Trial 4: Boxes blow over, trial aborted.
Trial 5: As soon as the grapes are displayed, the monkey gets up and walks away.
Trial 6: Finally a monkey that seems interested. Actually, a little too interested. As the second student is approaching the boxes to move the grapes, the monkey gets off his haunches and walks swiftly toward her. "Turn around!" Santos calls. The student pivots, pulls herself up to her full height and stares right at the monkey. It snarls menacingly back at her; she shrieks and runs to hide behind a colleague. The monkey grabs both grapes and runs away, chewing.
Students must commit to a month in Puerto Rico, but it is the prerogative of the professor to fly home at the end of the first week. Before Santos leaves, she makes some modifications to the false-belief experiment, and by the end of the month she hears that it's working better. In the months after returning to New Haven, she begins to formulate some tentative conclusions about what she has found: monkeys can gauge the knowledge and intentions of others when they correspond to their own perceptions of reality, but they cannot make the leap to the concept of a false belief.
So is the mental gap between monkeys and human beings closing or widening? In a sense, both: if Santos is right, monkeys manage to navigate complex social hierarchies, hiding from and deceiving others as necessary, all without an ability that human beings develop by the age of 4. The more she works with monkeys, the more Santos is convinced that their abilities are limited to specific contexts and tasks, such as competing for food or establishing dominance. It's rather like the honeybee dance, a fantastically ingenious way to communicate geographic information. Still, honeybees can't use it to talk about their feelings. "My guess," says Hauser, "is that we will eventually come to see that the gap between human and animal cognition, even a chimpanzee, is greater than the gap between a chimp and a beetle." Perhaps, Santos says. Monkeys can reason quite competently about human beings' intentions with respect to grapes, but only by imputing to them what they themselves experience: a readiness to grab and hoard whenever possible. She speculates that it is our capacity for language that enables us to understand mental states different from our own. We may not be hungry now, but because we have a word for the concept we can imagine what it feels like. "The more you hang out with monkeys," she says, "the more you realize just how special people really are."
Jerry Adler is a Newsweek senior editor specializing in science and medicine.
Sylwia Kapuscinski usually photographs human primates, and focuses on immigrants.