One of the classical means to test whether primates show an understanding of self and other is to test Theory of Mind, or to test the notion that the subject realizes that the Other has a mind which can have different information than the self. Theory of Mind tests have been used to determine when developmentally children realize that information is specific to the person's experience, and different people can have different information (perspective taking) or can be told different facts (deception) and they can believe something different than what actually happened or what is the truth. In this paradigm, often referred to as the Knower-Guesser paradigm, primates observe a human experimenter (in this case, Dee Dee Rupert, left)  hiding a treat in one of several locations but they themselves cannot see the hiding location due to a barrier being used. Then the human leaves and either re-enters (as the Knower) or another human enters (the Guesser, in this case, Clint Ulmer). The human who enters then points to one of the two or three cups that are inverted. The question for the primate subject? Should I choose the cup that the human points to? If they understand that the Knower actually hid the treat and thus knows where it is, it would make sense to follow the pointing gesture of the Knower. However since the Guesser was not present for the hiding, the Guesser's point could be potentially wrong, and thus they should not follow as readily the Guesser. If they have been trained to follow a point command made by a human and cannot consider Theory of Mind, they would follow the Knower and the Guesser at equal rates.

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The graph to the lower left shows our results from a total of 8 monkeys across a variety of conditions. The first two conditions demonstrate that monkeys follow the Knower much more than the Guesser, which suggests that they are thinking through what each human knows and the fallibility of the point. However, when the Guesser is present during the hiding and facing forward to see the hiding, then they should also follow the Guesser more (Guesser forward) than when the Guesser is present and facing backward and can't see the hiding (Guesser backward). Also the Guesser alone condition has the Guesser hiding the item and then pointing, which mimics a Knower role. As you can see, the monkeys often follow the Knower more, potentially because that person has been correct with their pointing more often, and they follow the Guesser less and somewhat independent of what the Guesser actually knows. 

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The moral of the story? Do a lot of testing to determine why the monkeys are making the choices they make! It may look like Theory of Mind but it is really the probability of winning attached to certain social identities, which is not the same thing!