Game theory uses deceptively simple challenges to provide insights into human decision making and cooperation. Many of the challenges force players to choose between (for example) taking a small but guaranteed payoff or a big payoff that will be lost unless another individual cooperates. The games themselves are often simple enough that they can be adapted to work with other primates so that researchers can determine which human behaviors are shared with our closer relatives. But that adaptation can significantly change the appearance of the game, raising questions about whether the results are actually comparable. So, to avoid this problem, a group of researchers attempted to test humans and primates using a single game that was largely the same for all species tested.

The game they used is technically called the assurance game, but is also known as the "stag hunt" by researchers. Participants are given a choice: they can take a small reward that's guaranteed (a rabbit), or risk choosing a big reward (the stag) that can only be obtained if a partner has chosen the stag as well. In the case of the primates (chimps, capuchins, and humans), the hunted animals were replaced by different quantities of money or food, depending on whether the primate was human.

To even things out as much as possible, the authors replaced the rabbit and stag with different colored poker chips. To avoid letting the humans' ability with language give them an unfair advantage, the participants were given minimal instructions—nothing about the rules of the game, and only a brief mention of what to expect once they sat down; the primates weren't trained on the game, either. The primates, in contrast, had extensive experience with being used in tests that ended with them getting a reward. To keep that from influencing the results, the humans were recruited from among a pool of college students that had previously been involved in economics experiments.

In a lot of ways, this does help to even out matters. But it also changes what the game's all about. When you know the rules, it's a simple test of cooperation and risk/reward (will both individuals risk the stag for a big payoff). Without them, all the primates need to perform an organized set of experiments to identify the pattern of behavior that gives them the biggest reward. Only then can issues of risk and trust come into play.

The authors expected to see differences among the species, and they did, but the differences suggested there was a lot more going on than simple mental horsepower. The capuchins are the most evolutionarily distant from humans, and they did worst from the perspective of maximizing returns: only one of the six pairs of partners chose the high-reward combination more often than they would by chance. But, individually, they made nonrandom choices. One capuchin seemed to like the "stag" chip and used it repeatedly; another switched its pattern when they switched it to a new partner. That last bit could be rather significant, since other primates have social hierarchies that they'd be aware of, which wouldn't apply to college students who don't know each other.

The chimps used in the work came from two different research centers, and exhibited what might be called cultural differences. At one center, most pairs tended to consistently pick matching or anti-matching chips, even after they saw the high reward provided by matching the right kind of chip. At a different center, two pairs of chimps (of four tested) quickly settled into the pattern that gave them a big payoff. But these had been used in cooperative experiments in the past, and might be more inclined to understand what was going on. In another pair, one individual consistently played stag, but got undercut by her partner choosing randomly.

Those of you expecting the humans to show their fellow primates how to play the game: prepare for some disappointment. Less than 20 percent of the human pairings managed to consistently get a high payoff. In contrast, 30 percent never managed to do that. Like some of the primates, most of the students picked a color and stayed loyal to it, and some of the pairings consistently matched their chips, even if it only ensured a low-payoff. Overall, most of the participants seem to have failed to understand the rules of the game, or simply couldn't be bothered enough to do so.

Overall, humans did better than chimps, and chimps than capuchins. But it's not as if the humans blew away the competition, and some pairings of the other species seemed to figure the rules out and work with them. Thus, the result seems to suggest that the mental capacity to figure out the rules of the game might be a spectrum. Humans may be at one end, but they haven't made a radical departure in ability compared to their primate peers.

Despite the best efforts of the authors to equalize cross-species influences, it's not clear that the results are free of individual peculiarities and, at least outside of the humans, social influences. The primates raised here share housing and are part of the same social group. Those same things tend to apply to college students, but the typical college class is much larger and (probably) less hierarchical than a primate social group. It's also impossible to tell whether any of the participants simply didn't care about the rewards enough to learn the rules of the game, or play it to maximize rewards if they did. The fact that you can't distinguish these two possibilities also limits the conclusions you can draw from the work.

Overall, it's an interesting first effort, but a much larger study would seem to be needed before any conclusions can really be extracted from the data. In the end, the value of the paper may be in simply highlighting the limitations we face when making cross-species comparisons of behavior.

PNAS, 2011. DOI: 10.1073/pnas.1016269108 (About DOIs).