Rational tool use and tool choice in human infants and great apes.

G. Gergely, H. Bekkering, and I. Király (2002) showed that 14-month-old infants imitate rationally, copying an adult's unusual action more often when it was freely chosen than when it was forced by some constraint. This suggests that infants understand others' intentions as rational choices of action plans. It is important to test whether apes also understand others' intentions in this way. In each of the current 3 studies, a comparison group of 14-month-olds used a tool more often when a demonstrator freely chose to use it than when she had to use it, but apes generally used the tool equally often in both conditions (orangutans were an exception). Only some apes thus show an understanding of others' intentions as rational choices of action plans.