Biology Now

On Being Human

Article Highlights:
• New research suggests that a characteristic that separates humans from our closest primate relatives is our sense of fairness.
• Unlike humans, when chimpanzees played an “ultimatum game” designed by researchers, the chimps’ decision-making was influenced by rational thought rather than by a sense of fairness. 
• This article can serve as a discussion point as an introduction to the study of evolution or animal behavior.

Contents:

• Introduction
• Human Sense of Fairness
• The "Ultimatum Game"
  
• Chimpanzees and Fairness
• Chimpanzees as "Rational Maximizers"
• Conclusion
• Classroom Connections for Teachers

 

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Introduction

What makes us human? What, if anything, clearly distinguishes our species from all other organisms on the planet? Philosophers, humanists, artists, and scientists have pondered this question for centuries, but without much resolution. Human language is often pointed to as a defining characteristic separating us from all other species, and this idea seems apt given the remarkable subtlety, complexity, and cultural dependence of our speech as compared to other animal signals. But even here, as researchers plumb the depths of animal communication more deeply (my own area of expertise), many fundamental underpinnings of speech­—including the brain mechanisms that support its learning, perception and production—are now recognized as not being unique to humans after all.

Human Sense of Fairness

A recent article in Science magazine, one of the world’s premier scientific journals, raises another possibility. In this paper, ­­­­Keith Jensen, Josep Call, and Michael Tomasello from the Max Plank Institute of Evolutionary Anthropology in Leipzig, Germany, suggest that another characteristic separating humans from our closest primate relatives is our sense of fairness (1). This conclusion might seem surprising at first; after all, we all know people who don’t play fair, and we marvel at the cooperation we observe in other animal societies, from the well-ordered corporate structure of an ant colony to the cooperative hunting of a lion pride or the cooperative defense against such predators observed in an elephant group. But examples of highly cooperative and altruistic behavior in animal societies usually can be explained by the fact that members of the group are relatives or, if they are not closely-related, they at least live in stable social groups where they know each other well and interact with each other over a long period. If two animals are relatives, they share genes, and it makes sense to a biologist that they should behave altruistically towards each other because, in so doing, they help pass the same genes to the next generation through a process known as “kin selection.” If two animals are not relatives, but live together in a stable social group, it still makes sense for them to behave altruistically toward each other because they will learn through repeated interactions to help others who help them in return, a process we refer to as “reciprocal altruism.”

The "Ultimatum Game"

But we humans often behave altruistically towards individuals we've never seen before and are unlikely to ever see again. As part of this generosity towards complete strangers, we appear to have a strong sense of fair play that cannot easily be accounted for by kin selection or reciprocal altruism. Consider the following scenario: A person is given $100 and is asked to share some portion of the money with a complete stranger. If the stranger accepts the offered division, then both parties get to keep whatever portion of the $100 was decided on; if the stranger doesn’t accept the offer, however, both parties get nothing. At first you might think that any offered portion of the $100 greater than zero would be accepted by the stranger—after all, getting any money seems to be better than getting no money at all, which is the outcome if the offer isn’t accepted. Remarkably (at least from a biologist’s point of view), we humans don’t behave this way! Instead, if we perceive that the offered division is unfair, then we’d rather take nothing at all. The scenario just described is referred to as the “ultimatum game.” In studies of humans across different cultures and ages, it turns out that the tipping point for what seems fair is approximately an 80:20 division (2). If the stranger is offered less than $20 of the $100 to be divided, he or she will not accept the offer, even though this means they’ll get nothing when they could have gotten something. Apparently, it just doesn’t seem fair enough!  Thus, we humans are not what can be called “rationale maximizers,” responding to any economically advantageous offer ($10 is not as good as $30 dollars in this deal, but it’s certainly better than nothing). Instead, our behavior reflects a sensitivity to fairness—we’d rather have nothing at all if the offer isn’t perceived as being fair.

Chimpanzees and Fairness

What about other animals? Do they have a comparable sense of fair play?  It appears that one of our closest animal relatives, the chimpanzee, does not. Make no mistake about it—chimps are highly social animals. They live in large and complex groups, they cooperate in all sorts of ways, and they even punish individuals who “cheat” in social interactions, such as sharing a source of food. But they don’t appear to have as much of a sense of fair play as we humans do. In their study, Jensen and his colleagues developed a clever way to have chimpanzees play a version of the “ultimatum game.” One chimp, the “proposer,” is able to reach a rope that can pull a sliding tray halfway toward its cage. A second chimp, referred to as the “responder,” sits in a cage adjacent to the proposer chimp. When the proposer pulls the tray half-way toward its cage, the tray also moves half-way toward the responder cage, and once this has happened, the responder can now reach a rod that will pull the tray the rest of the way to both the proposer’s and the responder’s cages. The tray actually has two containers, one that can be reached only by the proposer chimp when the tray is moved all the way toward the cages, and the other that can be reached only by the responder chimp. For either chimp to gain access to their container, the proposer must first move it halfway and then the responder must move it the rest of the way to the cages. The containers, of course, contain something of value to the chimps—not cash as in our human example, but raisins, which are a coveted treat.

What makes this setup like the “ultimatum game” is that there are actually two sets of containers that can be pulled toward the cages. The sets differ in the distribution of raisins in the two containers. One set might have five raisins in each container, for example, while the other set has eight raisins in the container that the proposer would get and two raisins in the container that the responder would get. Both chimps can see the distribution of raisins in the containers before the test starts. By testing different pairs of chimps with a variety of different distributions of raisins in the two containers, the experimenters were able to get chimps to play a version of the “ultimatum game”—the proposer chimp can pull a tray with a more-fair or a less-fair distribution of raisins, and the responder chimp can decide to “accept” the offer by pulling the tray the rest of the way toward the cages. If humans played this game with $10 bills instead of raisins, we would expect that the responder would only pull the tray the rest of the way toward the players if the proposer offered more than two $10 bills and kept fewer than eight $10 bills for themselves. If chimps exhibit a similar sense of fair play, then we would predict the responder chimp would only bring the tray in the rest of the way if the proposer offered more than two raisins out of the ten total.

Chimpanzees as “Rational Maximizers”

Not so for the chimps, as it turns out. If the proposer chooses a tray having even just one raisin for the responder, the responder will pull the tray all the way in, gaining its single raisin and allowing the proposer to gain access to the nine other raisins. This occurs even though the responder can see clearly that the alternative tray—the one not chosen by the proposer—has a fairer distribution of raisins. And it’s not just that the responder chimp feels compelled to pull the tray out of a sense of curiosity or boredom. If the proposer chooses a tray that offers no raisins at all to the responder, then the responder will not pull the tray the rest of the way. There’s no reason to, since there’s no reward at all for the responder in that case. The key difference between chimps and humans on this kind of test is that humans will only accept an offered division of some reward if the division is perceived as being relatively fair, while chimps will accept anything at all as long as they get something. Chimps behave like “rational maximizers” while humans behave more out of a sense of fairness.

Conclusion

Although the results of Jensen, Call and Tomasello’s experiment might diminish our view of the nobility of our evolutionary cousins a bit, it is also comforting to think that a sense of fair play is deeply intrinsic to what it means to be human. In spite of our foibles and occasional failings, our species has been remarkably successful in part because we live in complex societies, in which we must interact and cooperate with a bewildering number of other individuals, most of whom we will never know personally. Unselfish behavior can be explained by biologists through kin selection and reciprocal altruism, both of which surely apply to the human condition, but neither of which can fully explain the extent of our ability to cooperate with complete strangers. How our sense of fair play arose remains an open question. However, the fact that we somehow acquired an ingrained sense of fairness somewhere in our evolutionary history may well have been a key to the subsequent rise of human societies, which over the course of several thousands of years led to the modern world in which we live today. And all this, just because we know how to play fair.

Classroom Connections for Teachers

Animal behavior is a great way to excite students about biology, as is clear from the perennial popularity of nature shows on television, and teachers can capitalize on this interest by connecting behavior to other aspects of a typical high school curriculum. The study by Jensen and colleagues might be used to introduce a section on animal behavior, but it could also be used as an engaging way to introduce a section on evolution more generally. A student thinking about evolution for the first time is likely to come back to the age-old question of how humans differ from other animals. This paper provides an interesting new suggestion, based on a clever and relatively straightforward experiment.  In fact, this work could be used as a fun way to talk about experimental design in general.