Would you tip your waitress if you knew you’d never return to her restaurant? Probably, because that’s how most of us are socialized. But what if you knew the waitress would never know if you left a tip? Without the incentive of her approval, would you still be generous?

Researchers in the University of Miami’s Evolution and Human Behavior Laboratory who set out to answer that question found that we humans, who learned long ago to instinctively be generous and fair to others, can quickly unlearn that cooperative behavior when encountering strangers if we know we won’t benefit from our actions.

Lead author William H.B. McAuliffe, a Ph.D. candidate in psychology, and senior author Michael E. McCullough, professor of psychology, say their study, published October 22 in the journal Nature Human Behavior, supports the theory that our ingrained cooperative spirit is a remnant of our evolutionary past. When we lived in small groups, we knew every person in our social circle—or someone who knew them—and we never knew who we might need to help us. Over time, we automatized the decision to be kind out of self-interest.

“We are actually walking around with Stone Age minds,” said McCullough, director of the lab in the College of Arts and Sciences Department of Psychology. “Our minds still think how we treat everyone we meet could have consequences—that everyone we run across and are either mean to or nice to will somehow pay us back. We have a natural karma built into us because our minds have evolved into thinking that what goes around really does come around.”

But their study, “Experience with anonymous interactions reduces intuitive cooperation,” shows that the “cognitive shortcut” we have built into our brains to be generous or fair can be easily switched off if we learn there won’t be any payback, either positive or negative. The researchers demonstrated this point by exposing 200 volunteers to a social environment devoid of any incentive or punishment for how they treated others, and tracking how their behavior changed over time.

The volunteers, who came to the laboratory in small groups on two separate occasions about a month apart, were asked to play three games that required them to make decisions about investing money and sharing the windfalls with others in the room, and eventually with a charity. But, sitting at consoles with headphones, the participants did not interact with each other. They made all their decisions and collected all their winnings anonymously and privately.

During the first round, the study showed, participants behaved predictably: Acting on habits shaped by their everyday experiences, they split windfalls with strangers fairly and shared about half their earnings with charity. But on their return visit about a month later, they weren’t as generous, sharing, on average, about 20 percent less.

“After acclimating to the situation, they realized this was extraordinarily different from the situations they find themselves in everyday life,” McAuliffe said. “They realized, ‘What I do doesn’t really matter. It has no social consequences. Nobody is going to pat me on the back if I am generous. No one is going to think I’m stingy if I’m not.’ So, when they come back, they don’t act on that cognitive shortcut because they’ve learned that the same rules don’t apply.”

McCullough, who has devoted his career to shedding light on human behavior by examining our evolutionary past, said the study could explain why big-city dwellers have a reputation for being more hurried and less friendly to strangers than small-town folk.

“I think what this study says isn’t that generosity towards strangers is part of what humans evolved into, but instead that we evolved in a world where there really weren’t strangers,” McCullough said. “We knew everybody. They knew us, and if we didn’t know everybody directly, we knew somebody they knew, so if we were bad to someone they could say, ‘That is a terrible person.’ Now we live in cities with millions of people and you can legitimately encounter a stranger and say ‘I’ll never see that person again—and get away with treating them poorly.’ That’s less so in small towns, where almost everybody does know everybody.”

In addition to McAuliffe and McCullough, the study co-authors include recent UM psychology graduates Daniel E. Forster, now with the U.S. Army Research Laboratory, and Eric J. Pedersen, now with the Department of Psychology and Neuroscience at the University of Colorado Boulder.