One of the other key papers in the book in which the original finding was published ("The Psychology of Abilities, Competencies, and Expertise") was the role of

One of the other key papers in the book in which the original finding was published ("The Psychology of Abilities, Competencies, and Expertise") was the role of increasingly skillful teachers. If you don't have a series of progressively competent teachers who are all good at providing motivation you also won't see the kind of performance increases that lead to professional status.