Do only humans play fair? Nope, chimpanzees do too…


Is the sense of fairness uniquely human?

So begins a paper published in the Proceedings of National Academy of Sciences by Frans B. M. de Waal and colleagues. The question is indeed worth asking since the commonly held view is that only humans have a highly developed sense of morality and hence the concept of behaving fairly to others would also be uniquely human.

However, this new research adds up to the growing pile of evidence that morality is not something exclusive to humans but other primates too have a complicated sense of morality.  The current research here depends on a study conducted on two sets of participants – chimpanzees (Pan troglodytes) and human children on a modified ultimatum game. Before continuing lets try and understand, the concept of ultimatum game.

What is an Ultimate Game?

Humans often strange decisions which seems irrational from an economical perspective. For instance, some person might decide to decrease his/her own wealth greatly to donate it to charity. Now, this suggests that we humans, often care about others more than we care about ourselves.  To understand these decisions, more carefully economists design games which test and try to understand the economic decision making in humans.

The ultimatum game is a game often played in economic experiments in which two players interact to decide how to divide a sum of money that is given to them

The first player proposes how to divide the sum between the two players, and the second player can either accept or reject this proposal. If the second player rejects, neither player receives anything. If the second player accepts, the money is split according to the proposal. The game is played only once so that reciprocation is not an issue.

What did the authors do in the present study?

The authors used this ultimatum game with certain modifications and also a dictator game        (essentially similar like ultimatum game but participants cannot reject an unfair offer) to understand how chimpanzees and children ( ages 3-4 years) share tokens which acted essentially like money that could be exchanged for food. The main aim of the present study was to investigate how sensitive chimpanzees are to reward distribution when their partner can affect it. If they are sensitive to partner effects, their choices in the UG should resemble those of humans.

What the authors did find ?

Chimpanzees and children were similarly sensitive to ultimatum game. In a simple choice task resembling the Dictator Game, with either a passive partner (chimpanzees) or while alone (children), both species preferentially chose a “selfish” offer that brought the majority of rewards to themselves. In the Ultimatum Game condition, in contrast, respondents could affect the outcome (by accepting or rejecting the offer), and both species shifted their choices to a more equitable distribution. This shift is similar to the way adult humans change their offers between Dictator Games and Ultimatum Games.

A video from Emory University showing chimpanzees playing the ultimatum game

What does this finding signify ?

The findings reinforces the current wealth of data emerging from economic games, neuroimaging and theoretical work that the concept of morality is not unique to humans.Hence, the human tendency to share might have a more ancient evolutionary history than previously thought.

More on this:

  1. Introduction to Ultimatum Games 
  2. The evolution of fairness: explaining variation in bargaining behaviour, Lamba S, Mace R, Proceedings of the Royal Society B: Biological Sciences, 2013.
  3. Chimpanzees coordinate in a negotiation game, Melis AP, Hare B, TomaselloM, Evolution of Human Behaviour, 2009.
  4. Monkeys reject unequal pay, Brosnan SF, de Waal FBM, Nature, 2003.
  5. Cooperation in Primates and Humans: Mechanisms and Evolution, Peter Kappeler, Springer Publications, 2006.