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Your research with rhesus macaques in Puerto Rico suggests that they are capable of discerning human intentions. How did you realize that they were studying you even as you were studying them?

The monkeys were good at deception. Nowadays when out in the field we eat lunch inside a cage that we bring along, in part because the monkeys have gotten very good at stealing our lunch. And it’s not just the food we eat but the fruit we use as stimuli in our testing sessions. Many days we went home early because the monkeys ate all the fruit. We would be doing number experiments where one plus one is supposed to equal two. But with only one lemon left, they can’t add one plus one and we can’t test their addition skills. In order for the monkeys to have taken the lemons when we didn’t notice, they must have paid attention to what we could see and what we couldn’t see. They’d have to wait for the moment we were no longer looking, and step in and take the lemons away.

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