Unwilling versus unable: infants' understanding of intentional action.

Infants experienced a female adult handling them toys. Sometimes, however, the transaction failed, either because the adult was in various ways unwilling to give the toy (e.g., she teased the child with it or played with it herself) or else because she was unable to give it (e.g., she accidentally dropped it). Infants at 9, 12, and 18 months of age reacted with more impatience (e.g., reaching, looking away) when the adult was unwilling to give them the toy than when she was simply unable to give it. Six-month-olds, in contrast, showed no evidence of this differentiation. Because infants' behavioral responses were appropriately adapted to different kinds of intentional actions, and because the adult's actions sometimes produced results that did not match her goal (when having accidents or failed attempts), these findings provide especially rich evidence that infants first begin to understand goal-directed action at around 9 months of age.