How scientists taught monkeys the concept of money. Not long after, the first prostitute monkey appeared

You may have thought things like currency or money are concepts known solely by man – something which differentiates humans from animals. Some might have a sense of ownership, besides of course territory, but trading and the likes haven’t been observed in any other species besides homo sapiens. However, in 2005, an economist/psychologist duo from Yale managed to teach seven capuchin monkeys how to use money, and I’m pretty sure from here on some of you might be able to guess what happened from there on.

Monkey Business

“The capuchin has a small brain, and it’s pretty much focused on food and sex,” said Keith Chen, a Yale economist who along with Laurie Santos, a psychologist, are the two researchers who have had made the study. ”You should really think of a capuchin as a bottomless stomach of want,” Chen says. ”You can feed them marshmallows all day, they’ll throw up and then come back for more.”
It’s exactly these selfish desires that they tried to exploit and experiment with great success after teaching capuchins to buy grapes, apples and Jell-O. The economist wanted to study the incentives that motivated specimens to behave in a way, while the psychologist analyzed the behavior itself.

Chen’s monkey correlations to human economics attempts go from farther back when he was a Harvard graduate, and additionally shows some more interesting facts. He worked there with Marc Hauser, a psychologist, on a project which studied altruism behaviors in monkeys. They chose cotton-top tamarins for this. At first they put two in different cages, each with a lever. When the lever was pulled, the neighboring monkey would receive food. If not altruism, it was still a form of cooperation which was put to the test  – the typical tamarin pulled the lever about 40 percent of the time.

The most interesting part comes about at the time when researchers paced the game a bit harder. Now, they instructed a monkey to always pull the lever (mindless altruist), and an other to never pull it (ego-monkey). The two were then inserted in the game with other monkeys. At first, the mindless altruist was pulling the lever every time, never missing a cage for its food, while the other tamarins responded in the same way  50 percent of the time. The other monkeys soon understood, though, that the mindless altruist was just pulling the lever anyway, indifferently of whether it was reciprocated or not – their response dropped to 30 percent of the time. The ego-monkey was exposed to the harshest treatment, as expected – very harshly. “[The other tamarins] would just go nuts,” Chen recalls when she was introduced with all the other. ”They’d throw their feces at the wall, walk into the corner and sit on their hands, kind of sulk.”


When Chen and Santos first started their study, they didn’t have a particular goal in mind. It was just as simple as giving a monkey a dollar and see what would happen, which was exactly the case, instead of the dollar, however, a silver disc with a hole in its center was employed a means of currency for the capuchins. It took several months of repetition for the capuchins to learn that they could exchange such a token for fruit. After they understood this, each monkey was given 12 tokens to decide on how to spend it in her best interest on food valued at different prices. Researchers observed that the monkeys could very well budget. Researchers then changed the market and put Jell-O at a lower price, to see if monkeys would buy less grapes and more Jell-O. They acted exactly like the current laws of economics dictate for humans as well.

They then taught them how to gamble, and saw they made the same irrational decisions a human gambler would make as well. The data generated by the capuchin monkeys, Chen says, ”make them statistically indistinguishable from most stock-market investors.”

The capuchin monkeys understood money, not only used it

Do they understand the value of money or do the monkeys just follow nice treats? Well, on a particular day, a researcher cut circular slices of cucumber, similar to the discs that were handed out to the capuchin as money, and fed them to the monkeys instead of the usual cube-like shape. One of the monkeys took a slice, chewed a bit on it, and then immediately went to one of the researchers to see if she could buy something tastier with it. Oh, and then again there’s stealing too. Not a single monkey saved any of the tokens, but most of them tried to subtract a few more tokens when they were handed out. The monkeys were given tokens one at a time by inserting them in a separate chamber from that of their living quarters, but on one occasion everything sprung into chaos when a capuchin tried to make a run for it with a tray filled with tokens and ended up back with all the other monkeys. That was a tough time for researchers.

Something else happened then too, tough , in what’s maybe the most evident form of one’s grasp upon currency. The idea is that you can use money as a form of currency to exchange for goods or services, as in not just food. Well, one of the researchers, during the chaos event, observed how one of the monkeys exchanged money to another for sex. After the act was over, the monkey which was paid immediately used it to buy a grape…

There you have it folks, sounds familiar? In almost all aspects, capuchins manage to understand money and use it in a manner not too different from a plain old homo sapiens. The study, titled “How Basic Are Behavioral Biases? Evidence From Capuchin Monkey Trading Behavior“, can be read here.