Bildet er fra en demonstrasjon i Washington DC i april 1971

The Long Peace most likely began during the Vietnam War

Two statisticians at the University of Oslo have blown a hole in Steven Pinker's famous theory that the Long Peace dates from 1945 onwards. But Pinker is excited about the new calculations, which suggest that this more peaceful period instead began in 1965 – during the Vietnam War.

Nils Lid Hjort og Céline Cunen med kurven som viser at noe endret seg i 1965

Nils Lid Hjort and Céline Cunen with the graph that reveals a change point in 1965, during the Vietnam war. Photo: Bjarne Røsjø/UiO Download picture.

In his 2011 book, The Better Angels of Our Nature, the internationally famous psychologist and popular-science author Steven Pinker looks at history over several millennia and observes that the world has become gradually more peaceful. He has also described the period since World War II as “the Long Peace”, due to the decline in the number of interstate wars.

But now Pinker seems to be having second thoughts, after reading a blog post written by Professor Nils Lid Hjort of the Department of Mathematics at the University of Oslo (UiO), following a seminar at the Peace Research Institute Oslo (PRIO).

Céline Cunen, a PhD candidate at the Department of Mathematics, was responsible for some of the analytical work that forms the basis for Hjort’s blog post. But neither Cunen nor Hjort could have anticipated Pinker’s reaction: He was very excited about the blog and posted a brief tweet – to his 367,000-plus Twitter followers – praising the new analysis. He also sent an email describing the two researchers’ analysis as “fascinating and sophisticated” and their message as “beautifully presented”.

Steven Pinker thinks that it is interesting to consider 1965 as a possible starting point for the historical change he has identified, which has led to a reduction in the number of people killed in interstate wars.

 Pinker’s 2011 book is about much more than just the period since World War II. He argues that there has been a continuous decline in the relative levels of virtually all types of violence, and he writes of a “humanitarian revolution”, driven by democracy, trade and information.

Professor Michael Spagat, a British economist who has published a number of scholarly articles on armed conflict and who authors the blog War, Numbers and Human Losses(link is external), is also very enthusiastic. “That’s such a good article. Really informative and spectacularly well written,” wrote Spagat in an email to his contacts at PRIO.

“Starstruck” statisticians

Steven Pinker

Steven Pinker wrote that 1965 is a very interesting starting point for The Long Peace. (Picture from Wikimedia Commons)

As well as the scholarly content of Hjort’s blog, both Pinker and Spagat are enthusiastic about the way it is presented, putting heavyweight academic argument alongside references from popular culture. Indeed, the very first link in the list of references takes one to YouTube and a recording by “Boys, B.”, which presents the highly relevant message “Wouldn’t it be nice if”.

Wouldn't it be nice if we were older

Then we wouldn't have to wait so long

And wouldn't it be nice to live together

In the kind of world where we belong

Both Cunen and Hjort are flattered, and even a little starstruck, by this overwhelming international attention, and they are already at work on a paper for formal publication. But – what is it they've actually done?

“It began when one of Steven Pinker’s leading critics, the computer scientist Aaron Clauset from the University of Colorado in the United States, was due to visit Oslo to give a lecture at a seminar organized by PRIO and UiO,” explains Professor Hjort. “The two institutions had recently embarked on a partnership with the goal – as stated in the letter of intent – of furthering Oslo’s role as a global knowledge centre for the prevention and resolution of armed conflict.

Aaron Clauset has analyzed data concerning all interstate wars from around 1820 until today, and has concluded that the Long Peace has not endured long enough to be considered permanent. According to Clausen, this peaceful period needs to last perhaps another 100 or 150 years before it is possible to say that the world has undergone lasting change.

The most obvious changepoint is in 1965

Hjort’s original plan was for his lecture at the seminar to be of a rather general nature. He wanted to comment on various aspects of Clauset’s analysis and criticism of Pinker’s work, but without “getting into the data”. Nevertheless, some days before the seminar, he looked at the data and existing models and realized that there might be more to uncover.

He spoke to several of his PhD students, especially Céline Cunen, about the topic and the more detailed analysis. Cunen had previously used statistical methods to investigate several historical topics, including a comparison of the Wars of the Roses and the TV series Game of Thrones. In addition, Cunen and Hjort have developed a particular method for running a change-point analysis. In this they have been assisted by Gudmund Hermansen, one of Hjort’s postdoctoral fellows.

“I offered immediately to do this change-point analysis,” explains Cunen. “We got hold of PRIO’s data for all major interstate wars from 1823 until today. We saw fairly quickly that the clearest change-point occurred in 1965. The number of people killed in interstate wars is lower after 1965 than before 1965.”

Pinker tweeted an hour-and-a-half later

Steven Pinkers tweet

Steven Pinker's tweet an hour and a half after discovering Nils Lid Hjort's blog post.