I’ve just posted a new paper with Stijn van Weezel on the decline-of-war debate. This is part of an ongoing discussion which involves such people as Steven Pinker, Nassim Nicholas Taleb and Aaron Clauset.

Here is our abstract:

For the past 70 years, there has been a downward trend in the size of wars, but the idea of an enduring ‘long peace’ remains controversial. Some recent contributions suggest that observed war patterns,including the long peace, could have come from a long-standing and unchanging war-generating process, an idea rooted in Lewis F Richardson’s pioneering work on war. Aaron Clauset has tested the hypothesis that the war sizes after the Second World War are generated by the same mechanism that generated war sizes before the Second World War and fails to reject the ‘no-change’ hypothesis. In this paper, we transform the war-size data into units of battle deaths per 100,000 of world population rather than absolute battle deaths – units appropriate for investigating the probability that a random person will die in a war. This change tilts the evidence towards rejecting the no-change hypothesis. We also show that sliding the candidate break point slightly forward in time, to 1950 rather than 1945, leads us further down the path toward formal rejection of the no-change hypothesis. Next, we expand the range of wars to include not just the inter-state wars considered by Clauset (2018) but also intra-state wars. Now we do formally reject the no-change hypothesis. Finally, we show that our resultsdo not depend on the choice between two widely used war datasets.

Here is one of the key figures from the paper.

The X axis is war sizes expressed as the number of battle deaths divided by world population at the time they occurred. The Y axis for the figure on the left gives p values for tests of the hypothesis that war sizes after 1945 are generated in the same way that wars sizes before 1945 were generated. The Y axis on the right gives the same information but using 1950, rather than 1945, as the break point.

We can read off the picture on the right, for example, that wars killing more than 45 people per 100,000 of world population after 1950 have been much less common than such wars were before 1950. Indeed, the estimated probability that the pre-1950 war generation mechanism continued to operate after 1950 for wars of sizes above 45 per 100,000 is only around 0.05. So we can even formally reject a hypothesis that nothing changed after 1950 for wars of sizes 45 per 100,00 and above.

Please have a look at these slides for a more leisurely development of what I just said in the last two paragraphs. The slides explain all the key ideas in the paper. The only limitation of the slides is that they do not separate out the relative importance of each step in the argument.

There are two broad take-home points from this work:

The evidence is fairly strong for a decline in war thesis We should not focus exclusively on 1945 as the only potential break point.

On point 2 I know of two separate research teams who have papers specifically on the change-point issue. I believe that neither of these papers are yet in the public domain but I expect that they will be soon.

This topic is heating up!