From this news release, is this Science paper, “The Perception of Rational, Goal-Directed Action in Nonhuman Primates” where Justin Wood of Harvard’s Psychology department Justin Wood, Graduate Student at Harvard Pyschologyand colleagues., figure out that primates expect one another to act rationally. How?

Well Wood and crew setup two sets of experiments that tested the behavioral response of over 120 primates, including cotton-top tamarins, a type of New World monkey, rhesus macaques, a Old World monkey and chimpanzees to represent the apes.

I have been confused about the design of the first experiment, which the tested the primates by presenting them with two potential food containers. One container was grasped by the scientists on purpose and the other container was accidentally grasped by flopping their hand on top of it. I probably didn’t do a great job translating the setup, so forgive me.

The most sense I can make out of what was tested here is the ability to understand or differentiate goal-oriented and accidental behavior. And the results show that in all three species, the primates picked out the the food container that was purposefully grasped more often than the container upon which the hand was flopped. This indicates that the primate inferred goal-oriented action on the part of the experimenter when he or she grasped the container, and was able to understand the difference between the goal-oriented and accidental behavior.

In the second experiment, the primates were also presented with two potential food containers but this time the researchers sought to answer if the primates can infer others’ goals under the expectation that other individuals will perform the most rational action allowed by the environmental obstacles. In one situation, an experimenter touched a container with his elbow when his hands were full, and in another scenario, touched a container with his elbow when his hands were empty. And low and behold, the primates looked for the food in the container indicated with the elbow more often when the experimenter’s hands were full.

The primates considered, just as a human being would, that if someone’s hands are full then it is rational for them to use their elbow to indicate the container with food, whereas if their hands are empty it is not rational for them to use their elbow, because they could have used their unoccupied hand.

Wood comments on his paper,

“A dominant view has been that non-human primates attend only to what actions ‘look like’ when trying to understand what others are thinking. In contrast, our research shows that non-human primates infer others’ intentions in a much more sophisticated way. They expect other individuals to perform the most rational action that they can, given the environmental obstacles that they face.

This study represents one of the broadest comparative studies of primate cognition, and the significance of the findings is reinforced by the fact that these results were consistent across three different species of primates. The results have significant implications for understanding the evolution of the processes that allow us to make sense of other people’s behavior.”

What sorta significance? Well this study kinda sorta implies that the ability to engage in this type of rational action perception evolved as long as 40 million years ago, with non-human primates, and that it is not a human only response… possibly this sort of behavior was positively selected for in the primate lineage and ultimately folds into the social brain hypothesis. Speaking of which, you may also wanna check out the evolution of the social brain, which also debuted in the latest Science, while you are at it.