Playing by the same rules reduces the differences between humans, chimps and monkeys

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You’ve been asked to compete against some of your friends in a game of skill, but you realise something is amiss. They’ve been given precise instructions and details about the game’s mechanics. You’ve been given a couple of pieces and left to figure things out on your own. On this uneven playing field, no one could fairly compare your performance with that of your friends. This seems obvious, but it’s a problem that plagues a lot of research into the behaviour of humans and other animals.

Scientists will often test monkeys and apes with tweaked versions of psychological games that were originally designed to test humans. The goal is simple: understand the similarities and differences between our mental abilities and those of our closest relatives.

But these comparisons are tricky. Frans de Waal, who studies the behaviour of apes and monkeys, says, “Humans are tested by their own species and the apes by a different species (us). Humans understand everything the experimenter says or explains, whereas the ape needs to figure these things out based on experience. The paradigm really doesn’t permit the comparisons that have been made, especially the negative assessment of ape capacities.”

Sarah Brosnan form Georgia State University agrees. “As humans, we surely design tasks that are more intuitive to us than to other species. We don’t know whether humans perform differently from other species absent these advantages. Are human-specific abilities, including language, added ‘on top’ of other primate abilities, making us fundamentally similar in our outcomes. Or are we fundamentally different from the rest of the primates?”

To answer these questions, Brosnan developed a decision-making game that would level the playing field between humans, chimpanzees and capuchin monkeys. As far as possible, all three species played the game under the same conditions, and they played more similarly than you might expect.

Brosnan used an “Assurance game”, also known as a “Stag Hunt”. It’s named after a scenario involving two people on a hunt. Without the other knowing, each must choose to hunt a stag or a hare. A stag takes two people to kill but a single hunter can take a hare. It’s best that both players cooperate to hunt the stag to net the biggest rewards. Failing that, they should both hunt hares to get a prize, albeit a smaller one.

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Brosnan recruited eight capuchins, 24 chimps and 52 students, all of whom had taken part in similar games, where they had been paid in food or money. The all played the Stag Hunt while sitting next to each other in pairs of the same species. Each player had to hand over one of two tokens – one representing each choice. When both tokens were in, Brosnan held them up so the players could see what the other chose. She then handed out rewards– fruit for the monkeys and chimps, and quarters for the students, in the amounts described in the table on the right.

Brosnan designed the experiment with the chimps and monkeys in mind, and modified the details for the students, reversing what normally happens in these studies. For a start, the students received next to no instruction. Brosnan only told them that they would make decisions with red and blue chips, and be paid in quarters. She also paid them after each round, and sat them next to each other. This is what normally happens in chimp and monkey experiments, but humans are usually paid in a lump sum at the end and paired anonymously via computers.

The capuchins behaved in the least structured way. Only one pair (out of six) made Stag-Stag choices more often than chance, and most had no strategy at all. The chimps fared better. A similar proportion leant towards the Stag-Stag option, but many of the other others consistently matched their partner, playing Hare-Hare as often as Stag-Stag. As you might expect, the students did best of all. They arrived at the Stag-Stag choices more often than the chimps or capuchins, and they were the only species to frequently play Hare-Hare.

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The order of the three species in the cooperation league table isn’t surprising. Chimps take part in coordinated social activities that are more complex than those of capuchins, such as group hunts. The social life of humans is more complicated still.

But despite their apparent success, the three species weren’t that dissimilar. For example, only around one in five of the students arrived at the Stag-Stag choice. That’s more than the chimps and monkeys, but only just. When the playing field is levelled, there isn’t a vast gulf separating our cooperative decisions from those of our closest relatives. There are differences, yes, but differences of degree.

De Waal says, “The outcome across species is more similar than different. In this, the study follows in the footsteps of imitation studies on apes.” While earlier studies found that apes are reticent to imitate humans, more recent experiments have shown that they’re perfectly capable of imitating other apes. Brosnan adds, “These results provide initial evidence that, as with other behaviors, humans’ behavior in cooperative economic games may be part of an evolutionary continuum. [Our] decision-making strategies emerged from those of our common ancestors.”

This is not to say that humans don’t have unique mental skills that separate them from other primates. Indeed, Brosnan may have hamstrung the human volunteers to a greater extent than the other two species, by omitting uniquely human advantages such as language. For example, we know from previous work with the stag hunt that people choose the Stag-Stag route more often if they have instructions.

But Brosnan’s experiment shows that these human-specific abilities lie on top of foundations that we share with other primates. We don’t run on a completely different operating system; we’ve just added plug-ins to an old shared one.

Reference: Brosnan, Parrish, Beran, Flemming, Heimbauer, Talbot, Lambeth, Schapiro & Wilson. 2011. Responses to the Assurance game in monkeys, apes, and humans using equivalent procedures. PNAS http://dx.doi.org/10.1073/pnas.1016269108

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