The idea that war is on the decline — that is, that there are fewer wars today and fewer people are dying from them than ever before — is hard for a lot of people to believe (including Republican presidential candidates).

And yet the data makes a very compelling case that that's true:

Those numbers were put together by Steven Pinker, a Harvard psychologist whose book The Better Angels of Our Nature makes the strongest case yet that the world is getting progressively more peaceful. Pinker's argument has come under fire recently, with some arguing that it's way too soon for anyone to say we've turned the corner from an era of war.

I spoke with Pinker this week to discuss some of the reasons why, specifically, he thinks the world has gotten so much safer, especially in the past 70 years. We talked about the idea that war just isn't as profitable as it used to be, why Vladimir Putin and ISIS seem to think differently, and what world leaders should do if they actually want to make sure the unprecedented peace of the past 70 years holds. What follows is a transcript of our conversation, lightly edited for length and clarity.

Zack Beauchamp: One story you hear from political scientists for why there's been less war recently that it's just less profitable —countries don't gain very much, economically or politically, from taking over new land anymore. Does that seem right to you?

Steven Pinker: Yes, it's one of the causes. It's the theory of the capitalist peace: when it's cheaper to buy things than to steal them, people don't steal them. Also, if other people are more valuable to you alive than dead, you're less likely to kill them. You don't kill your customers or your lenders, so the arrival of the infrastructure of trade and commerce reduces some of the sheer exploitative incentives of conquest.

This is an idea that goes back to the Enlightenment. Adam Smith and Montesquieu extolled it; it was on the minds of the founders when they built incentives for free trade into the Constitution.

I don't think it's the entire story of the decline in war. But I do think it's part of the story. There was a well-known study from Bruce Russett and John Oneal showing statistically that countries that engage in more trade are less likely to get into militarized disputes, and countries that are more integrated into the world economy are less likely to get into trouble with their neighbors.

ZB: Is it just that pairs of countries are trading with each more, or has something fundamentally changed about the global economy?

SP: It's both. Countries that trade with each other are less likely to pick fights with each other. Independently, individual countries that get more integrated into the global economy are less likely to make trouble.

But one of the reasons I say this is only part of the answer is that in the Oneal and Russett analysis, it counted for some percentage of the variance in militarized dispute, but only a chunk. They found independent contributions from democracy and membership in the international community (namely, the number of international organizations and treaties that a country has signed on to).

Quite across from calculations of interest that are tilted by international markets and institutions, there's the idea of norms: what you consider and don't consider as a legitimate possible move. [Some scholars] argue that the main factor is that war has been delegitimated — at least among the great powers and the developed states — as a thinkable option.

In the 19th century, there was this cliché from [Carl] von Clausewitz that war was just the continuation of politics by other means: you consider whether to go to war [like any other policy option]. Now that's just not something decent leaders do.

Finally, cost-benefit calculations depend on what counts as a "cost." If you lose several tens or hundreds of thousands of your own citizens, is that a cost? And how big a cost is it? Now, increasingly, that counts as a cost: leaders are less likely to see their young men as cannon fodder, which means countries are willing to endure other costs to avoid that one. That's a result of the rise of humanistic sentiments, as opposed to nationalistic or ideological ones.

ZB: Russia under Vladimir Putin seems to be a clear exception to this pattern — clearly, Putin thinks waging war in Ukraine is worth the cost in international sanctions and opprobrium, not to mention lives lost. Why do you think that is?

SP: I think there's been backsliding. Gorbachev clearly felt it: that's why he didn't call out the tanks when the Berlin Wall fell. He clearly wanted to avoid military confrontations that would result in hundreds and thousands of deaths.

Putin is definitely backsliding, and he's quite explicit about it. He places a high value on recapturing Russian national grandeur. That's a value that obviously can be at odds with preservation of lives.

ZB: So the point here is that what's "rational" for a leader to do depends on what they want to accomplish.

SP: Rational analysis of costs does not designate what the costs and benefits are. Let's say one of the benefits is national glory, and one of the costs is the lives of your citizens or, even more poignantly, the citizens of another country. How you weight the value of glory versus lives is going to affect you how think about the rational cost-benefit analysis.

ZB: This seems like a major problem in the way we talk about the causes of war in public discourse. We call Iran, Russia, or ISIS "irrational," but that's not really what's going on. Instead, they're acting based on a very different set of cost-benefit calculations than what we're used to.

SP: Yes, but I'd add that the analysis of values itself can be put under a rational spotlight. It's not as if values are exclusively a matter of taste and whim. I think one can say "are these values consistent with other values you claim, such as respect for your life and your family" and "are your factual beliefs about the coming of the messiah able to stand up to scrutiny," but it's a separate discussion.

ZB: Earlier you mentioned international institutions, like the UN, as things that seem to put a damper on violence. Is there a way to strengthen these institutions — or do they not matter? Is it just that the countries that tend to join a lot of these organizations are already democratic and integrated into the global economy?

SP: Whether you can actually [promote peace by getting countries to join organizations] is a really good question, and I suspect the answer is yes. The more you can jawbone countries to join international organizations and the international community, or get them to support these organizations financially, the greater the prospects will be for peace.

In particular, there are analyses that show that peacekeeping forces, whether they're blue-helmeted UN soldiers or more ad hoc coalitions, do tend to have a measurable effect in reducing the likelihood of a [newly peaceful] country's recidivism back into war. They don't work all the time, and there have been some famous failures, but to the extent that they're actually supported, that the world community arms them and trains them, they do have a beneficial effect.

ZB: In broad strokes, then, things are moving in a positive direction: the conflicts we see are terrible, but they're not fundamentally upsetting the longer trends toward peace. And yet that fact isn't transforming the American public discourse. When look at the Republican presidential campaign, for example, you have people saying, "We have never seen more threats against our nation and its citizens than we do today." Why do you think it's so hard to convince people that things are all right?

SP: Well, there is the peculiarity of the United States.

It's an outlier among Western democracies along a number of dimensions: the US has a higher rate of violent crime, it gets involved in more wars, it continues to have capital punishment, [and] has high rates of religious belief compared to other Western democracies.

Now, the US is a complex, heterogeneous country. But the more populist southern and southwestern areas are less shaped by the Enlightenment and more by a culture of honor: there are threats, and moral virtue consists in having the resolve to deal with them. A "manliness versus cowardice" mindset.

"The more populist southern and southwestern areas are less shaped by the Enlightenment and more by a culture of honor"

On top of that American peculiarity, the general style of punditry and analysis both in journalism and the government is event- and anecdote-driven, rather than trend- and data-driven. And we know from cognitive psychology — Daniel Kahneman and others — that people are overly impressed by big, noisy, memorable events as compared to slow, systemic trends. The natural tendency is to go with what you read this morning.

The United States is also in the unique circumstance of having such outsize military power that it has the dual demands of protecting its own interests globally but also being seen in the role of "global policeman." It's the only single country that can do that, but it has no official mandate for doing it.

ZB: So does that mean you're skeptical of America's global military presence, which some argue is a key reason the world is so peaceful today?

SP: It's not so much that I would personally argue for isolationism. But nor can we simply assume that the US always acts disinterestedly or wisely — the historical record is pretty clear on that.

So I'd like more thoughtful deliberation on America's role, and a strengthening of international institutions. The US thinking it can just go it alone and defy the rest of the democratic community has been shown to be a mistake.