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Can’t Read My Monkey Face

(Yes, that was a Lady Gaga reference) I’m extremely late for January 2011’s paper-a-month blog post, but better late than never!

This month’s paper actually dates to a little over 3 years ago. And, it actually isn’t about monkeys – its about chimpanzees (but “monkey” and “poker” have the same number of syllables, whereas “chimpanzee” has one more and hence throws off the reference). But, in it, they describe a very interesting experimental design to look into a very bold question: how do chimpanzees play the ultimatum game?

First off, what’s the ultimatum game? The ultimatum game is considered to be a test of fairness vs. rationality. The basic idea is that you have a setup with two people – a offer-maker and an offer-taker. The offer-maker makes an offer to split a prize (be it money, or food, or something else that both the offer-maker and the offer-taker find valuable) between the offer-maker and the offer-taker (i.e., so the offer-maker could say that he/she gets 90% whereas the offer-taker gets 10%). The offer-taker then decides whether or not to accept the offer or to reject the offer – whereby rejection means both offer-maker and offer-taker get nothing. (refer to dinosaur comics below)


Why is this an interesting test of fairness vs rationality? If humans were perfectly rational, then the offer-taker would accept any offer better than 0 and, knowing this, the offer-maker would offer the bare minimum piece to the offer-taker and take the lion’s share for him/herself. The reason for this is that a rational offer-taker would realize that as long as the offer gives something better than 0 – which is what you get if you reject the offer no matter what — the offer-taker is actually better off just taking the offer.

However, study after study shows that human beings aren’t perfectly rational – that faced between a grossly unfair, but objectively better outcome and nothing, that most humans will prefer the latter. In fact, studies have shown that the offer-taker tends to reject offers where they receive less than 20%. There are many evolutionary, sociological, and psychological interpretations, but at the end of the day the combination of valuing fairness and punishing unfairness is something which lets people live and work together.

While there are all sorts of interesting scientific and philosophical work being done to explore how humans play the ultimatum game, it does beg the question: do other creatures value fairness the way we do? In other words, if we could get other animals to play the ultimatum game, how would they play?

The researchers here conducted a fascinating experiment where they “taught” chimpanzees, who are not only our closest genetic relative but also known to work collaboratively on tasks such as hunting and territorial patrol, to play a slight variation of the ultimatum game. The setup is shown below in Figure 1a:


This variation of the ultimatum game requires the offer-maker (or “proposer” in the language of the paper) to select between two possible distributions of the prize (in this case, different amounts of raisins), visible to both the offer-maker and offer-taker (or “responder”) by pulling on one of two different ropes. The offer-taker then decides whether or not to accept the offer by pulling on a rod which brings the raisins within reach of both chimpanzees to eat (see figures 1b and 1c below).

Also, to help get a sense for the relative value of fairness, in each “round” of the experiment, the offer-maker always had a chance to offer a 8/2 split (with the offer-maker getting 80% of the spoils and the offer-taker getting 20%), and depending on which experimental group, the other option was either: 10/0 (offer-maker gets 100%), 8/2 (same as the control), 2/8 (offer-maker gets 20%, offer-taker gets 80%), and 5/5 (even split). This gave the researchers a chance to see how offer-makers and offer-takers would view the relative merits of other offers against that control.


Very clever experimental design, in my humble opinion! To insure that the chimpanzees knew what was going on, they were trained for some time to make sure they understood how to operate the apparatus and that their actions had consequences for their partner (i.e., by having the chimpanzees operate the apparatus and then be allowed to enter the partner’s chamber to eat the raisins).

So, what were the results? See for yourself (below in Figure 2). The left side shows every experiment run (~50 for each comparison), how often the offer-maker in each group proposed 8/2 vs. the other option, as well as how often the offer-taker rejected the offer.


The first thing that jumped out to me is that, with the exception of the 10/0 offer where the offer-taker gets absolutely nothing, the offer-takers reject very few of the offers.  The second thing that jumped out to me is that, with the exception of the 8/2 vs 10/0 decision, the offer-makers strongly preferred the offer where they got more (they overwhelmingly selected 8/2 over 5/5 and 2/8), suggesting that they first value their own well-being, but are also cognizant that the other chimpanzee has minimal reason to accept a 10/0 offer where they get nothing. These two observations support the conclusion of the research group, that chimpanzees are mostly rational maximizers, and don’t place too much weight on fairness.

The third thing which jumped out to me is very confusing – the chimpanzees were actually more wiling to reject 8/2 deals when the alternative was equivalent or worse (another 8/2 deal or a 10/0 deal) than when they had the much better options of a 5/5 or a 2/8 deal. This is either random experimental noise or suggests that chimpanzees like having a fairer option even when they don’t get it?

If I were to suggest next steps for the team, I’d ask them to probe deeper into that third point – because it suggests either there is a very interesting behavioral quirk about chimpanzees that is not well-understood today or that the chimpanzees didn’t fully understand how to play the game. I’d also recommend them to try a more direct measure of finding out if chimpanzees value altruism (a concept related to but not exactly the same as fairness): I’d love to see if chimpanzees are Pareto efficient (are they willing to be generous if it doesn’t cost them anything): are they more likely to propose a 8/6 over a 8/2. A positive result there would reinforce the finding here, that chimpanzees are actually altruistic – but they are rational maximizers first and foremost.

(Image credit – Dinosaur Comics) (remainder from Figures 1 and 2)

Paper: Jensen et al., “Chimpanzees are Rational Maximizers in an Ultimatum Game.” Science 318: 107-109 (Oct 2007) – DOI: 10.1126/science.1145850

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  1. Lena K. Lena K.

    Interesting read :-). I wonder though how, if at all, they dealt with the issue of repeat play. Obviously with the human version you would get very different results if you knew the game would be played multiple times (or, say if you did not know for certain that the game would be played only once. I assume they did not replace their chimps for every iteration (if they did, they must’ve had one heck of a research budget), and even besides that the process of training them implies effectively repeated trials, and it’s not like chimp A knows that it will never have to deal with chimp B again. This seems a huge confounding variable that is inherent in trying to interpret the results when doing this experiment with animals…

    • Ben Ben

      Thanks for commenting, Elena!

      You make a fair point — but I think what’s most likely is that chimp A is more inclined to believe they will deal with chimp B in the future (since N was so small and I doubt the researchers use a new set of chimpanzees for every chimp-related experiment they run); given that, wouldn’t the final conclusion of the chimps being not-particularly-concerned-with-fairness/altruism be re-inforced?

    • Mike Mike

      When you do this with humans, you usually take pains to make sure they’re playing with strangers. But in this case, they pulled chimps from the same “group” — so while they may not have been repeat players in the game, the chimps could always go beat each other up afterwards.

      (Also, it turns out that if you do it multiple times, a purely rational maximizer doesn’t change relative to a single iteration. In the last game, the rational maximizer will accept even the 9:1. But then that’s predetermined, and the second-to-last game become just like the last game… and so on.)

  2. […] Another science paper post? Yup, I’m trying to get ahead of my paper-a-month deadlines by posting February’s while […]

  3. Mike Mike

    Also, Ben, I suggest this for your next paper.

    “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.”

    “… it wouldn’t reflect well on anyone involved if the money turned the lab into a brothel.”

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