To answer that question, Jayachandran and her colleagues traveled to western Uganda, whose forests are home to chimpanzees and gorillas, and whose deforestation rates are the third highest in the world. In 2011, they selected 121 villages and offered a PES scheme to half of them, chosen at random. This experiment marks the first time the approach has ever been tested in a randomized trial, and its results were encouraging. Over two years, the program managed to halve the amount of fallen forest near the villages that participated in the scheme, compared to those that didn’t.

The program was also cheap and cost-effective. It took just $20,350 to pay all the enrollees, who (on average) bolstered their annual income by $56 a year—around 10 to 20 percent. “That’s the magic of the program,” says Jayachandran. “It’s enough money that there are financial pressures to not cut the forest, but cheap enough that wealthy countries can pay for it.”

“Sometimes, people ask why the government doesn’t just buy up the forests and make them into reserves,” she adds. “But people have their homes there. They’re integrated with the forests, so you have to figure out a way to let people live their lives.”

Other scientists found similar results when evaluating Mexico’s PES program after it had launched: There, payments also halved the loss of tree cover. “It is immensely useful to know that those results hold up in a fully randomized setting” and in a different continent, says Jennifer Alix-Garcia from the University of Wisconsin-Madison, who was involved in the Mexico analysis. “It suggests the existence of an effective environmental policy that can be applied in challenging institutional settings without adverse effects on households. What could be better?”

“I’m happy to see PES getting increased attention,” adds Katharine Sims from Amherst College, who was also involved in the Mexico study. “It has emerged as a key strategy for global forest conservation [and ] provide an important way of better aligning individual and social values.”

A local non-profit—the Chimpanzee Sanctuary and Wildlife Conservation Trust—designed and administered the program. Its staff traveled to the targeted villages, talked to their leaders, and offered them a contract that forbade them from cutting down mature trees, in exchange for an annual payment of $28 per pristine hectare. If they agreed, the team did on-the-ground spot-checks to make sure that that they were holding up their end of the deal, and assessed any differences in tree cover using satellite images.

On average, the team found that tree cover fell by 4.2 percent in the villages that were invited to take part in the PES scheme, and by 9.1 percent in the business-as-usual group. Best of all, the team found no evidence that the villagers were gaming the system. “You might expect that the people signing up in droves are the ones who were planning to conserve trees anyway,” says Jayachandran. But, in fact, the enrollees’ past behavior suggested that they would actually have cut down more trees than the typical landowner.