'Unwilling' versus 'unable': capuchin monkeys' (Cebus apella) understanding of human intentional action.

A sensitivity to the intentions behind human action is a crucial developmental achievement in infants. Is this intention reading ability a unique and relatively recent product of human evolution and culture, or does this capacity instead have roots in our non-human primate ancestors? Recent work by Call and colleagues (2004) lends credence to the latter hypothesis, providing evidence that chimpanzees are also sensitive to human intentions. Specifically, chimpanzees remained in a testing area longer and exhibited fewer frustration behaviors when an experimenter behaved as if he intended to give food but was unable to do so, than when the experimenter behaved as if he had no intention of giving food. The present research builds on and extends this paradigm, providing some of the first evidence of intention reading in a more distant primate relative, the capuchin monkey (Cebus apella). Like chimpanzees, capuchin monkeys distinguish between different goal-directed acts, vacating an enclosure sooner when an experimenter acts unwilling to give food than when she acts unable to give food. Additionally, we found that this pattern is specific to animate action, and does not obtain when the same actions are performed by inanimate rods instead of human hands (for a similar logic, see Woodward, 1998). Taken together with the previous evidence, the present research suggests that our own intention reading is not a wholly unique aspect of the human species, but rather is shared broadly across the primate order.