In 2005, an economist and a psychologist from Yale University taught seven capuchin monkeys how to use money. From then, the first ever prostitute monkey appeared.

Economist Keith Chen wanted to study the incentives that motivate other species to behave in such a way, while his friend, psychologist Laurie Santos, analyzed the monkeys’ behavior.

When Chen was still a Harvard graduate, he worked with psychologist Marc Hauser in a project to study the altruism behaviors of monkeys. They put two cotton-top monkeys in separate cages, each with a level. When the monkey pulled the lever, their neighboring monkey received the food. In conclusion, the typical monkey pulled the lever 40 percent of the time.

Chen and Hauser made it harder by instructing the monkey (mindless altruist) to always pull the lever, while the other to never to pull it (ego-monkey).

The two were then sent to the other monkeys. Other tamarins responded like the mindless altruist 50 percent of the time, but after understanding the game, they became indifferent and their response dropped to 30 percent.

Years later, Chen wanted to further the study with Santos. At first, they gave capuchin monkeys a disc instead of a dollar, as a means of currency.

After several months, the monkeys understood that they could exchange the disc for fruit. Each of them was given 12 tokens each to decide how they will spend those on food, which were valued at different prices.

They observed that the monkeys know how to budget their tokens. Researchers sometimes changed Jell-O’s at a lower price to see if they will buy some grapes more or the Jell-O’s. According to their report, the monkeys still acted the way economics dictate for humans, too.

Some monkeys went back to each researcher looking for a better-tasting food and offering a higher amount of money. There were others who tried to steal token from another. On one occasion, a capuchin tried to steal a tray filled with tokens and ended up with all the others.

During the chaos, the researchers saw something else. Indeed, they taught the monkeys how to use the tokens to buy food, but on their own, the monkeys figured they could use the tokens in exchange of services, too.

One of the monkeys exchanged his token for sex. After the act, the paid monkey used the token to buy a grape – the first time they realized a “prostitute monkey” appeared.

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