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.

Aww, Yuck! The Science of Disgust

disgust Yuck, Puke, all the different things one can think about when feeling disgust. Like seeing cockroaches crawling all over the floor, or when seeing dead bodies or in some very weird cases like mine, when eating pineapple slices with a hint of salt on top. It is one of myriad of other emotions we humans have. But, what is disgust ? Is it any similar to disliking anything? What is the evolutionary origin of such an emotion? And also, what role does disgust play in our modern human society? So, lets try and understand this very strange and in some ways, highly useful behaviour.

What is DISGUST ?

Wikipedia says – Disgust is a type of aversive reaction that involves withdrawing from a person or object with strong expressions of revulsion whether real or pretended”. Simply speaking, disgust is a feeling of repulsion which one feels towards myriad of different objects, or people. It can come in various contexts – like the sight of garbage piling up, rotting food or faeces. In some of these cases one doesn’t even have to teach the growing child about them. So, is it something innate? Well, indeed it is one of the universal, basic emotions which helps the organism to survive by keeping away from ingesting harmful substances. Hence, it is classified in some contexts as a disease avoidance mechanism. However, disgust also has a social connotation in humans. For example, a music virtuoso might feel a badly composed music disgusting, or for a gourmet chef, an improper blend of spices would be disgusting.   Also, as disgust is an emotion it also has physiological implications. Unlike other emotions, for example anger or fear which increase our heart beat, disgust decreases it.

Evolution of Disgust

Evolutionarily speaking, disgust is a very important emotion since it allows the organisms to avoid contaminated food that may cause harm to them.It has been thought that disgust is an important component of behavioural immune system, which is responsible to keep us away from any harmful pathogens i.e, an avoidance mechanism. Early on, this would have been quite beneficial to us and help us survive. Though tracing the early history of disgust, is quite impossible since behaviours don’t fossilize. But recent studies in Caenorabhditis elegans, a highly simple roundworm exhibits avoidance behaviour towards various pathogenic strain of bacteria like, Bacillus thuringiensis, Pseudomonas aeroginosa, Serratia marcescens. Other species have also shown to exhibit hygiene maintenance, for example – Ants, Bees, Bullfrog (Rana catesbeiana), Fishes, Bats, Birds, and various mammals. Hence, the earliest form of disgust can be found in the hygiene maintenance behaviour of all the different organisms. As i said earlier, disgust is an instinct which is present in humans too. In a web-based experiment Valerie A Curtis, tested the hypothesis that disgust has evolved to keep humans away from harmful, pathogenic environments. Here is the link to the test-

The test asked people to score how disgusting they found a series of photos. Within the photo series, the authors randomly mixed seven sets of pairs of images, made to be similar in appearance, but in each case one of the pictures contained a disease threat, while the other did not. The results from the scores submitted by over 40,000 people show :

  • Images of objects holding a potential disease threat were reported as significantly more disgusting than similar images with little or no disease relevance.
  • The pattern of response was found to be similar across all regions of the world.
  • Females reported a higher disgust sensitivity than the males.
  • A constant decline in 

    disgust sensitivity over the life course was also reported.

Hence, it is safe to conclude that disgust is probably common to all humans in all the myriad of cultures and it serves as an avoidance mechanism towards all the objects which were associated with a disease risk in the evolutionary past.

Morality & Disgust ?

In humans, various studies have found these disgust elicitors:

  • decaying food
  • dead organisms
  • body products (urine, faeces, saliva, sexual fluids)
  • various organisms (cockroaches, fleas, rats etc)
  • unhygenic environments
  • any signs of infection

Tyber et al, have classified disgust in three parts-

  • Pathogen disgust – disgust aimed at keeping us away from harmful organisms or unhygienic environments.
  • Sexual disgust – disgust which keeps us away form dangerous sexual partners or behaviours. Example of these would be avoidance of costly mates or inbreeding.
  • Moral disgust – disgust keeping us away from breaking social norms. As Mark Pagel in his new influential book – Wired for Culture , writes that its the capability of humans to form strong social associations and hence the diverse cultures which we have separates us apart from other animals. So, in this context moral disgust can be used to keep us away from various social transgressions and would potentially include behaviors such as lying, theft, murder, and rape. Hence, this kind of disgust would lead us to avoid forming associations with norm-violating individuals within groups.

Disgust has always served as a very important rhetorical tool for discrediting, undermining or demonizing an opponent or a group of people with whom you don’t agree. An easy way to do those things would be to portray someone as infecting the integrity of your own social group. The effects of appealing to disgust towards certain situations is quite subliminal and not at all a conscious effort. Jones and Fitness have coined the term “moral hypervigilance” to describe the phenomenon that individuals who are prone to physical disgust will also be prone to moral disgust. An example of this would be how our culture often refers to criminals as “slime” and criminal activity as “stinking”.

So, this leads us to question: If a large proportion of a population sees a behaviour to be disgusting, then is that a good reason to think that behaviour/practice is immoral“?

The answer to this would be an emphatic NO. Since different cultures have different sensitivities and these lead to differences in the practices people see as disgusting. hence, attaching moral values to disgust would be very wrong. And yes, it would be “MORALLY” wrong to do it.

More on this-

  1. Yuck!: The Nature and Moral Significance of Disgust by Daniel Kelly
  2. Pathogenic bacteria induce aversive olfactory learning in Caenorhabditis elegans, Zhang Y, Lu H, Bargmann CI., Nature, 2005.
  3. The genetics of pathogen avoidance in Caenorhabditis elegans, Schulenburg H, Ewbank JJ., Mol Microbiol., 2007.

  4. Disgust as a disease-avoidance mechanism, Oaten M, Stevenson RJ, Case TI., Psychol Bull. 2009.
  5. The effect of disgust conditioning and disgust sensitivity on appraisals of moral transgressions, Bieke David, Bunmi O. Olatunji, Personality and Individual Differences, 2011.
  6. Microbes, mating, and morality: Individual differences in three functional domains of disgust, Tybur, Joshua M. Lieberman, Debra; Griskevicius, Vladas ,Journal of Personality and Social Psychology,2009.
  7. Evidence that disgust evolved to protect from risk of disease, Val Curtis, Robert Aunger and Tamer Rabie, Proceedings of Royal Society of London B, 2004