It’s no secret that humans are sensitive to unequal rewards. In today’s turbulent economic times people are more sensitive than ever. For example, in recent months there has been considerable public outcry over the governments continued multi-billion dollar bailouts for industry, while the average individual is still looking for some relief. It has been suggested that this sense of fairness is necessary for the evolution of cooperation. After all, if we didn’t respond negatively when we felt we were being cheated, the best strategy in life would be to cheat.
This type of aversion to inequity was long thought to be uniquely human until a few years ago researchers here at Living Links discovered similar reactions in capuchin monkeys. Since this time, inequity aversion has also been found in other nonhuman primates including chimpanzees and cotton-top tamarins.
Recently, Range and colleagues found that dogs also had sensitivity unfair situations. They gave the dogs a simple task, lifting their paws to “shake.” Each dog was paired with a familiar partner who completed the same task. When both dogs were rewarded, they completed the task successfully. However, when conditions were changed such that a one dog received a reward for completing the task but the other did not, the unrewarded dog stopped participating (as shown in the picture). If both dogs presented their paws but neither was rewarded, or there was only one dog present, they performed at a much higher rate. Thus, it was only when one dog saw another getting a reward for the same task in which they themselves were not rewarded, that they went on strike. This suggests that the dogs were sensitive to the unfair reward distribution.
So what do humans, apes, monkeys and dogs all have in common that would support the evolution of inequity aversion? All of the species (or their close relatives) are highly cooperative. While a cooperative society may lead to a sense of fairness, what is particularly interesting to me is that the species studied have different levels of sensitivity to inequality. For example, humans tend to respond negatively to all inequity, even when we receive more than our counterparts. In contrast, monkeys only go on strike when they receive a lower quality reward than their partners. This study demonstrated that dogs are sensitive only when their reward is completely absent; they would still work for a lower quality reward (when they were rewarded with a piece of bread but their partner got a piece of sausage). As more studies on this topic are conducted, it will be interesting to see if there is an evolutionary continuity to a sense of fairness.
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