Living in big cities among strangers can make people quickly 'switch off' their tendency to be generous and fair to others, especially when such cooperative behaviour would not directly benefit them.
Researchers from the University of Miami in the US suggest that although humans learned long ago to instinctively be generous and fair to others, they can quickly unlearn that cooperative behavior when encountering strangers.
The study, published in the journal Nature Human Behaviour, 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 and we never knew who we might need to help us. Over time, we automatised the decision to be kind out of self-interest," said William H B McAuliffe, a PhD candidate at University of Miami.
"We have a natural karma built into us because our minds have evolved into thinking that what goes around really does come around," said McCullough.
However, the study 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 will not 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 behaviour 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.
However, 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.
However, on their return visit about a month later, they weren't as generous, sharing, on average, about 20 per cent less.
"After acclimating to the situation, they realised this was extraordinarily different from the situations they find themselves in everyday life," McAuliffe said.
"So, when they come back, they don't act on that cognitive shortcut because they've learned that the same rules don't apply," he said.
McCullough 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.