age, or even since the Middle Ages. He looks at murder, war, capital punishment, even violence against animals.

But in a working paper released yesterday, Pasquale Cirillo and Nassim Taleb, the latter the author of The Black Swan: The Impact of the Highly Improbable, contend that Pinker has it wrong. Well, more precisely (lest I incite a riot with demagoguery) they challenge the notion that the great powers have enjoyed a distinctly long peace since World War II.

This paper launches a second round in a war of words (and equations) over the long peace. Previously, Taleb said the The “Long Peace” is a Statistical Illusion. Pinker suggested Taleb was Fooled by Belligerence.

Everything I know about the history of violence, I learned from Pinker’s TED talk. And I have not mastered the mathematical methods marshalled by Cirillo and Taleb. But I do know something about them, having written a program to do them. I think Taleb’s latest salvo mostly misses its mark. Here’s why:

  • Cirillo and Taleb invoke extreme value theory, which is designed for studying the probabilities of events such as big floods, geomagnetic storms, and world wars. Their applications of EVT models (Pareto/power law, Lomax, Generalized Pareto) dominate the paper. But these assume rather than show that the aggregate casualty rate of warfare has held steady over the centuries. EVT models can allow for time trends but the ones used here don’t.
  • Cirillo and Taleb do state and defend their assumption of no time trend. But this analysis amounts to three paragraphs on page 6. They observe that
    • Gaps between big wars are really long. E.g., after rescaling historical casualty figures in proportion to today’s world population, a war costing at least 10 million lives has hit only once a century on average (Table II). Only 70 years after World War II, they contend, it is too soon to declare that this time is different. (And here one can see how Taleb’s experience with finance bolsters his skepticism of such declarations.)
    • A simple measure of correlation over time in war deaths finds almost none. Fewer deaths in one year does not predict fewer deaths 10 or even 100 20 years later (figure 8). This cuts against the notion of trend.

    To me these facts seem relevant but inadequate to overturning Pinker’s conclusion, which synthesizes a much larger body of scholarship drawing on multiple types of evidence to document the recent, historically anomalous disappearance of violent conflict between great powers. To my knowledge there have been no ~million-death-or-greater wars between great powers since 1945. Cirillo and Taleb say globally, over 21 centuries, wars of at least that magnitude have happened every 26.71 years (again, scaling to current world population). So on the evidence presented, the long peace still looks improbable.

  • It seems to me that if Cirillo and Taleb want to rule out a time trend trend break according to their own standard of evidence, then they should introduce a post-1945 dummy in their EVT models and test whether it is statistically distinguishable from zero.

Presumably both sides would agree that among great powers, the last 70 years have been unusually peaceful. And both would agree that the future is uncertain. The rupture appears to come in the readiness to extrapolate forward from the last 70 years. Whether this rupture is great, or whether Taleb is jousting with a caricature of Pinker, I am not sure. I would find Cirillo and Taleb more compelling if they documented exactly which of Pinker’s words they find to violate the truth.

Update: further thoughts.