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Yale-New Haven Hospital’s Monkeys


Are animals as stupid as human beings? Will they indulge in trading if trained? Will a dog exchange a bone with another dog for some favour like sex? Keith Chen, a professor of behavioural economics, wanted to know. So he conducted an experiment which came to be known as the Yale-New Haven Hospital’s monkey experiment. He was shocked by the results. And the hospital had to ask him to leave the monkeys alone.

Chen conducted his research on a group of monkeys. His choice was the capuchin, which is a cute, little, brown monkey with a small brain that is highly focused on food and sex. (Not very unlike many human beings, you are tempted to think.)

Chen, along with Venkat Lakshminarayanan, worked with seven capuchins at a lab set up by psychologist Laurie Santos at Yale-New Haven Hospital. The monkeys lived together in a large cage. At one end of the cage was a smaller cage which was the testing chamber, where one monkey at a time would enter to take part in experiments.

First, Chen and his colleagues taught the monkeys to use money. They gave them silver coins which they could use for buying food. Give the coin back to the researcher and the monkey gets the goodies. The monkeys learnt to buy the food of their choice by giving the coins to the particular researcher holding their choice food.

Then Chen experimented with price variation. How would the monkeys behave if he raised or lowered the prices of food items? To Chen’s surprise, the monkeys behaved quite like human beings. When the price of a particular food rose the monkeys bought less of it and when the price fell they bought more. The monkeys were rational enough.

What about their irrationality? To test that, Chen set up two gambling games. Coin toss was the gamble. Head or tail? A very common human gamble. The monkey was shown a grape first. Depending on the coin flip, the monkey would get that grape or a bonus one as well. In the second game, the capuchins were shown two grapes and if the coin flip went against him one grape would be taken away.

In the first game, a bonus is won. In the second, something is lost. Actually there is no difference in the final outcome. In the both the gambles, the final average number of grapes won by each monkey would be more or less the same. Yet we all have a natural aversion to loss and an equally natural preference for gain. What about the monkeys? Yes, they behaved just like us again. The monkeys abandoned the two-grape gamble and gathered around the one-grape researcher. The capuchins behaved as if the pain from losing a grape was greater than the pleasure from gaining one. That is quite irrational if you understand that there is no real gain or loss in the game. Yet ‘loss aversion’ is a strong economic behaviour of human beings. And of monkeys too!

Similar experiments were actually carried out with men before Chen came to the conclusion. He studied the behaviour of intra-day traders at stock markets and concluded that the data generated by the capuchin monkeys “make them statistically indistinguishable from most stock-market investors.”

The biggest surprise for Chen came soon enough. One morning the alpha male of the group did something unique. He scurried into the testing chamber as he had done many times, but on this day, instead of taking his 12 coins and going to buy food, he flung his coins into the main cage and ran after them. All the capuchins rushed to grab the coins. Each one, behaving just like normal humans, grabbed what he or she could. Chen and his colleagues were unsuccessful in their attempts to retrieve the coins from the monkeys. They had to give food in return for the coins the capuchins had grabbed illegally. The monkeys learnt that crime pays.

What shocked Chen, however, was not this. He watched one male capuchin going to a female with the coin he had grabbed. He offered the coin to her which she accepted and then immediately he had sex with her. What Chen originally construed as altruism was in fact “the first instance of monkey prostitution in the recorded history of science.” [The quote is from Super Freakonomics by Steven D Levitt & Stephen J Dubner which is the source of this entire post.]

As soon as the copulation was over, the female monkey which had received the illegal coin went to Chen to buy grapes with it.

The hospital to which the capuchins belonged called a halt to the experiments. They did not want to irreparably damage the social structure of the capuchins.

The social structures are artificial constructs and they inescapably affect our behaviour patterns. Just imagine, for the sake of momentary delight, a social system in which people supported one another with understanding and empathy. Wealth wouldn’t be a major value there. Greed wouldn’t be a dominating vice. Selfishness and jealousy would be suppressed since they would make you look like hideous gargoyles on a majestic edifice. Not practical, you would say. Why? Because we have already been thoroughly corrupted by our existing social constructs with their warped notions.

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Previous post in this series: Xenophanes’s God

Tomorrow, the last: Zorba’s Secret



  1. A very interesting post discussing some very interesting experiments...

  2. A very interesting experiment. Having a society with empathy and no wealth would as you say be unimaginable because yes we have been corrupted far too much.
    Deepika Sharma

    1. The social system alters people's behavior significantly. Memes, in Dawkins' sense, are as strong as genes.

  3. The 'system' corrupts us morally and we let that happen. Nothing can be more sad than this. In the above experiment , the monkeys seem so intelligent but when we extrapolate the behavior to humans, have we really stopped evolving and lost sense of rationality ?

    1. We are evolving only at the intellectual level,probably. The heart belongs to the ape still.

  4. This post was entertaining and disheartening in equal measure.

    The more I read about the experiment, the more human-like the capuchins behaved.

    Maybe we are all capuchins in a giant lab--like the dragonflies you mentioned in a comment on my blog once.

    1. I live with the hope (very faint and appearing like a delusion) that one day human beings will realise their folly and correct their systems.

  5. It's true that we are manipulated by our societies, i remember my dad telling me " it would have been good if we didn't evolve." Your writing tells me why he had that different look on his face while saying it.

    1. The evolution gave us heaven in the next life at least 😅


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