From Ukraine, Syria and Gaza to the centenary of the First World War in 2014, news junkies and students of history cannot help but wonder if war is a perpetual feature of civilization. German philosopher Immanuel Kant wondered as much in a 1795 essay entitled “Perpetual Peace,” concluding that citizens of a democratic republic are less likely to support their government in a war because “this would mean calling down on themselves all the miseries of war.” Ever since, the “democratic peace theory” has had its supporters. Rutgers University political scientist Jack Levy, in a 1989 essay on “The Causes of War,” reasoned that the “absence of war between democratic states comes as close as anything we have to an empirical law in international relations.” Skeptics point out such exceptions as the Greek and Punic wars, the War of 1812, the U.S. Civil War, the India-Pakistan wars and the Israel-Lebanon War. Who is right? Can science answer the question?

In their 2001 book Triangulating Peace, political scientists Bruce Russett and John Oneal employed a multiple logistic regression model on data from the Correlates of War Project that recorded 2,300 militarized interstate disputes between 1816 and 2001. They assigned each country a democracy score between 1 and 10, based on the Polity Project, which measures how competitive its political process is, as well as the fairness of its elections, checks and balances of power, transparency, and so on. The researchers found that when two countries scored high on the Polity scale, disputes between them decreased by 50 percent, but when one country was either a low-scoring democracy or an autocracy, it doubled the chance of a quarrel between them.

Kant also suggested that international trade (economic interdependency) and membership in international communities (transparency and accountability) reduce the likelihood of conflict. So in their model Russett and Oneal included data on the amount of trade between nations and found that countries that depended more on trade in a given year were less likely to have a militarized dispute in the subsequent year. They also counted the number of intergovernmental organizations (IGOs) that every pair of nations jointly belonged to and ran a regression analysis with democracy and trade scores. Overall, democracy, trade and membership in IGOs (the “triangle” of their title) all favor peace, and if a pair of countries are in the top 10th of the scale on all three variables, they are 81 percent less likely than an average pair of countries to have a militarized dispute in a given year.

How has the democratic peace theory held up since 2001? With all the conflict around the world, it seems like peace is on the rocks. But anecdotes are not data. In a 2014 special issue of the Journal of Peace Research, Uppsala University political scientist Håvard Hegre reassessed all the evidence on “Democracy and Armed Conflict.” He stated that “the empirical finding that pairs of democratic states have a lower risk of interstate conflict than other pairs holds up, as does the conclusion that consolidated democracies have less conflict than semi-democracies.” Hegre is skeptical that economic interdependence alone can keep countries from going to war—the “Golden Arches Theory of Conflict Prevention” popularized by Thomas Friedman's observation that no two countries with McDonald's fight—unless their economies are in democratic nations. He wonders, reasonably, if there might be some other underlying factor that explains both democracy and peace but does not suggest what that might be. I propose human nature itself and our propensity to prefer the elements of democracy. Peace is a pleasant by-product.

Whatever the deeper cause may be, the long-term trends are encouraging. According to Freedom House, there were no electoral democracies (with universal suffrage) in 1900, 69 in 1990, and 122 in 2014—63 percent of the 195 countries in the world. That's moral progress. The other 37 percent—particularly the theocratic autocracies desirous of thermonuclear weapons and bent on bringing about Armageddon—means we must remain vigilant. Otherwise we run the risk that Kant's perpetual peace will dissolve into the source of his essay title's inspiration: an innkeeper's sign featuring a cemetery. This is not the type of perpetual peace toward which most sentient beings strive.