“What we found is an alternative evolutionary path towards fair behavior,” said study researcher Patrick Forber, a philosopher at Tufts University in Medford, Mass. …

Under these mixed-up conditions, spite evolved — much to the researchers’ surprise, Forber said. And with spite in play, strange things started happening. First, rational and fair players disappeared. Spiteful players rejected rational players’ unfair offers, essentially spiting them out of the game. Fair players got duped by spiteful players, who always took their nice offers, but never returned the favor.

Only one type of player could survive the onslaught of spite: the easy rider. These players made fair offers, so spiteful players had no cause to punish them. But they also accepted what they could get from the spiteful, which kept them in the game.

The findings are theoretical, but they open up the question of how fairness evolved in humans, Forber said. (And in chimpanzees, which also value fairness in the Ultimatum Game.)

“It could be that [fairness] wasn’t a solution to solving problems of cooperation,” he said. “Instead, it was a solution of handling these anti-social types.”