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Science in Mind

Why are people willing to help future generations?

Scientists examine how sacrifices for the future are weighed

Why do people make sacrifices today that benefit future generations?

The question underlies environmental policies of all kinds, but scientists don’t fully understand the motivations that would lead people to cut emissions today to prevent catastrophic consequences of climate change after we are dead. Why bother?

Drawing on wide-ranging expertise in evolutionary biology, psychology, math, and economics, a team from Harvard and Yale universities has crafted an experiment that begins to unravel how people weigh such decisions.

The results, published Wednesday in the journal Nature, reveal some of the important elements of systems that enable people to act for the greater good, and have important implications for policy makers. They help explain, for example, why the Kyoto global warming treaty was ineffective at reducing global carbon emissions.


The study shows how easily the generous intentions of a majority can be disrupted if rules and institutions are not in place to make sure that everyone sacrifices — a few cheaters can ruin it for everyone.

“The question is a very important one for how humans, as a species, maintain what is given to them,” said Martin Nowak, a mathematician at Harvard University who led the study. “In some ways, I believe intelligent life on earth requires the ability to learn to cooperate with future generations.”

What Nowak’s study found is, in part, discouraging: If groups of people were left up to their own devices to divvy up a resource, they exhausted it completely in just three generations. But impose a democracy where the votes of the majority bind everyone to some common behavior, and the resource remained intact indefinitely.

Earlier research on the puzzling phenomenon of cooperation had revealed how surprisingly complex it is, and revised the longstanding image of people as cold, calculating, and self-interested. An implicit assumption about why cooperation happens so frequently, however, was that such behavior would have rewards for the individuals involved — either through direct payback or through the building up of reputation.


“When I and other people study cooperation, we have this idea built into it that the reason I cooperate with you is that down the road you’ll treat me fairly,” said David DeSteno, a psychology professor at Northeastern University who was not involved in the research. But in the case of posterity, that question was less clear. Short of time travel, future generations have no way to pay us back, and would the same motivations for cooperation exist when the recipients of the generosity were merely hypothetical?

To examine cooperation with future generations, the researchers recruited participants online. Volunteers were divided into groups of five and told that there was a common pool of 100 units of an unnamed material and they could each take up to 20 units. As long as they collectively left 50 in the pool, the resource would be fully replenished back to 100 units, allowing the “next generation” — another group of five people — to play.

Although the majority of players chose to harvest a sustainable amount from the pot — up to 10 units each — there were always defectors who took more, meaning within a few short generations there would be nothing left for future game players. However, when the researchers had the players vote on how much to take and the middle value was imposed on the whole group, the resource was sustainable over at least 12 generations.


The researchers also tried a “partial voting” scenario in which three of the players voted and the vote applied to them, but two other people could make up their own minds. That approach failed to keep the resource intact.

That, Nowak suggested, could give some insight into why some nonbinding international agreements with good intentions will fail. For example, the Kyoto Protocol set targets for emission reductions but depended on countries deciding to join it. The findings also suggest that in the United States, politicians may need to have more faith in people’s willingness to make sacrifices for the future, Nowak said. We may be willing to pay higher taxes or change our lifestyles as long as we don’t feel others are getting away with something.

Jochem Marotzke, a director at the Max Planck Institute for Meteorology in Germany, said the study was compelling and explains why policies may work within a country but may not solve global problems.

“When we try to apply this to the climate change issue, there’s no world democracy, there is no world government” to ensure all countries make sacrifices, Marotzke said.

In his own work, Martozke said he has sometimes seen “very, very little willingness to think of the next generations.”

Kimberly Wade-Benzoni, associate professor of management at the Fuqua School of Business at Duke University, has found that simple interventions, such as subtly reminding people of their own mortality or asking them about their legacy, can make people more generous to future generations.


But her work may also point to an inherent challenge.

“Uncertainty about the future is one of the barriers to intergenerational beneficience,” Wade-Benzoni said. “One of the things that gets in the way is people say, ‘We don’t know what the future is going to look like. We don’t know if they’re even going to like trees. We’ll have technology to solve the problems.’ ”

Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.