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ROBERT SIEGEL, host:

From NPR News, it's ALL THINGS CONSIDERED. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

Showing your humanity usually means performing an act of kindness or charity. Treating someone humanely means treating him fairly and with dignity. But are these traits really uniquely human?

In our ongoing series The Human Edge, we've been asking how and why humans became the dominant species on earth. And today, NPR's Joe Palca explores where traits like curiosity and fairness come from.

JOE PALCA: Humans catch on to fairness at an early age. I demonstrated this in 2003 with the help of two young boys, Sam(ph), who was nine at the time, and his brother Jacob(ph), who was six. The demonstration began with two piles of books on the kitchen table.

Jacob, you see this pile of books here? If you take these books upstairs and put them in your bookshelf, I'll give you a dollar, okay?

JACOB: Okay.

PALCA: In a few moments, Jacob was back in the kitchen.

JACOB: Give me a dollar.

PALCA: Here's your dollar.

JACOB: Thank you.

PALCA: Now Sam, if you take those books up to your bookcase, I'll give you 50 cents.

(Soundbite of laughter)

SAM: No way.

PALCA: No way indeed. Kids clearly get the concept of equal pay for equal work.

Psychologist Sarah Brosnan argues that traits like fairness and curiosity are essential for social creatures to survive and live together. But where did these traits come from? Brosnan is trying to find out.

Ms. SARAH BROSNAN (Psychologist): Let me see. Who do we have here? This is Wilma(ph), and she's got a little boy Widget(ph) running around in here somewhere.

PALCA: Wilma and Widget are capuchin monkeys. They're outdoors in a caged enclosure at the Language Research Center, a part of Georgia State University.

Ms. BROSNAN: The nice thing about testing these guys is they're living in a normal social environment. So they spend the vast majority of their day out here running around playing together, and then we just separate them out for the testing.

PALCA: The monkeys climb over branches in the cage, swing from the top of the cage, wrestle with each other. When it's time for testing, they go indoors.

Brosnan's graduate student, Audrey Parrish, is testing two capuchins, Liam and Logan. The test tries to get at the concept of fairness in capuchins. Here's how it works: Audrey hands Liam a granite token, then he hands it back to get a food reward - not too tricky. Audrey alternates between Liam and Logan.

Now here's the twist. Sometimes each monkey gets the same reward, and there are two kinds of rewards: a scrumptious, extremely desirable grape, or a ho-hum piece of only somewhat desirable cucumber. Think ice cream cone versus celery stick.

Logan was perfectly happy to exchange the token for a cucumber when his pal Liam is getting a cucumber, too.

Ms. BROSNAN: The question is now how is Logan going to respond to that cucumber when Liam is getting a grape?

PALCA: What Brosnan finds is more often than not, a capuchin offered the crummy reward after his partner gets the good one says in essence, no way, and refuses to hand back the token.

Ms. BROSNAN: What we're really testing is, how do you respond when you're the one who gets the lower salary, not how do you respond when you hear there's a discrepancy between salaries in the environment? So they don't necessarily have to have an ideal of fairness or an idea of the way the world should work. All they have to care about is that they got less than someone else.

PALCA: Brosnan sees this work as evolutionary proof that animals have some of the same complex social rules that humans do.

Clive Wynne isn't so sure. Wynne is an animal psychologist at the University of Florida. He says you don't have to invoke ideas like fairness or inequity to explain the capuchins' behavior.

Mr. CLIVE WYNNE (Animal Psychologist, University of Florida): There's an older concept and more basic concept of frustration that humans share with many other species: the tendency to act up if something they were expecting to receive is not given to them.

So if a child is in the habit of receiving a piece of chocolate for completing their homework, and then they don't get their piece of chocolate, they may throw a tantrum. And that kind of frustrative behavior is seen in any number of different species and have been shown back in the 1920s in monkeys.

PALCA: The person who did that monkey work was a psychologist at Yale named Otto Leif Tinklepaugh. I'm not going to say any more about him or his work; I just thought he had such a great name, I wanted to share it.

Sarah Brosnan says whether or not you accept terms like fairness or inequity to explain what the capuchins did in her test, she insists you can see unmistakable echoes of human behaviors in her capuchins.

Take curiosity. Brosnan says look at what the capuchins did the first time they saw me and my recording gear: They all came over to have a look.

Ms. BROSNAN: They're curious about you. You're new. They haven't seen you. They haven't seen a microphone before. So they want to see what it is. Is it going to do anything to them like give them food, or is it going to be a threat?

PALCA: Brosnan says curiosity, that desire to explore your world, is key to human culture. We went beyond curiosity about food and threats and began to wonder where we came from and why the stars twinkle in the night.

You can also see beginnings of another important human social activity in capuchins: the desire to play, to do things that have no immediate payoff.

Ms. BROSNAN: You're not acquiring food. You're not mating. You're not defending yourself from a predator.

PALCA: But saying play is purely social is not to suggest it isn't important. It helps juveniles learn the limits of acceptable behavior in their groups.

Brosnan doesn't believe play is a behavior inherited from monkeys in a genetic sense.

Ms. BROSNAN: But instead is a behavior that all sorts of intelligent, socially living species that live in complex social groups and need to know their ways around them have evolved.

PALCA: What humans and their big brains bring to the table is an ability to do more with these socially learned behaviors, to be curious about more things in our environment and extend concepts like fairness and inequity to make more complex societies.

Ms. BROSNAN: That probably explains why we're building city-states, and other species are still in groups of 200.

PALCA: In other words, we had the human edge.

Joe Palca, NPR News.

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