AFP

PEOPLE love to catalogue the traits they think characteristic of their species. Some, such as language, are obvious. Others, such as patience and a sense of fairness, are more subtle. These traits, however, did not spring into existence fully formed. They evolved—and to understand human evolution it would help to know their genetic underpinnings and the order in which they evolved.

One way of looking at these questions is to compare people with their closest relatives, great apes such as chimpanzees. Another is to compare them with each other. Three studies published this week, which take one or other of these approaches, have cast light on the evolution of both patience and fairness. It turns out that patience is older than fairness. It also turns out that although the propensity to be fair varies a good deal from one person to the next, that variation is rooted in genetics rather than culture.

The origin of virtues

The essence of patience is the ability to delay the gratification of an appetite in favour of a greater ultimate reward. Past tests of the degree to which animals other than people can delay their gratification have focused on birds and monkeys. Both groups can delay gratification if a bigger reward is on offer, but only for a few seconds.

Birds, however, are remotely related to humans, and even monkeys are not as close as apes. In Current Biology, Marc Hauser of Harvard University and his colleagues compare chimpanzees and humans directly. Both, it turns out, can be patient to a high degree. In fact chimps are more patient than people.

The human participants in Dr Hauser's experiment were allowed to choose a preferred food, such as raisins or chocolate. The chimpanzees were simply offered grapes—which they usually like. Otherwise the experimental conditions were identical. The choice was between one unit of goodies immediately and three after two minutes. Chimpanzees were nearly four times more likely to wait for the big reward than humans were. This suggests not only that the trait of patience predates the split between humans and chimpanzees, some 4m years ago, but that the trait seems more characteristic of chimps than people.

When it comes to fairness, though, it is a different story. Economic theory has contrived a species it calls Homo economicus—a “rational maximiser” who grabs what he can for himself. But, curiously, he makes no appearance in the ultimatum game, a classic economics experiment.

In this game, two players, a proposer and a responder, divide a reward. It could be a cake. It could be cash. It could even be a bunch of grapes. The game is so named because the proposition is an ultimatum. The responder can either accept the division or reject it. If he rejects it, both players receive nothing.

Homo economicus would accept any division in which his share was not zero. But that is not what happens. Scores of studies have run the ultimatum game across cultures and ages. Universally, people reject any share lower than 20%—apparently to punish the greed of the proposer. People do not act like Homo economicus. Instead, they are the arbiters of fairness.

To find out if chimpanzees share this sense of fairness, Keith Jensen and his colleagues at the Max Planck Institute for Evolutionary Anthropology, in Leipzig, designed a way for chimps to play the ultimatum game. Their version started with a pair of trays far from the players' cages. Each tray had ten raisins divided in different ways between two pots—say eight and two, or five and five. One chimp was allotted the role of proposer. He could choose one of the trays, pulling it by way of a rope just halfway to the cage. The other, the responder, could then choose to pull on a rod, bringing the tray close enough for both to get the raisins, one pot for each. If the responder chose not to pull the tray closer within a minute, the offer was considered rejected, and the game concluded.

The result, which Dr Jensen reports in Science, is that chimps are simply rational maximisers—Pan economicus, if you like. Though proposers consistently chose the highest possible number of raisins for themselves, responders rarely rejected even the stingiest offers.

This is a telling outcome. A number of researchers in the field of human evolution think that a sense of fairness—and a willingness to punish the unfair even at some cost to oneself—is humanity's “killer app”. It is what allows large social groups to form. Without it, free-riders would ruin such groups, because playing fair would cease to have any value. Dr Jensen's previous experiments have shown that chimpanzees are willing to punish actual thieves. But his new data add weight to the theory that the more sophisticated idea of fair shares, which underpins collaborative behaviour, appeared in the hominid line only after the ancestors of the two species split from one another.

Nor, according to the third of this convenient trilogy of papers, is a sense of fairness rooted in culture. Rather, it is genetic—as it would have to be in order to evolve. Paradoxically, discovering this relies on the fact that not everyone possesses it to the same degree.

As they write in the Proceedings of the National Academy of Sciences, Bjorn Wallace of the Stockholm School of Economics and his colleagues have shown this by playing the ultimatum game with twins. They used the classic trick of neutralising the effect of upbringing and exposing that of genetics by comparing identical twins (who share all their genes) with fraternal twins (who share half).

Each twin of a pair played the ultimatum game, both as proposer and as responder. Dr Wallace found, in the case of identical twins, a striking correlation between the average division that each member of a pair proposed and also between what they were willing to accept. In other words, their senses of what was fair were similar. No such correlations were seen in the behaviour of fraternal twins.

Besides showing that a sense of fairness has a genetic basis, this result also raises a question: why should the sense of what is fair be so variable? It may be that in a population of the fair, the unfair prosper while amongst the unfair, the fair are better off. The result would be an equilibrium in which various attitudes to fairness do just as well as each other. But why, exactly, that should be the case is a subject for another day's research project.