Santos did not originate the idea that has fueled several breakthroughs in the past decade, but she has been one of the most imaginative and successful in applying it. The concept, known as "domain specificity," holds that the cognitive abilities of primates evolved for particular tasks and can be tested only in a context that is meaningful to the animal itself. The early theory-of-mind experiments tried to enlist monkeys or chimps in begging for food, sharing it or cooperating to find it—behaviors, says Santos, that do not come naturally to them. As she and co-author and Yale colleague Derek E. Lyons put it in a recent paper in the journal Philosophy Compass, "though primates are social creatures, they are not exactly sociable ones." Colleagues say Santos has a talent for thinking like a monkey. Her experiments cleverly elicit and exploit primates' natural gifts for competitiveness, stealthiness, hoarding and deceit.
Trial 2: This time Olivia is the one facing away, and the monkey, better versed in theory-of-mind, makes a dash for her grape.
Rhesus macaques, especially juveniles, are capable of simulating cuteness, but it's not their defining characteristic. Scrappy and long-limbed, with pink hairless faces framed by gray or brown fur, they fight convincingly among themselves. At least two here appear to have lost limbs in their perpetual struggle for rank, and they will stand up to a human being if the stakes are high enough—a grape, for example. They have been known to carry a variety of herpes that can be fatal to human beings, and scattered around the island are first-aid stations holding disinfectant kits to be used in case of a bite. (On the other hand, a single human visitor with active tuberculosis could wipe out the entire colony.) Santos recognizes many of the individual monkeys here by sight or by the letter-and-number code tattooed on their chests, but she says she has never been even tempted to name them.
She has somewhat more affection for the 11 capuchin monkeys in her lab at Yale, who are named after characters in James Bond movies (Goldfinger, Jaws, Holly Goodhead). Her work with them involves experiments on "social decision-making." She equips them with tokens they can trade for food and studies the development of their rudimentary economy. Like human beings, they are loss-averse: if the going price is two grapes for a token, they prefer to trade with an experimenter who shows them one grape and then adds one, compared with one who shows three and takes one away. They are also sneaky. After swapping for an apple, she says, they will sometimes take a bite of it, then present the untouched side to the researcher and try to sell it back. And they have an entrepreneurial bent. At times they would offer their feces in exchange for a token, behavior that baffled the researchers until a student pointed out that every morning someone comes into the cage and scoops out the droppings—which may have given them the idea that people value them.
Trial 3: Katharine faces away again, and the monkey sidles up and grabs her grape, just as science would predict. Then it does a quick sideways dash and snatches up Olivia's as well. the experiments done so far are tests of first-order knowledge: the monkey sees the human experimenter either facing or facing away from the grape. Now Santos intends to test whether macaques possess the more sophisticated concept of "false belief"—the recognition that another individual may be mistaken. The classic test for this in people is the "Sally-Anne" experiment. The subject watches "Sally" put a ball in a box, then leave the room. While she's gone, "Anne" moves the ball to a different box. The experimenter asks the subject: Where will Sally look for the ball? The expected answer from adults is the first box, where Sally last saw it. Children younger than about 4, and those with autism, more often say the second box, where the ball actually is; they cannot conceive that Sally has a false belief.
To test if monkeys are capable of false belief, Santos has devised an experiment involving two grapes, three open boxes and four researchers, including Santos herself and someone to record the whole thing on video. Again, the premise is that the monkeys are more likely to steal things that, from their point of view, are unguarded. The protocol is as follows: the three boxes are arranged side by side on the ground with their open sides facing the monkey, and a student puts one grape in each of two boxes—B and C, say. Then she stands behind the boxes and turns her back, and a different student moves the grapes—into A and B. The monkey now knows where the grapes are, but the first student does not. When she turns and faces the monkey, which box is the monkey more likely to rob? If the monkey understands "false belief," it will expect the student to be guarding boxes B and C, and so will be more likely to steal from A.
"Make sure you don't both have your backs turned to the monkey at the same time," Santos warns the students. "Some of these monkeys will just rush the boxes."
Trial 1: After finally locating a suitable monkey, setting up the boxes and going through the pantomime with the grapes, Santos drifts back into the trees and watches as the monkey languidly scratches itself. Almost ostentatiously, it seems, the animal turns and looks out over the rocks to the sea.
Trials 2 and 3: No approach.
With her students, Santos tramps up and down the now-familiar hills, across a rocky isthmus, to the sounds of wind and crashing waves, chattering monkeys and the continual bang of metal lids slamming on the chow bins. Santos tries to enlist one young monkey gnawing a biscuit, only to be stared down by a nearby male that was about to mount a different female. "Don't worry," Santos says placatingly as she backs away, "she's gonna mate with you, I promise."