Humans are vulnerable to a very peculiar psychological weakness: if you give a thing a label because of a contingent property it possesses, they will tend to associate the label with the thing instead of the property. Eventually, they end up attributing the label to the thing, and the property as inherent to it.

In other words, if you tell kids that they're smart because of their accomplishments, they'll adopt 'smartness' into their identity, and eventually will cease putting effort into accomplishing because they'll believe they don't need to.