"The argument was that this is a uniquely human phenomenon," says Frans de Waal, a professor of psychology at Emory University in Atlanta and a researcher at the Yerkes National Primate Research Center.
That changed in 2003 when he and a colleague named Sarah Brosnan did a study on monkeys. Monkeys had to hand a small rock to researchers to get a piece of food in return. Monkeys were happy to do this to get a piece of cucumber. But the monkeys would suddenly act insulted to be offered cucumber if they saw that another monkey was getting a more delicious reward, a grape, for doing the same job.
"The one who got cucumber became very agitated, threw out the food, threw out the rock that we exchanged with them, and at some point just stopped performing," says de Waal.
In that experiment, the monkeys considered the fairness of two different types of payment. But when Range and her colleagues did a similar study with their trained dogs, testing to see if dogs would become upset if they only got dark bread when other dogs received sausage, they found that dogs did not make that kind of subtle distinction. As long as the dogs got some kind of food payment, even if it wasn't the yummiest kind, the animals would play along.
