Studying Life’s Inequities

What humans and animals have in common.


By Marlene Cimons, National Science Foundation

Social scientists know that most people get upset if they are paid less than somebody else for doing the same job, or if they get the smallest share of a divided prize. It’s human nature to feel resentment, even anger, when somebody gets more than you do—or is it? 

Researchers don’t entirely understand the origins of this competitive indignation, whether it is solely cultural and unique to humans, or whether it is an evolutionary phenomenon that we share with animals, or a combination. But they are trying to find out.

“We evolved as a species that has to work together, and we are intrinsically interested in what other people are getting,” said Sarah Brosnan, assistant professor in the Georgia State University department of psychology.  “I think we have evolved to care about what the other person gets because we work together during many of our daily activities. This may extend to our most recent common ancestors, too.”

Brosnan has been studying primate species to see if they too react negatively to unequal rewards, and how their decision-making is affected. Thus far, in at least some species, the animals’ reaction seems similar to that of humans, that is, it is important for the animals to get as good a reward, if not better, than their partners.

This likely suggests that the response is not necessarily a hallmark of being human and implies “that when a species needs to work together, there is selection to be attentive to relative outcomes,” meaning equal rewards, she said.

The research is important because cooperation, that is, working together, is pivotal to many animals’ survival, including among humans, and is a major component to relationships among countries, essential to peace negotiations, international trade agreements and other economic policies. The results should help clarify how decision-making is affected by unequal rewards such as, among other things, food and salary.

“If animals work together in a dangerous environment to take down prey, then they all better get some of the meat or it wasn’t worth it,” she said. Similarly, “when you’re forming policy and making decisions, such as in trade agreements, you always have to keep in mind the perception of equality, or people won’t sign off on them.”

The work is funded over five years by a $677,462 from the National Science Foundation as part of the American Recovery and Reinvestment Act of 2009. Among other things, the money is supporting two postdoctoral fellows “which is helping to develop their careers,” Brosnan said, and is expected to create positions for several additional graduate student research positions in the coming years.

The researchers hypothesize that, over time, nature provides ways for individuals to know when they need to find new working partners, a phenomenon that has in part become linked to rewards. 

“Those who could recognize their rewards weren’t as good as the others and went off to find new partners did better,” she said.  “The behavior became entrenched, without an understanding on their part. Natural selection doesn’t require you to maximize what you get, but to do better than other individuals. One of the things people forget is that these need not be rational responses, but are evolved propensities, deeply ingrained.  They’re not just a cultural overlay, but something we have been selected to do.”

Her team has been conducting a series of studies on seven primate species, including those known to work cooperatively, and several that do not.  The scientists pair two from the same social group, then ask them to perform a specific task.

“We give them a token, and ask them to give it back,” she explained.  “If they do, they get a piece of food. The critical test is how they respond if one primate gets a better reward than the other. One might get a grape, the other a piece of cucumber—the grape being the better reward—the more sugar, the better.”

The responses vary, depending on the species. “For some, we see a strong reaction,” she said.  “For others, it looks like they don’t respond to inequities. These latter species don’t get upset when their partner gets a grape and they get a cucumber.”