I learned something from reading this article about a classic experimental finding in developmental psychology:

So you keep hiding the toy in “A” and the baby keeps searching for the toy in “A.” Simple enough. But what happens if you suddenly hide the toy in “B”? Remember, you’re hiding the toy in full view of the infant. An older child or an adult would simply reach for “B” to retrieve the toy. But not the infant. But, despite having just seen the object hidden in the new “B” location, infants between 8 and 12 months of age (the age at which infants begin to have enough motor control to successfully reach for an object) frequently look for it under box “A,” where it had previously been hidden. This effect, first demonstrated by Jean Piaget, is called the perseverative search error or sometimes the A-not-B error.

The result is robust to many variations on the experiment and the full article goes through some hypotheses about the error and a new experiment that turns them on their head.

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