In both biology and economy, game-theory models are used to understand human behavior, mainly concerning cooperation. These models usually incorporate gains, benefits, and reciprocity. And this reciprocity seems to be very important. Basically, if you do something for me, I’ll return the favor (in such a way that the benefits for me outweigh the costs). But human beings also show generosity towards complete strangers in one-shot encounters. How, then, can this be explained?

A new paper, published in the Proceedings of the National Academy of Sciences, argues this one-shot generosity is “the necessary by-product of selection on decision systems for regulating dyadic reciprocity under conditions of uncertainty.” What this means, is that the uncertainty of being indeed a one-shot encounter spurs us to ‘play safe’ and be nice, just in case we would meet the beneficiary of our actions again. If so, a mutually beneficial relationship might ensue, because we were nice the first time. So, this translates into, ‘I don’t know if I’ll meet you again, but just in case, I’ll be nice this time’.

There are two main assumptions here. First, the costs of potentially mistaking a repeated encounter for a one-shot one, are greater than the cost associated with the single ‘nice’ action. Secondly, there is uncertainty about whether or not the encounter will be repeated. Based on these assumptions, the authors developed models to test their hypothesis. These models were based on the well-known Prisoners' dilemma (see figure 1), and comprised two important components: a cognitive one and a motivational one. The cognitive component reflects the ability to assess whether it’s a one-shot encounter or a repeated interaction, and the motivational component reflects the motivation to act according your beliefs. So, if you think you’re in a one-shot interaction, defect, if not, cooperate.


Figure 1: Prisoners' Dilemma.


Two sets of models were developed. In one set, the cognitive component could evolve, while the motivational one was set. The other set had a set cognitive component, and a motivational one that could evolve.

The results of both sets of models were quite similar. If the average number of interactions between two individuals was five or higher, this will lead to the development of generosity in one-shot encounters. According to the authors, this conditions is certainly met in human beings.

...we suspect that selection producing one-shot generosity is likely to be especially strong for our species. The human social world — ancestrally and currently— involves anabundance of high-iteration repeat interactions and high-benefit exchanges.

The overall conclusion of the authors is:

In short, the conditions that promote the evolution of reciprocity —numerous repeat interactions and high-benefit exchanges— tend to promote one-shot generosity as well. Consequently, one-shot generosity should commonly coevolve with reciprocity. This statement is not a claim that direct reciprocity is the only force shaping human cooperation —only that if reciprocity is selected for (as it obviously was in humans), its existence casts a halo of generosity across a broad variety of circumstances.

While this is an interesting idea concerning the potential origin of human generosity in one-shot encounters, modeling human behavior is always tricky. At best, it’s a proxy for reality. Luckily, the authors are not blind for this restriction, as they note:

Although it is difficult to precisely map parameters in simplified models to real-world conditions,…

In their overall conclusion (see second quote), the authors also note that reciprocity needn’t be the only force shaping human cooperation. And indeed, culture shouldn’t be ignored. Most of us are taught that being generous is a good quality, and are usually encouraged to behave generous. It is not inconceivable that the matter is complex (as most matters in life usually are), involving interactions between our biological roots, reciprocity, uncertainty and culture.



Delton, A.W.; Krasnow, M.M.; Cosmides, L. and Tooby, J. (2011). Evolution of direct reciprocity under uncertainty can explain human generosity in one-shot encounters. Proceedings of the National Academy of Sciences. Published onlinebefore print, July 25. doi:10.1073/pnas.1102131108. (Click here for the article)