THE extraordinary success of Homo sapiens is a result of four things: intelligence, language, an ability to manipulate objects dexterously in order to make tools, and co-operation. Over the decades the anthropological spotlight has shifted from one to another of these as the prime mover of the package, and thus the fundament of the human condition. At the moment co-operation is the most fashionable subject of investigation. In particular, why are humans so willing to collaborate with unrelated strangers, even to the point of risking being cheated by people whose characters they cannot possibly know? Evidence from economic games played in the laboratory for real money suggests humans are both trusting of those they have no reason to expect they will ever see again, and surprisingly unwilling to cheat them—and that these phenomena are deeply ingrained in the species's psychology. Existing theories of the evolution of trust depend either on the participants being relatives (and thus sharing genes) or on their relationship being long-term, with each keeping count to make sure the overall benefits of collaboration exceed the costs. Neither applies in the case of passing strangers, and that has led to speculation that something extraordinary, such as a need for extreme collaboration prompted by the emergence of warfare that uses weapons, has happened in recent human evolution to promote the emergence of an instinct for unconditional generosity.

Leda Cosmides and John Tooby, two doyens of the field, who work at the University of California, Santa Barbara, do not agree. They see no need for extraordinary mechanisms and the latest study to come from their group (the actual work was done by Andrew Delton and Max Krasnow, who have just published the results in the Proceedings of the National Academy of Sciences) suggests they are right. It also shows the value of applying common sense to psychological analyses—but then of backing that common sense with some solid mathematical modelling.

Be seeing you

Studying human evolution directly is obviously impossible. The generation times are far too long. But it is possible to isolate features of interest and examine how they evolve in computer simulations. To this end Dr Delton and Dr Krasnow designed software agents that were able to meet up and interact in a computer's processor.

The agents' interactions mimicked those of economic games in the real world, though the currency was arbitrary “fitness units” rather than dollars. This meant that agents which successfully collaborated built up fitness over the period of their collaboration. Those that cheated on the first encounter got a one-off allocation of fitness, but would never be trusted in the future. Each agent had an inbuilt and heritable level of trustworthiness (ie, the likelihood that it would cheat at the first opportunity) and, in each encounter it had, it was assigned a level of likelihood (detectable by the other agent) that it would be back for further interactions.

After a certain amount of time the agents reproduced in proportion to their accumulated fitness; the old generation died, and the young took over. The process was then repeated for 10,000 generations (equivalent to about 200,000 years of human history, or the entire period for which Homo sapiens has existed), to see what level of collaboration would emerge.

The upshot was that, as the researchers predicted, generosity pays—or, rather, the cost of early selfishness is greater than the cost of trust. This is because the likelihood that an encounter will be one-off, and thus worth cheating on, is just that: a likelihood, rather than a certainty. This fact was reflected in the way the likelihood values were created in the model. They were drawn from a probability distribution, so the actual future encounter rate was only indicated, not precisely determined by them.

For most plausible sets of costs, benefits and chances of future encounters the simulation found that it pays to be trusting, even though you will sometimes be cheated. Which, if you think about it, makes perfect sense. Previous attempts to study the evolution of trust using games have been arranged to make it clear to the participants whether their encounter was a one-off, and drawn their conclusions accordingly. That, though, is hardly realistic. In the real world, although you might guess, based on the circumstances, whether or not you will meet someone again, you cannot know for sure. Moreover, in the ancient world of hunter-gatherers, limited movement meant a second encounter would be much more likely than it is in the populous, modern urban world.

No need, then, for special mechanisms to explain generosity. An open hand to the stranger makes evolutionary as well as moral sense. Except, of course, that those two senses are probably, biologically speaking, the same thing. But that would be the subject of a different article.