According to a new study, people unlike chimpanzees will reject an offer of water, even when they are severely thirsty, if they perceive the offer to be unfair.
The findings have important implications for understanding how humans make decisions that must balance fairness and self-interest.
It's been known for some time that when humans bargain for money they have a tendency to reject unfair offers, preferring to let both parties walk away with nothing rather than accept a low offer in the knowledge that their counterpart is taking home more cash.
Researchers at the Wellcome Trust Centre for Neuroimaging at UCL wanted to see whether humans would similarly accept unfair offers if they were bargaining for a basic physiological need, such as food, water or sex.
The team recruited 21 healthy participants and made 11 of them thirsty by drip-feeding them a salty solution, whilst the remainder received an isotonic solution that had a much smaller effect on their level of thirst. To obtain an objective measure of each individuals need for water, the team measured the salt concentration in their blood. The participants' subjective perception of how thirsty they were was assessed using a simple rating scale.
The participants then separately took part in an ultimatum game. They were given instructions that two of them had been randomly selected to play a game to decide the split of a 500ml bottle of water that could be consumed immediately. One of them would play the part of 'Proposer' and decide how the bottle should be split. The other would be a 'Responder' who could either accept the split and so drink the water offered to them, or reject the split so that both parties would get nothing. The participants knew that they would have to wait a full hour after the end of the game before they would have access to water.
In reality, all of the participants played the part of the Responder. They were presented with two glasses of water with a highly unequal offer that they were told was from the Proposer: the glass offered to them contained 62.5ml, an eighth of the original bottle of water, and the other contained the remaining seven eighths that the Proposer wanted to keep for themselves. They had fifteen seconds to decide whether to accept or reject the offer.
The team found that, unlike chimpanzees, the human participants tended to reject the highly unequal offer, and here that was the case even if they were severely thirsty. The participants' choices were not influenced by how thirsty they actually were, as measured objectively from the blood sample. However, they were more likely to accept the offer if they subjectively felt that they were thirsty.
"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," explained Dr Nick Wright, who led the study.
"However, we also show this fairness motivation is traded-off against self-interest, and that this self-interest is not determined by how their objective need for water but instead by their subjective perception of thirst. These findings are interesting for understanding how subjective feelings of fairness and self-interested need impact on everyday decisions, for example in the labour market," he added.