Logan knows when he's been handed a raw deal. He and Liam, the monkey next to him, just did the exact same trick and Liam was rewarded with a grape. Logan got a cucumber. Any monkey knows grapes are more prized than that boring old cucumber slice Logan received. So he simply tosses that cucumber slice out of his cage.

"They're more than happy to eat cucumbers until they see their partner getting a grape, in which case they don't like their cucumbers any more," notes Georgia State psychologist and neuroscientist Sarah Brosnan.

She conducted a similar and more formal study with chimpanzees that helped prove that the sense of inequity and fairness runs deep in the primate family tree. We humans may not get all 'spun up' over cucumbers and grapes like our monkey cousins, but we do share their innate sense of fair play.

"This tells us a lot about human situations," Brosnan explains. "For instance, you may be perfectly happy with your salary until you find out someone who's a year your junior with fewer publications has a higher one, in which case, suddenly your salary isn't OK at all."

Brosnan decided to test another common trait. With support from the National Science Foundation (NSF), she set out to see if cooperation, like fair play, is baked into our DNA. To test the notion, she assembled a team of researchers. Senior research scientist Michael Beran helped her test chimpanzees and rhesus and capuchin monkeys. Bart Wilson, professor of law and economics at Chapman University in California, orchestrated similar games for humans to play.

"I'm interested in how people make decisions in strategic and social interactions," explains Wilson. "People are very sensitive to their surroundings and the environments in which they make these decisions, and so this was an opportunity, working and talking with Sarah Brosnan, to explore the real differences and similarities that humans might have with nonhuman primates."

To level the playing field, no instructions were given on how to play the game. Players were paired with a team member. Sometimes they saw their partners, sometimes they didn't. Monkeys and humans (students) sat in front of computer screens, each given a choice to select a red or a blue square. The human teams used a computer mouse to make their selections. The monkeys used a joystick. If both members of a team chose the red square, both won big. If they couldn't get on the same page, they got little or nothing. Monkeys received tasty treats. Students got cash.

"You get a dollar apiece for red and red," says student game player Hanna Grierson.

"We're getting really interesting results both on the monkey side and on the human side," says Beran.

Monkeys and humans had the same success rate--they cooperated about 70 to 80 percent of the time. That suggests that cooperative behavior runs deep in the nature of primates. The only time humans outscored monkeys was when the students employed our key advantage--language.

"We didn't tell humans that they could talk to each other or not," says Brosnan. "All the people who talked about the game figured out the cooperative interaction."

It turns out our language skills set us apart from our primate cousins. "Which is probably what's allowed us as humans to develop really complex cooperative enterprises like economies and nation states that other species simply haven't been able to do," notes Brosnan.

That's enough to make Logan and Liam envious. Or maybe it's just sour grapes.

This story was originally written for Science Nation and was republished with permission here. Video: Science Nation, Miles O'Brien/Science Nation Correspondent, Ann Kellan/Science Nation Producer.