Pay one monkey with a delicious grape and another with a ho-hum cucumber for the same amount of work, and the monkey who got the lesser reward will probably quit working for you. He may even throw the vegetable back at you, even though monkeys are usually happy to receive cucumbers, says Sarah Brosnan, PhD, a psychology professor at Georgia State University.

That experiment by Brosnan and collaborator Frans de Waal, PhD, published in 2003 in Nature (Vol. 425, No. 6,955), was one of the first to show that animals may have an appreciation for fairness—a moral sense that many researchers previously thought was the sole domain of humans. Since then, a slew of intriguing results suggest that animals—particularly those that depend on cooperation for their survival—may have some of the same underpinnings of justice and altruism as we do.

For example, a chimpanzee is quick to let another chimp out of his cage, and they happily retrieve out-of-reach objects for their human handlers. Monkeys will spontaneously share rewards with others who worked toward the goal. And, in the fairness realm, dogs will quit participating in a task if they see another animal receive a better reward.

"Fairness and altruism didn't develop de novo in humans," Brosnan says. "It's likely something that began in social species, including primates, and evolved to us."

However, interpreting animal behavior through human eyes can be problematic, observes Marc Hauser, PhD, a Harvard psychology professor and evolutionary biologist. In the cucumber-grape study, for example, the monkeys could have tossed the cucumbers simply because they were frustrated they didn't get a grape once they saw it. Moral outrage may never have entered the equation.

To determine which explanation makes more sense, researchers must uncover the underlying mechanisms or moral behavior, Hauser says.

"That is basically the name of the game," he says. "You are looking for behavioral or neurobiological signatures that are the same across species."

Equal work, equal pay

Research indicates that some animals—particularly ones that hunt together—divvy up the rewards of a spoil. One study even found that animals will occasionally deliver a better reward to a collaborator than they themselves receive. In a 2006 study by Brosnan and her colleagues in the American Journal of Primatology (Vol. 68, No. 7), two capuchin monkeys had to work together to pull a tray of food to their cages. But before they began pulling, the monkeys had to decide which one would receive a grape and which one would get a less-coveted apple slice. Instead of fighting over the grape, or always letting the dominant monkey eat it, the animals generally alternated roles across trials, so they both earned some grapes and some apple slices, Brosnan found. In the few cases where the dominant monkey hogged the good food, the other monkey tended to quit participating—she'd rather go without a reward than be paid unfairly.

That tendency to share rewards, says Brosnan, probably developed as a result of the way capuchins work together to hunt raccoon-like animals called coati. "If we are hunting, and I am not giving you much of the kill, you would be better off finding another partner," she says.

Monkeys that were sensitive to unfair situations were more likely to demand their fair share of meat, survive and ultimately reproduce, thereby passing to their offspring a genetic predisposition to seek equitable situations, Brosnan hypothesizes.

Dogs, too, go on strike if they get underpaid, according to a 2008 study in the Proceedings of the National Academies of Science (Vol. 106, No. 1) by Friederike Range, PhD, of the University of Vienna in Austria. In it, trained dogs were happy to "shake hands" with a human for no reward—that is, unless they saw another dog receive treats for the behavior. An intriguing result, though some criticized the study for allowing the animal's owners to be present. The owners possibly gave the dogs subtle cues—a phenomenon known as the "Clever Hans" effect, after a horse that answered math problems by stomping its hoof. The animal's owner would become tense as the horse neared the correct number of stomps, accidentally signaling it was time to stop with his posture and facial expressions.

Cross-species comparisons

No matter how methodologically sound the studies turn out to be, the animals' behavior, so far, bears only a passing resemblance to human responses to unfairness, some researchers say. In the case of the cucumber-rejecting monkeys, for example, the monkeys were only harming themselves when they passed up the food. In contrast, humans put in similar situations will take the lesser reward unless the rules of the game are set up so that, if they reject the lesser reward, they can keep the other player from a reward as well, notes Hauser.

"Some of the cases that have been coming up in the animal literature do show some of the same signs as humans and in some ways they seem different," he says.

It makes sense for species that hunt cooperatively to notice when they receive less than others, says Felix Warneken, PhD, a psychology professor at the Max Planck Institute for Evolutionary Anthropology, in Germany. However, only adult humans seem to notice when others get less than they deserve, he says.

"[Young] children don't mind if they get more than others," says Warneken. "But somewhere between the age of 4 and 8, children acquire these equality rules that they think everyone should receive the same."

Humans may need this advanced sense of fairness because, as a species, we occupy a tough ecological niche, says Michael Gurven, PhD, a University of California, Santa Barbara, anthropology professor. While other primates can survive off of vast amounts of relatively low-calorie roughage, humans must seek hard-to-come-by, high-calorie bonanzas—searching for honey, for example, and hunting animals that are faster than we are. As a result, it takes a lot of training and practice before people can break even on their caloric needs: Among the Ache tribe of Paraguay, a hunter-gatherer society studied by Gurven, people start hunting enough to feed themselves by the time they are 19 and they don't reach their peak productivity until age 40.

"Food sharing is crucial in human societies," says Gurven. "Human history wouldn't look anything like it does now if it weren't for the existence of widespread food exchange at a level you would never see in primates."

As a result, we've developed social rules that build on our primal inequity aversion, and we teach our children to be uncomfortable with situations where we receive more than others, Warneken says.

Happy to help

While human standards of fairness seem to require explicit teaching, a tendency to help others may be inherent in both children and chimps, suggests research by Warneken, in a 2006 issue of Science (Vol. 311, No. 5,765). Warneken and his colleagues put 18-month-old infants and young primates in a variety of situations where they could assist adult humans. For example, the adults tried to grasp an out-of-reach object that the children and chimps could reach, or the adults dropped an item through a small hole in a box and unsuccessfully attempted to retrieve it, while the children or chimps could get the item by using a door in the box that the adult was unaware of.

In the easy task, both the children and the chimps spontaneously retrieved objects for the adults, but the chimps required more prompting—the experimenters had to look back and forth between the items and the chimps to request help from the animals. In the harder tasks, where it wasn't immediately clear what the adults were attempting to do, only the children pitched in.

The results, says Warneken, show that both children and chimps spontaneously assist others, though they differ in their ability to interpret situations and realize that another animal needs help.

But chimps aren't always so helpful, notes Jennifer Vonk, PhD, a psychology professor at the University of Southern Mississippi. Her research, published in 2008 in Animal Behaviour (Vol. 75, No. 5) gave chimps the opportunity to use a stick to knock food toward their cages and also toward another chimp's cage. After an initial learning period, the chimps often knocked food toward themselves, but they didn't use that skill to deliver food to other chimps, even as the animal begged and attempted to grasp the food.

"They just seemed indifferent," she says.

Yale psychology professor Laurie Santos, PhD, and her colleagues got a contrasting result with capuchin monkeys. In a 2008 study published in Current Biology (Vol. 18, No. 21), they found that, when given the choice of delivering an average or a special reward (an apple vs. a marshmallow) to a monkey in an adjacent cage, the monkeys more often chose to deliver the tasty marshmallows, for no apparent reason or reward.

The pattern of results, thus far, is perplexing, says Hauser. Why would capuchins be more generous than chimps?

"What you find is the evidence in some of these cases is on the weak side—it often shows puzzling patterns across different species and tasks," he says.

But one overarching theme is emerging: Many animals, in addition to humans, seem to act with the others' well-being in mind, Santos says.

"All pro-sociality is not unique to humans," she says. "Our close living primate relatives are very nice to each other, though perhaps not as consistently as humans."