Humans walk away from an unfair offer rather than accept an unequal share, a new study exploring the perception of fairness in humans found.

Humans have an acute sense of fairness and would readily reject an offer if the share offered to them was unequal. Researchers from Wellcome Trust Centre for Neuroimaging at UCL found that even when bargaining for basic needs, humans can walk off from an unequal offer. They also found that sometimes, self-interest can offset the need for a fair play in humans.

Chimpanzees, our closest relatives, are known to take up offers that are highly unequal because they have no sense of fairness.

For the study, they recruited 21 healthy participants. 11 of these participants were kept on salt drips to increase their thirst and others were kept on isotonic drip (that wouldn't increase thirst).

Salt concentration in the blood was measured to assess the thirst level of the participants. To get a subjective view about their thirst, researchers gave them a questionnaire and asked them to rate their thirst.

Participants were then asked to play an ultimatum game. They were told that two of them would be the 'proposer' and the rest would be 'responders.' The proposer would get the chance to split the 500ml water in any way they like and the responder had to say whether they accept the deal or not. Responders had 15 seconds to make the decision. They were also aware that they would have to wait for an hour more if they rejected the deal.

During the game, the responders were shown two glasses of water; one with 62.5 ml (for the responder) while the other with 437.5 ml (for the proposer). At least half of their participants were objectively thirsty because they were on a saline drip. Researchers purposefully showed the glass of water so that they could trigger the appetite in the participants. To reject an unfair deal, participants had to overcome a physiological urge to drink water.

The research team found that humans would reject an unequal offer unlike chimpanzees who'd take up an offer even when they got unequal share.

"Whether or not fairness is a uniquely human motivation has been a source of controversy. These findings show that humans, unlike even our closest relatives chimpanzees, reject an unfair offer of a primary reward like food or water - and will do that even when severely thirsty," said Dr. Nick Wright, lead author of the study.

Participants in the study were more likely to take up an unequal share if their subjective idea of thirst was greater than people who believed that they weren't severely thirsty.

"These findings are interesting for understanding how subjective feelings of fairness and self-interested need impact on everyday decisions, for example in the labor market," said Wright.

Another study from PLoS ONE found that perception of fairness changes when the participant is told that they own an asset that they'd have to share with others. An owner of a property will have more negative reaction to both unequal (when the other party gets bigger share) and equal shares.

The present study is published in the journal Scientific Reports.

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