The Evolution of Fairness

I have had the pleasure of meeting Franz de Waal on two occasions.  The first time was during an NSF Summer Institute at Dartmouth and the second was when he graciously agreed to sit on a panel I organized.  He is a very nice fellow. 

Since 2003, de Waal and Dr. Sarah Brosnan have been studying fairness behavior in monkeys.  Their basic research questions, as I understand it, is to what extent do nonhuman primates recognize and respond to situations of fairness and unfairness and how can this be explained by evolutionary theory?  The pair have published a paper surveying the literature and their findings in Science.  I don’t have access to the paper yet, but I have just read a summary of it at phys.org.  Here is Brosnan’s description of their hypothesis:

"This sense of fairness is the basis of lots of things in human society, from wage discrimination to international politics," Brosnan said. "What we're interested in is why humans aren't happy with what we have, even if it's good enough, if someone else has more. What we hypothesize is that this matters because evolution is relative. If you are cooperating with someone who takes more of the benefits accrued, they will do better than you, at your expense. Therefore, we began to explore whether responses to inequity were common in other cooperative species."

Obviously, the question of the evolution of a sense of fairness in monkeys and nonhuman apes bears of the question of the natural history of human fairness.  Human beings are extraordinarily cooperative animals.  Our capacity for cooperation is possible in large part because we are capable of a sense of obligation toward others and a tendency toward righteous indignation when others fail to oblige in return. 

It is not immediately obvious how either capacity emerges in our evolutionary history.  The sense of obligation means that we give unto others when we don’t have to do so, which entails a cost in resources.  If I share what I have in my hands, I have less for myself.  The indignation means that I may refuse to accept a share from another that I regard as unfair, thus getting nothing (and perhaps getting into a fight) rather than getting something.  In a situation where resources and needs are marginally related (which was our situation for most of our time as a species on this earth) getting something rather than nothing would seem like the obviously better choice. 

It turns out that the indignation part of the equation is easier to explain.  Here is a summary of their 2003 paper in Nature. 

In this study, brown capuchin monkeys became agitated and refused to perform a task when a partner received a superior reward for that same task. To view video footage of the study, visit http://www.youtube.com/watch?v=meiU6TxysCg. Since then, Brosnan has tested responses to inequity in nine different species of primates, including humans. She has found that species only respond to inequity when they routinely cooperate with those who are not related to them.

Here I am speculating on what de Waal and Brosnan have found and what it might mean.  Animals that cooperate with non-kin face a problem that animals who cooperate only with closely related individuals mostly do not.  Instincts for cooperation will be selected for only if they advance the reproductive success of the cooperator.  If cooperative associations reap benefits that are not available to conspecific non-cooperators, then the one will outbreed the other.  The species will evolve toward greater cooperation. 

However, what is true between the cooperators and non-cooperators will be true within the population of cooperators if some routinely exploit others.  Let us consider an over-simplified scenario in which two version of a key gene are evenly distributed across a population of cooperators.  One version (RI1) codes for righteous indignation whenever that animal doesn’t receive a fair share of the benefits of cooperation and the other version of the gene (RI2) does not.  Assuming that RI2 results in a smaller payoff from cooperation (and it only has to be a very small difference) then RI2 will gradually disappear from the general population. 

The really neat thing about this is that it doesn’t matter how big the payoff is.  Whether the population is small or large, struggling or flourishing, within the population of cooperators, the indignant will increase and the complacent will wither away.  So when someone says “it’s not the money, it’s the principle of the thing”, they are expressing something that has a long evolutionary history.  An inherited sense of morality will maintain itself in the population only if it cares as much about the principle as the profit. 

The evolution of obligation is rather more difficult.  It is easier to see why an animal would be offended by a smaller share than feel an obligation to share when she has more than her partner. 

Responding to getting less than a partner is not the only aspect of fairness. For a true sense of fairness, it also matters if you get more. Brosnan and de Waal hypothesize that individuals should be willing to give up a benefit in order to reach equal outcomes and stabilize valuable, long-term cooperative relationships. Thus far, this has only been found in humans and their closest relatives, the apes.

A willingness to share equitably with others when you could take more for yourself means that you are (more or less consciously) concerned about maintain a reputation as a good cooperator.  That requires a more sophisticated psychology than we see in most primates. 

I close by noting that Plato was right.  The idea of justice is as real as the idea of a triangle.  The one governs the architecture of certain animal societies as much as the other governs the architecture of roofs.