“Monkeys in Mino near Osaka have learned to steal purses and wallets and take out the coins use them to buy drinks and snacks from vending machines.”

Monkey Business: Keith Chen’s Monkey Research
by Stephen J. Dubner and Steven D. Levitt  /   June 5, 2005

Adam Smith, the founder of classical economics, was certain that humankind’s knack for monetary exchange belonged to humankind alone. ”Nobody ever saw a dog make a fair and deliberate exchange of one bone for another with another dog,” he wrote. ”Nobody ever saw one animal by its gestures and natural cries signify to another, this is mine, that yours; I am willing to give this for that.” But in a clean and spacious laboratory at Yale-New Haven Hospital, seven capuchin monkeys have been taught to use money, and a comparison of capuchin behavior and human behavior will either surprise you very much or not at all, depending on your view of humans.

The capuchin is a New World monkey, brown and cute, the size of a scrawny year-old human baby plus a long tail. ”The capuchin has a small brain, and it’s pretty much focused on food and sex,” says Keith Chen, a Yale economist who, along with Laurie Santos, a psychologist, is exploiting these natural desires — well, the desire for food at least — to teach the capuchins to buy grapes, apples and Jell-O. ”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.” When most people think of economics, they probably conjure images of inflation charts or currency rates rather than monkeys and marshmallows. But economics is increasingly being recognized as a science whose statistical tools can be put to work on nearly any aspect of modern life. That’s because economics is in essence the study of incentives, and how people — perhaps even monkeys — respond to those incentives. A quick scan of the current literature reveals that top economists are studying subjects like prostitution, rock ‘n’ roll, baseball cards and media bias.

Chen proudly calls himself a behavioral economist, a member of a growing subtribe whose research crosses over into psychology, neuroscience and evolutionary biology. He began his monkey work as a Harvard graduate student, in concert with Marc Hauser, a psychologist. The Harvard monkeys were cotton-top tamarins, and the experiments with them concerned altruism. Two monkeys faced each other in adjoining cages, each equipped with a lever that would release a marshmallow into the other monkey’s cage. The only way for one monkey to get a marshmallow was for the other monkey to pull its lever. So pulling the lever was to some degree an act of altruism, or at least of strategic cooperation.

The tamarins were fairly cooperative but still showed a healthy amount of self-interest: over repeated encounters with fellow monkeys, the typical tamarin pulled the lever about 40 percent of the time. Then Hauser and Chen heightened the drama. They conditioned one tamarin to always pull the lever (thus creating an altruistic stooge) and another to never pull the lever (thus creating a selfish jerk). The stooge and the jerk were then sent to play the game with the other tamarins. The stooge blithely pulled her lever over and over, never failing to dump a marshmallow into the other monkey’s cage. Initially, the other monkeys responded in kind, pulling their own levers 50 percent of the time. But once they figured out that their partner was a pushover (like a parent who buys her kid a toy on every outing whether the kid is a saint or a devil), their rate of reciprocation dropped to 30 percent — lower than the original average rate. The selfish jerk, meanwhile, was punished even worse. Once her reputation was established, whenever she was led into the experimenting chamber, the other tamarins ”would just go nuts,” Chen recalls. ”They’d throw their feces at the wall, walk into the corner and sit on their hands, kind of sulk.”

Chen is a hyperverbal, sharp-dressing 29-year-old with spiky hair. The son of Chinese immigrants, he had an itinerant upbringing in the rural Midwest. As a Stanford undergraduate, he was a de facto Marxist before being seduced, quite accidentally, by economics. He may be the only economist conducting monkey experiments, which puts him at slight odds with his psychologist collaborators (who are more interested in behavior itself than in the incentives that produce the behavior) as well as with certain economist colleagues. ”I love interest rates, and I’m willing to talk about their kind of stuff all the time,” he says, speaking of his fellow economists. ”But I can tell that they’re biting their tongues when I tell them what I’m working on.”

It is sometimes unclear, even to Chen himself, exactly what he is working on. When he and Santos, his psychologist collaborator, began to teach the Yale capuchins to use money, he had no pressing research theme. The essential idea was to give a monkey a dollar and see what it did with it. The currency Chen settled on was a silver disc, one inch in diameter, with a hole in the middle — ”kind of like Chinese money,” he says. It took several months of rudimentary repetition to teach the monkeys that these tokens were valuable as a means of exchange for a treat and would be similarly valuable the next day. Having gained that understanding, a capuchin would then be presented with 12 tokens on a tray and have to decide how many to surrender for, say, Jell-O cubes versus grapes. This first step allowed each capuchin to reveal its preferences and to grasp the concept of budgeting.

Then Chen introduced price shocks and wealth shocks. If, for instance, the price of Jell-O fell (two cubes instead of one per token), would the capuchin buy more Jell-O and fewer grapes? The capuchins responded rationally to tests like this — that is, they responded the way most readers of The Times would respond. In economist-speak, the capuchins adhered to the rules of utility maximization and price theory: when the price of something falls, people tend to buy more of it.

Chen next introduced a pair of gambling games and set out to determine which one the monkeys preferred. In the first game, the capuchin was given one grape and, dependent on a coin flip, either retained the original grape or won a bonus grape. In the second game, the capuchin started out owning the bonus grape and, once again dependent on a coin flip, either kept the two grapes or lost one. These two games are in fact the same gamble, with identical odds, but one is framed as a potential win and the other as a potential loss. How did the capuchins react? They far preferred to take a gamble on the potential gain than the potential loss. This is not what an economics textbook would predict. The laws of economics state that these two gambles, because they represent such small stakes, should be treated equally. So, does Chen’s gambling experiment simply reveal the cognitive limitations of his small-brained subjects? Perhaps not. In similar experiments, it turns out that humans tend to make the same type of irrational decision at a nearly identical rate. Documenting this phenomenon, known as loss aversion, is what helped the psychologist Daniel Kahneman win a Nobel Prize in economics. The data generated by the capuchin monkeys, Chen says, ”make them statistically indistinguishable from most stock-market investors.”

But do the capuchins actually understand money? Or is Chen simply exploiting their endless appetites to make them perform neat tricks? Several facts suggest the former. During a recent capuchin experiment that used cucumbers as treats, a research assistant happened to slice the cucumber into discs instead of cubes, as was typical. One capuchin picked up a slice, started to eat it and then ran over to a researcher to see if he could ”buy” something sweeter with it. To the capuchin, a round slice of cucumber bore enough resemblance to Chen’s silver tokens to seem like another piece of currency.

Then there is the stealing. Santos has observed that the monkeys never deliberately save any money, but they do sometimes purloin a token or two during an experiment. All seven monkeys live in a communal main chamber of about 750 cubic feet. For experiments, one capuchin at a time is let into a smaller testing chamber next door. Once, a capuchin in the testing chamber picked up an entire tray of tokens, flung them into the main chamber and then scurried in after them — a combination jailbreak and bank heist — which led to a chaotic scene in which the human researchers had to rush into the main chamber and offer food bribes for the tokens, a reinforcement that in effect encouraged more stealing.

Something else happened during that chaotic scene, something that convinced Chen of the monkeys’ true grasp of money. Perhaps the most distinguishing characteristic of money, after all, is its fungibility, the fact that it can be used to buy not just food but anything. During the chaos in the monkey cage, Chen saw something out of the corner of his eye that he would later try to play down but in his heart of hearts he knew to be true. What he witnessed was probably the first observed exchange of money for sex in the history of monkeykind. (Further proof that the monkeys truly understood money: the monkey who was paid for sex immediately traded the token in for a grape.)

This is a sensitive subject. The capuchin lab at Yale has been built and maintained to make the monkeys as comfortable as possible, and especially to allow them to carry on in a natural state. The introduction of money was tricky enough; it wouldn’t reflect well on anyone involved if the money turned the lab into a brothel. To this end, Chen has taken steps to ensure that future monkey sex at Yale occurs as nature intended it.But these facts remain: When taught to use money, a group of capuchin monkeys responded quite rationally to simple incentives; responded irrationally to risky gambles; failed to save; stole when they could; used money for food and, on occasion, sex. In other words, they behaved a good bit like the creature that most of Chen’s more traditional colleagues study: Homo sapiens.

Keith Chen
email : keith.chen [at] yale [dot] edu


Cost of coitus: Male monkeys pay for sex  /  Jan 2, 2008

Selling sex is said to be humankind’s oldest profession but it may have deep evolutionary roots, according to a study into our primate cousins which found that male macaques pay for intercourse by using grooming as a currency. Michael Gumert of Nanyang Technological University in Singapore made the discovery in a 20-month investigation into 50 long-tailed macaques in Kalimantan Tengah, Indonesia, New Scientist reports on Saturday. On average, females had sex 1.5 times per hour. But this rate jumped to 3.5 times per hour immediately after the female had been groomed by a male — and her partner of choice was likely to be the hunky monkey that did the grooming.

Market forces also acted on the value of the transaction. If there were several females in the area, the cost of buying sex would drop dramatically — a male could “buy” a female for just eight minutes of nit-picking. But if there were no females around, he would have to groom for up to 16 minutes before sex was offered. The work supports the theory that biological market forces can explain social behaviour, the British weekly says. “There is a very well-known mix of economic and mating markets in the human species itself,” said Ronald Noe of France’s University of Strasbourg. “There are many examples of rich old men getting young attractive ladies.”

Macaque monkeys ‘pay’ for sex
by Colin Barras / 02 January 2008

Sex has probably been a commodity for as long as human society has existed, and perhaps even longer. The “oldest profession” seemingly has pre-human evolutionary roots. “When the opportunity arises, male macaque monkeys groom females to ‘pay’ for sex,” says Michael Gumert of Nanyang Technological University, Singapore. Gumert looked at research on a 50-strong group of long-tailed macaques in Kalimantan Tengah, Indonesia, that covered a 20-month period. He found there was an increase in sexual activity after bouts of male-to-female grooming. On average, females had sex 1.5 times per hour, but immediately after being groomed by a male partner, this rate jumped to 3.5 times per hour. After grooming, the female was also less likely to offer herself to males other than her grooming partner (Animal Behaviour, DOI: 10.1016/j.anbehav.2007.03.009).

“My interest in this study stemmed from Trivers’s theory of reciprocal altruism,” says Gumert. In the early 1970s, Robert Trivers suggested that an organism will provide a service benefiting another, as long as it gets something back at a future date. But Gumert suspected that if the payback involved sex, the value would vary depending on the context – like all commodities in economics – a finding not predicted by reciprocal altruism. Sure enough, if there were several females in the area, the value of sex would drop – a male could “buy” a female for just 8 minutes of grooming. But if there were fewer females than males in the area, a male would have to groom his partner for up to 16 minutes before sex was offered.

A two-player interaction, such as is usually considered in reciprocal altruism studies, doesn’t make sense in this “general mating market”, says Ronald Noë of the University of Strasbourg, France. Noë and Peter Hammerstein of Humboldt University in Berlin, Germany, formulated biological market theory to better explain the kind of social behaviour Gumert identified in macaques. Market forces have a powerful influence on behaviour, says Noë. “There is a very well-known mix of economic and mating markets in the human species itself,” he says. “There are many examples of rich old men getting young attractive ladies.”

Yet prior to Gumert’s study, the evidence that market forces influence mating in nature has been scant – the only other clear example was in wood mice. “Many studies that fail to find biological market effects were performed in captivity,” says Gumert. “It is quite possible that the confinements of captivity alter or remove the effects of a social market.” For instance, there is no migration within captive communities, so the value of commodities such as sex remains stable, which makes market forces difficult to identify, he says. Gumert says macaque males are very “short-termist” in their thinking. “Some work is showing that monkeys really don’t have the capacity to wait for long-term trades and therefore trades probably only occur in the immediate sense,” he says.

Michael D. Gumert…

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Monkeys pay for sex too
by Jennifer Viegas  /  21 December 2007

Male macaques exchange grooming for the right to mate with females whose fur they have cleaned, a study has found. The findings, from a study of long-tailed macaques, presents the first evidence of its kind that a “social market” influences sexual interaction in a non-human primate. The research by Michael Gumert, of the Division of Psychology at Nanyang Technological University in Singapore, and colleagues, has been accepted for publication in the journal Animal Behaviour.”I found that the amount of grooming a male performs on a female during a sexual interaction is related to the supply/demand ratio of females per male around the male-female pair at the time of the grooming,” says Gumert. Put another way, male monkeys – especially lower status ones – have to groom more to get more action when fewer females are around. Grooming in macaques involves using the teeth and hands to pick through the fur of the recipient to remove dirt, tangles and parasites. The activity often sexually excites the monkeys, particularly the males, so many scientists suspect it evolved into foreplay in humans.

Indonesian study
Gumert, analysed a wild population of long-tailed macaques at Tanjung Puting National Park in Indonesia, from 2003 to 2005. During this period he documented 243 male-to-female grooming sessions, most of which were directed at females who were receptive to mating. The “grooming before sex” bouts lasted anywhere from a few seconds to a half hour or more, with the durations frequently linked to either the number of potential other partners or to the status of the groomer or recipient. According to Gumert, “rank does not remove the market, it only skews it.”

“Powerful individuals can take more and give less than low-ranked individuals can,” he says, suggesting that such corruption of the fair trade ideal appears to be an inherent facet of primate social life that can apply to everything from monkey sex to human politics. High-ranking females can also skew the system because, in the case of macaques, they demand more attention before they agree to mate. Since males often have their work cut out for them, they also try to first “flirt” with females, using facial gestures before they approach. “Being anthropomorphic, this may be like winking or smiling,” says Gumert. “The male bows and bobs his head, raises the eyebrows and smacks his lips at the female.” He also found that females will groom males at times, but that this behaviour doesn’t appear to be linked to sex. Gumert suggests it instead may serve to forge bonds with certain males, which could later protect the female’s offspring from other aggressive males without such a vested interest in her family.

“Well done” study
Professor Frans de Waal, a psychologist from Yerkes Primate Center at Emory University in Atlanta says the new study is “very well done and nicely applies the biological market concept to something new – exchange of grooming for sex, or sex for grooming.” De Waal adds: “We all know that primate males often do a bit of grooming before they mate with females, and that they groom very little if the female isn’t fertile, but it is good to see such a thorough, quantified account of it.” Gumert experienced a similar fair trade in his own life, when he married an Indonesian woman in a traditional village ceremony. He provided nuptial gifts to her family, as well as a small dowry. “I received no material gifts [in return],” says Gumert, “but I did get to marry my wife.”

Payment for sex in a macaque mating market
by Michael D. Gumert / 5 November 2007.

Abstract: “In primate sexual relationships, males and females can cooperate through social trade. Market-like trading of sexual activity has been theorized, but no data have yet been presented that clearly show its existence. I collected data to test whether biological market theory could account for exchanges of male-to-female grooming and sexual activity in longtailed macaques. I explored male-to-female grooming, rates of sexual activity, and grooming–mating interchanges, which were male-to-female grooming bouts that directly involved mating. Male-to-female grooming mainly occurred when females were sexually active, and males groomed females longer per bout when mating, inspection, or presentation of female hindquarters was involved. Moreover, male-to-female grooming was associated with an increase in female rates for all forms of sexual activity, where in contrast, female-to-male grooming was associated with decreased rates of mating in the groomed males. Males did not preferentially mate with swollen females or invest more grooming in them during grooming–mating interchanges, as swellings did not seem to be a reliable indicator of female fertility. Rank status was correlated with grooming payment during grooming–mating interchanges in favour of higher-ranked males and females. In support of a biological market interpretation, the amount of grooming a male performed on a female during grooming–mating interchanges was related to the current supply of females around the interaction. The results provided evidence of a grooming–mating trade that was influenced by a mating market.”

You scratch my back, honey, and then maybe… / 04 December 1999
“Female wood mice demand to be groomed in return for sex, according to scientists at the University of Oxford. Grooming in mammals is usually a reciprocal behaviour, where both parties devote roughly equal amounts of time to it for mutual benefit, such as removing parasites. However, Pavel Stopka and David Macdonald analysed hundreds of hours of film showing wood mice grooming and found that males spent more time grooming females than vice versa ( Ethology, vol 105, p 982). Stopka and Macdonald believe that a “biological market” operates, where females demand grooming in return for sex, as there are more males wanting sex than fertile females. “Wood mice are promiscuous, and grooming is the only resource males offer,” says Stopka.”

by S. P. Henzia & L. Barrett / 21 June 2002.

Abstract: “We used data from adult female chacma baboons, Papio cynocephalus ursinus, to provide the first test of hypotheses on interchange trading and the structure of a biological market (Noë & Hammerstein 1994, Behavioral Ecology and Sociobiology,35, 1–11) within a primate group. The interchange commodities selected were grooming and handling of infants less than 3 months of age. Patterns of grooming in relation to infant handling showed strong evidence for interchange. Grooming for infant access was initiated by potential handlers and was significantly likely to be nonreciprocated. More critically, the data show that infant ‘supply’ created a market effect: grooming bout duration (the price ‘paid’ for handling) was inversely related to the number of infants present in the group. In addition, there was an inverse relationship between grooming bout duration and the rank distance between mothers and handlers, suggesting that higher-ranking mothers could demand a higher price for infant handling. Where rank distance was high, females were able to handle infants without grooming. Dominance could thus be used to disrupt the infant market effect. If biological markets models are to be fully applicable to primate groups (and those of other social mammals) then the potentially distorting effect of dominance needs to be incorporated into the framework.”

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