The Frontal Cortex

Chimps Are More Rational

Or at least they play the ultimatum game more rationally than humans:

German researchers have demonstrated chimpanzees make choices that protect their self-interest more consistently than do humans.

Researchers from the Max Planck Institute of Evolutionary Anthropology in Leipzig studied the chimp’s choices by using an economic game with two players. In the game, a human or chimpanzee who receives something of value can offer to share it with another.

If the proposed share is rejected, neither player gets anything.

Humans typically make offers close to 50 percent of the reward. They also reject as unfair offers of significantly less than half of the reward, even though this choice means they get nothing.

The study, however, showed chimpanzees reliably made offers of substantially less than 50 percent, and accepted offers of any size, no matter how small.

The researchers concluded chimpanzees do not show a willingness to make fair offers and reject unfair ones. In this way, they protect their self interest and are unwilling to pay a cost to punish someone they perceive as unfair.

Here’s the Science paper. I think it will be interesting to disentangle two different variables which might be at work here. The first variable is theory of mind (TOM). The authors conclude that the lack of “other-regarding preferences,” the inability to simulate the mind of another chimp, is partly responsible for the rational behavior of the primates. Further evidence for this line of argument comes from studies of autistic adults playing the ultimatum game, who also act like rational maximizers.

The second variable is what we’ll call “the fairness instinct”. People have a strong aversion to inequitable outcomes. The authors of this study argue that chimps might have a lessened aversion to acts of unfairness. But there are several other studies which have concluded that the fairness instinct is a basic element of primate morality. Consider this experiment from Franz Waals and Sarah Brosnan, who trained brown capuchin monkeys to give them pebbles in exchange for cucumbers. Almost overnight, a capuchin economy developed, with hungry monkeys harvesting small stones. But the marketplace was disrupted when the scientists got mischievous: instead of giving every monkey a cucumber in exchange for pebbles, they started giving some monkeys a tasty grape instead. (Monkeys prefer grapes to cucumbers.) After witnessing this injustice, the monkeys who only received cucumbers went on strike. Some started throwing their cucumbers at the scientists; the vast majority just stopped collecting pebbles. The capuchin economy ground to a halt. The monkeys were willing to forfeit cheap food simply to register their rage at the arbitrary pay scale.

This labor unrest among monkeys illuminates our innate sense of fairness. It’s not that the primates demanded equality – some capuchins collected many more pebbles than others, and that never created a problem – it’s that they couldn’t stand when the inequality was a result of injustice. Humans act the same way. When proposers do something to deserve their riches – like score high on a trivia quiz – nobody complains. But when they get rewarded for no reason and then refuse to fairly distribute their reward, people get furious.

The obvious answer is that both variables (TOM and the fairness instinct) are at work when people play the ultimatum game. My guess is that TOM simply extends the fairness instinct and allows us to anticipate how other people will respond to unfair offers. Because we can simulate their mind, we know the unfair offer will make them angry, and so we make a fair offer.


  1. #1 Ted
    October 10, 2007

    Jonah, Your post and this study begs a question I’ve been pondering in regard to the research on autism and other developmental disabilities that I’m involved in as to whether the TOM piece of the equation is always necessarily a deficit. Might it not be a branch of our species that has evolved that more single mindedly defends our own interests? I sometimes look at the central coherence theory of autism the same way. Mightn’t it be a useful deviation in our varied gene pool if everyone were not expected to excel in the same way and if this difference were well applied? The joke about the apparent rise in diagnoses of Asperger’s these days is that 40 years ago we just called them all ‘engineers.’
    Not to trivialize the misfortune of those whose TOM or central coherence differences can be accompanied by mental retardation, severe language problems, sensory difficulties and many other symptoms. These deviations may have their uses when not part of a more crippling collection of symptoms which we group under one diagnosis today, but which could one day be re-defined as co-morbidities.
    Interesting post, as usual. BTW, I’m enjoying your book.

  2. #2 amybuilds
    October 10, 2007

    I wonder how the “fairness instinct” develops in primates and humans. At what age does it appear? How is it’s development affected by environment? In my observation the “fairness instinct” is stronger in some and weaker in others. There are always those who are willing “to cut off their nose to spite their face” and others who are more tolerant of inequity. What causes the spectrum? I wonder how an environment of scarcity might skew the results of the ultimatum game.

  3. #3 Sandeep Gautam
    October 11, 2007

    Amybuilds, the sense/instinct of fairness may be partly genetic. I review a new study in my blogpost “fairness in your genes” regarding the same . although environmental factors would also be at work, but much of the variation can be explained by genetics.

    Johan, great article as usual. I had completely missed upon the ToM angle, when I first read about this study.

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