Is it human nature to show restraint today, so that strangers might benefit tomorrow? Well, that depends on the human.
Researchers studied this question by telling subjects that they and four other players were allowed to take up to $20 from a shared pot of $100, and that if they collectively took less than $50, the next round of players would receive $100 as well.
It turns out that most people will restrain themselves. But a small percentage of people will always take more than their fair share - not because they want more than other players, or because they cannot delay gratification, but because these few players assume that others will try to take advantage of them. It's a sort of defensive greed.
The researchers hoped to understand whether humans are prone to make sacrifices in order to share with hypothetical people of the future. They thought it might reflect important real-world situations, such as limited fisheries that would collapse if people caught as much as they could sell. They also thought it applied to climate change, given that climate scientists agree that to keep global temperatures from wreaking havoc in the future, people must immediately begin to use much less fossil fuel than our planet provides.
The Paris Agreement - from which United States President Donald Trump recently announced the US would withdraw - was set up to curb the use of fossil fuels, with the goal of preventing global temperatures from rising more than 2 deg C above pre-industrial levels.
Do people have the kind of restraint it would take to cut back for the sake of future generations? The experiment would suggest that individually, most of us do, but collectively, we do not - because the few "defensively greedy" people take so much that they negate others' sacrifices.
About two-thirds of players took only $10, which is a fair share of the $50 sustainable limit when there are five players. But about a third took the maximum allowed, $20. Even if four players restrained themselves and took only $10, that fifth person grabbing the full $20 would send their total over the $50 sustainable limit. (Details were published in a 2014 Nature paper, Cooperating With The Future.)
The experiment came up in a conversation I had in April with its lead researcher, Harvard mathematics and biology professor Martin Nowak. He said people who took the maximum amount assumed others would not bother to restrain themselves. It does not look like our old conception of greed as acquisitiveness. It is more a product of fear and a fragile ego; the people who took the full $20 were afraid to play the chump.
Compare this to what Mr Trump said when he announced the US would withdraw from the Paris climate agreement: "This agreement is less about the climate and more about other countries gaining a financial advantage over the United States." (The agreement was non-binding, so it is true others could do nothing while remaining part of the pact, but then, so could the US - not much of a "financial advantage".) This kind of behaviour is called conditional cooperation.
Harvard behavioural scientist Oliver Hauser, who was one of the paper's co-authors, said people are more likely to cooperate if they know everyone else is doing the same. That makes perfect sense. If you know that even one other player is taking the full $20, and you assume others will take at least $10, there's no reason for you to take less than $20. For you to show restraint would not benefit anyone.
However, researchers found they could create a sustainable system when people got together and voted on the amount they should each take. When there was voting, volunteers almost universally agreed to take only $10, and the game then sustained itself indefinitely. In each round, players took only $50.
Oxford University evolutionary biology professor Stuart West has done similar studies in humans and other animals, and cautions that people tend to want to find evidence that humans are more altruistic than other species. But it is not the case, he said. Ants and slime moulds are more prone to self-sacrifice for the good of the group.
Animals, including humans, can be good citizens or jerks. Some birds will fake a distress call in order to get everyone else out of the way and eat all the food. That behaviour can pay off as long as only a few birds do it.
In a recent paper, Professor West and his colleagues showed that humans will sometimes make short-term sacrifices to cooperate as a way to angle for alliances that might serve them in the future. So there can be a selfish side to what at first looks like selflessness. For many of us, there are social rewards to expressing support for the Paris Agreement - and, as a bonus, there is no apparent cost to doing so.
The question now is how steep the price will be for those humans who are too fearful and defensive to cooperate.