June 14, 2005

Language, trade and sex

The orthographically diverse Ste(x)ens of 'Freakonomics' -- Stephen J. Dubner and Steven D. Levitt -- devoted their June 5 NYT column to language and monkey sex. Well, not really. It's about research by Keith Chen and Lauri Santos at Yale to teach capuchin monkeys to use money. After "several months of rudimentary repetition", the monkeys learned that one-inch silver disks with a central hole "were valuable as a means of exchange for a treat and would be similarly valuable the next day". Chen and Santos were then able to experiment with price shocks, wealth shocks, gambling games and so on. And along the way, the monkeys began on their own to exchange money for sex:

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.)

The authors primly observe that "it wouldn't reflect well on anyone involved if the money turned the lab into a brothel", and tell us that "Chen has taken steps to ensure that future monkey sex at Yale occurs as nature intended it", though we don't learn what methods the monkey vice squad uses. This is a curious intervention, since capuchins in the state of nature apparently trade food for sex as a matter of course, or at least share food and other favors, including sex, in ways that appear to involve calcuations of reciprocal benefits. Thus prostitution -- if that's what we should call this -- is OK in Yale's monkey cages, but only for in-kind payments, not for symbolic ones.

Kerim Friedman at Savage Minds picked up on the symbolic aspect of such exchanges:

...money is a form of symbolic communication. As such, it meets several of Charles Hockett’s design features for comparing animal and human communication: interchangeability, arbitrariness, discreteness, displacement, and learnability.

But I'm not sure that the silver disks are really relevant here. Dubner and Levitt start their article with the famous quote from Adam Smith's Wealth of Nations about how "Nobody ever saw a dog make a fair and deliberate exchange of one bone for another with another dog. 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" [Wealth of Nations, Book 1, Chapter 2]. What seems to contradict Smith is that capuchins sometimes trade food for sex: "this (marshmallow) is mine, that (redacted) is yours; I am willing to give this for that".

The silver disks don't really change this communication -- if it is one -- in any essential way. The displacement of reward from a grape to a coin makes the exchange seem more anthropomorphic, but this seems like a bit of a presentational trick to me. We're not so surprised to learn that monkeys (as well as other animals) can be taught (by "months of rudimentary repetition") to associate some arbitrary counter with food rewards, and to work for such tokens more or less as if they were pieces of the associated food. When a monkey, on his own, uses one of these counters in a natural exchange with another monkey as if it were a piece of food, this seems very human. But we could alternatively see this "displacement" as analogous to what happens when a monkey learns that marshmallows -- which don't occur in the state of nature -- are good to eat, and therefore begins to value them as if they were especially yummy pieces of fruit. For the monkeys, the coins are arguably just a more complicated kind of fruit, one which you've got to take to the experimenter to peel.

Adam Smith begins his chapter by wondering whether human's "propensity to truck, barter, and exchange one thing for another" is "one of those original principles in human nature of which no further account can be given; or whether, as seems more probable, it be the necessary consequence of the faculties of reason and speech". But normal capuchin sharing is apparently much more diffuse -- animals share food with others that they like or fear, or who have things they want. You could call this a general "propensity to truck, barter, and exchange one thing for another", but it's not very specific or contractual. It's not at all clear that the cited instance of Capuchin "prostitution" was more than another example of this general kind of "be nice to others so that they'll be nice to you" kind of behavior.

The main point of the Chen and Santos experiments is to show that capuchins can learn to engage in a wider variety of behaviors that look like human economic transactions. In my view, this is yet another interesting demonstration that non-human mammals have more of the basic abilities required for speech and language than one might have thought. Each discovery of this kind makes it all the more puzzling to me that non-human animals never seem to take those next few, small, logical steps towards effective communication, without "months of rudimentary repetition" in the laboratory.

[For some background on the relevant parts of capuchin monkeys' cognitive landscape, see Frans de Waal and Jason Davis, "Capuchin cognitive ecology: cooperation based on projected returns", Neuropsychologia 41 (2003) 221–228.]

Posted by Mark Liberman at June 14, 2005 06:08 AM