We often think of playing fair as an altru­istic behavior. We're sac­ri­ficing our own poten­tial gain to give others what they deserve. What could be more self­less than that? But new research from North­eastern Uni­ver­sity assis­tant pro­fessor of phi­los­ophy Rory Smead sug­gests another, darker origin behind the kindly act of fairness.

Smead studies spite. It's a conun­drum that evo­lu­tionary biol­o­gists and behav­ioral philoso­phers have been mulling over for decades, and it's still rel­a­tively unclear why the seem­ingly point­less behavior sticks around. Tech­ni­cally speaking, spite is char­ac­ter­ized as paying a cost to harm another. It yields vir­tu­ally no pos­i­tive out­come for the per­pe­trator. So why would evolution—which is sup­posed to weed out such behaviors—let spite stick around?

Smead's research, con­ducted in col­lab­o­ra­tion with Patrick Forber of Tufts Uni­ver­sity and recently pub­lished in the journal Pro­ceed­ings of the Royal Society B, sheds new light on this nefar­ious phenomenon.

A common means of studying social behav­iors is through sim­pli­fied models and games. One of these is called the ulti­matum game, in which a one player pro­poses a divi­sion of resources the other player can either accept or reject. Sup­pose each inter­ac­tion con­cerns the dis­tri­b­u­tion of 10 one-​​dollar bills. The first player could sug­gest that he take $5 for him­self and give the remaining $5 to the second player. That would be a fair play.

How­ever, that first player could also go for an unfair option in which he keeps $9 for him­self and gives just $1 to the second player. While the second player is worse off if he rejects the pro­posal (he's got ziltch in his pocket instead of $1), he almost always does so in real-​​world ver­sions of the game: It's just not fair.

But when Smead and his col­leagues decided to sim­u­late this game math­e­mat­i­cally to see how it would play out, they found that in fact the exact oppo­site hap­pens. Fair­ness usu­ally gets flushed out of the system since it's more ben­e­fi­cial for both the first player (the pro­poser) to sug­gest unfair offers and for the second player (the responder) to accept them.

"Evo­lu­tionary models don't match what we're observing in real life," Smead said. Clearly, he thought, there must be some­thing else going on.

In the new study, Smead and Forber con­sid­ered that the ulti­matum game is actu­ally quite unlike the real world. It's an extremely sim­pli­fied sim­u­la­tion of one of infi­nite ways that two indi­vid­uals could act. The researchers couldn't, for obvious rea­sons, make the game as com­plex and nuanced as real world social inter­ac­tions, but they could instead just add a little more nuance to it and see what happened.

So that's what they did. In their new ver­sion of the game, the researchers intro­duced some­thing called "neg­a­tive assort­ment." Think of assort­ment as the like­li­hood that a person you're inter­acting with is sim­ilar to you. In neg­a­tive assort­ment, that like­li­hood is low, so in the ulti­matum game the players would likely use dif­ferent strategies.

Here's where spite comes back into play. If you and I both commit to just making fair offers, but my strategy is to accept all offers—be they fair or unfair—and yours is to accept only fair ones, we are dif­ferent. A spiteful strategy would be to both make only unfair offers, but reject such offers when they come from the other person.

In the orig­inal ver­sion of the ulti­matum game, a spiteful player will usu­ally walk away with nothing and for­feit the game. But with neg­a­tive assort­ment, spite becomes common and actu­ally ends up pro­moting fair­ness. "Acting fairly pro­tects you from spite," Smead explained.

Think of it this way. A "gamesman" is someone who only makes unfair offers to ben­efit him­self but accepts what­ever comes his way because he believes it'll all wash out in the end. "Gamesmen become a target for spite because they're making unfair offers," Smead said. The "spiters" will reject those offers, even­tu­ally killing off the gamesmen.

But fair players will now do quite well in the pres­ence of spite. Since they don't make unfair offers, they don't risk being rejected by the spiteful players. Fair­ness actu­ally becomes a strategy for sur­vival in this land of spite.

"Real social life is com­pli­cated," Smead said. While his new ver­sion of the ulti­matum game is still a sim­pli­fi­ca­tion, it illu­mi­nates another pos­sible expla­na­tion for fair behavior that hadn't been con­sid­ered before.