Will Work for (Equal) Pay

Scales_of_Justice_(PSF)One important concept we possess at a young age is fairness (as any parent of multiple children will confirm!). Children especially do not hesitate to speak up (or, more often, whine) if they detect inequality, and much of our society focuses on achieving fairness for all. Given the sociality of some animal species, could these animals also have a concept of fairness?

Brosnan & de Waal (2003) investigated whether capuchin monkeys (who are considered very social) conceptualize fairness. (The term most researchers use is “inequality aversion”, since they’re not sure if it is actually the same as the human concept of fairness. For simplicity, I’ll continue to call it “fairness”.) They first taught the capuchins an exchanging task, where the researcher gives the monkey a token (e.g. a rock), which the monkey then hands back to the researcher in exchange for a treat.

Once the capuchins learned this task, Brosnan & de Waal wanted to see how they would react to getting a less desirable treat than another capuchin in the same task. It seems obvious that they would be upset – they’re doing the exact same task as the other capuchin, so they should also get the more desirable treat (equal “pay” for equal “work”). But think about the thought processes required for such a reaction: the capuchin must not only pay attention to the task the other capuchin does, but also recognize that it is the same task she does. Then she has to see what treat the other capuchin gets, and then realize that it is better than the treat she gets. Finally, and most importantly, she has to CARE about that disparity – this is what we call fairness (and what researchers call inequality aversion).

In order to study fairness in capuchins, Brosnan & de Waal did the exchange task with two capuchins that were in separate, side-by-side enclosures. There were no opaque barriers between the enclosures, so capuchins could clearly see each other doing the task. In the inequality condition of the exchange task, one of the capuchins would always receive a grape (a more preferred treat), and the other capuchin would always receive a piece of cucumber (a less preferred treat). As a control, the researchers also conducted sessions of the task where both capuchins got the same treat.

small__5546971563Brosnan & de Waal found that in the inequality condition, the capuchin receiving the cucumber often refused to do the exchange task, either by refusing to return the token or refusing to take the treat (here’s a short clip showing a hilarious example of this – I highly recommend watching it!). These “non-exchanges” occurred on about 40% of trials in the inequality condition, but only about 5% of trials in the control condition (when both capuchins got the same treat). So when the researcher wasn’t playing fair, the cheated capuchin often refused to play at all!

The researchers decided to do the same task, but with an even bigger disparity. This time, instead of merely giving the capuchins unequal pay for equal work, the capuchin receiving the grape didn’t have to work for it at all – she was just given the grape, while the other capuchin still had to exchange for the cucumber. In this condition, non-exchanges by the cucumber capuchin increased to around 80% of trials!

The results of these two experiments suggest that capuchins do have some concept of fairness, though it may not be exactly like the concept of fairness that humans have. In a final experiment, Brosnan & de Waal investigated the effect of the mere presence of a grape on the cucumber capuchin’s reaction. They tested only one capuchin at a time, making her exchange for a piece of cucumber, and placing a grape in front of the (empty) adjacent enclosure. The results were similar to the first experiment: the capuchin refused to exchange in about 40% of the trials.

This suggests that their concept of fairness may not require equal treatment relative to other capuchins. Rather, their concept may only require that a better reward exists, regardless of whether another capuchin is receiving that reward.

Overall, these results suggest that some kind of concept of fairness may be innate in primates (including humans). However, evolutionarily, the capuchins’ behavior seems a bit puzzling: If you’re trying to survive, you should take whatever food you can get, rather than refuse it because something better exists (especially if you wouldn’t get that better food anyway). How could this behavior be evolutionarily advantageous? (Hint: Think about why the researchers did this experiment with capuchins in the first place.)

A lot of work research on fairness in primates has been done since Brosnan & de Waal published their results. Here’s a recent review of that literature, for those who are interested (unfortunately, the article isn’t available for free, but hopefully those affiliated with a university or research institute can access it).

(The clip I linked to above is taken from de Waal’s fascinating TED Talk on moral behavior in animals – check it out!)


Brosnan, Sarah F., and Frans BM De Waal. “Monkeys reject unequal pay.” Nature 425.6955 (2003): 297-299.


One thought on “Will Work for (Equal) Pay

  1. Pingback: Brazil: capuchin monkeys have ‘stone age culture’ | Dear Kitty. Some blog

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