A-not-B error

A-not-B error is, in the theory of cognitive development of Jean Piaget, a particular error made by young children during substage 4 of their sensorimotor stage. The error is made by infants who have successfully uncovered a toy at location "A", and continue to reach to that location even after they watch the toy be moved in a nearby location "B". This demonstrates a lack of, or incomplete, object permanence.

Traditionally this phenomenon has been explained as the child seeing an image and remembering where it was, rather than where it is. Other accounts deal with the development of planning, reaching, and deciding things. There are also behaviorist accounts that explain the behavior in terms of reinforcement.