The Evolution of Fairness

I have been very interested of late in the logic of justice.  Plato’s Socrates assumed that justice was an idea, which is to say a logically coherent form.  Just as you know that a triangle has three angles and that the angles add up to 180 degrees, regardless of the dimensions of the plane figure,  if you understand what a triangle is, so you will know what the right thing to do in any context if you understand the logically idea of justice. 

My field of research involves the historical dimension of this idea.  How is that the dimension of justice emerged in the evolution of the human soul?  That it did emerge is not in doubt.  Even if you think that claims of justice are always or almost always confidence games, you might want to know why they work.  What are human beings charmed by the idea of justice?  To put that question in a more useful form: how did human beings become the kind of creatures that could be so charmed? 

Game theory, paired with evolutionary psychology, has come a long way toward answering that question.  The ultimatum game presents justice in a simple, logical dilemma.  In this game, two individuals (the Proposer and the Responder) must come to an agreement over the division of a benefit.  The one proposes a distribution: 90% me, 10% you, 50/50, etc.  The other must accept the distribution, or neither of them gets anything. 

Simple economic theory predicts that the responder will accept any distribution above 100/0, for something is better than nothing.  However, when the game is played with real persons, the proposer usually offers considerably more than the minimum and the responder usually rejects any offer that is not close to parity.  This is evidence that the human mind is robustly aware of the logic of fairness. 

So why does the partner at an advantage offer more than the minimum and why does the partner at a disadvantage refuse to accept an unfair distribution at the price of nothing?  It might make perfect sense to do so if the tables might be turned next time.  If I am generous to the responder now, he may be generous to me when he gets to be the proposer.  If he screws me now, just wait until next year.  I’ll get that bastard. 

That makes sense if proposers and responders are randomly assigned their roles with each turn.  Stéphane Debove, Nicholas Baumard, and Jean-Baptiste André move this logic one essential step forward in their essay “Evolution of equal division among unequal partners.”  Find it in Evolution 69-2: 561-569. 

Debove et. al. point out that a random assignment of roles in natural encounters is not plausible.  In nature, stronger individuals will find themselves in the role of proposer again and again, and vice versa.  If the stronger individual doesn’t have to fear that he will end up in the responder role, what incentive does he have to be generous? 

The answer is that the stronger individual is not competing only with the weaker ones for prizes; he is competing also with stronger individuals for partners.  If weaker partners are readily available, the cost of choosing any one partner is low.  It is in the advantage of the stronger individual to make as many partnerships as fast and as often as possible.  The easiest way to close those deals is to be generous. 

Our authors calculate that the generosity of the stronger proposers will decline as the supply of weaker responders goes down.  When fewer deals are possible, it pays to be choosy.  But even when weaker responders are few, and the cost of choosing is greatest, generosity will increase over time. 

This piece of research is not based on empirical evidence.  It is based on simple logic.  It makes the case that nice guys finish first, eventually, regardless of the particular situations.  Plato was right.  I note that the authors opened with a mention of Plato and Rousseau.