They found something surprising: One of the best predictors of a peace agreement’s success is simply whether the parties had prior agreements, even if those earlier cease-fires failed. Not even a war’s duration or its intensity can so reliably predict a peace deal’s outcome. Neither does the poverty or ethnic diversity of the combatants.

“Failures pave the way for better agreements down the road,” Professor Quinn said.

Over time, participants see cease-fires as less risky. If all sides come out feeling that they at least broke even, they grow more willing to make another deal. In Syria, with the status quo so terrible, breaking even doesn’t require much.

“These items could be as simple as a request that Assad refrain from calling opposition members terrorists in the press,” Professors Joshi and Quinn wrote in a Foreign Affairs article summarizing their research. “As soon as one party reciprocates, a peace process is underway. And with each successful round, just enough trust and good will may be generated to move on to the next item.”

This is trust not in the colloquial sense of proving personal integrity, but in the political science sense: Negotiators believe they understand one another’s incentives and can predict their behavior. Each side becomes more willing to make concessions, believing the other side will deliver on its end.

Take, for example, Yugoslavia, where there were 91 mediated truces or cease-fires from 1989 to 2000. Of those, 35 percent lasted less than a week and 13 percent lasted exactly a week.

Though each appeared to be a catastrophic failure, they culminated in the 1995 Dayton Accords, which ended the Bosnian war that was a subset of the larger conflict, as well as later deals.

We are already seeing possible hints of this in Syria. The tempo of cease-fires is increasing, with the terms expanding and the outside actors investing more political capital. Those gains are slight and the process of building trust is still fragile, so it remains unclear whether the cycle will catch.