O'Neill notes there's some evidence for an affirmative answer, but overall, if knowledge is measured by research productivity (which I concede is a questionable assumption), there appears to be no correlation between subject knowledge and teaching effectiveness. O'Neill notes that this finding upends some central assumptions about higher education:
But we don't want to consider that a good scholar is not necessarily an effective teacher. We don't want to think about this, since the whole idea of higher ed as we know it today largely rests on the assumption that good scholars are at least capable of teaching, if not proficient at it.
O'Neill has a diagnosis for this expertise-teaching gap: Intellectual mastery of a subject is abstract and theoretical, whereas teaching is a practical and concrete art. And there's no reason to expect that the former will produce the latter. Furthermore, O'Neill notes that experts tend to focus on what we might call the foreground of learning, the explicit steps one takes late in the learning process, while not attending to the background of learning, the easy-to-overlook habits and aptitudes that make learning possible:
Students need to own the book before they open it, know the room number before they can show up, know where the test is being given before they can pass it. You may counter: but those are trivial pre-requisites, not the "core of learning." And I say: trivial things matter when you do them poorly, and further, there are aspects of practice which are non-trivial and which the theoretically-minded tend not to notice.To O'Neill's mind, teaching is better modeled on cooking than chemistry, a "practical art" in which excellence is acquired slowly and in stages. I don't disagree with this, though O'Neill doesn't give the role of theory in teaching a fair shake. Teaching, he warns, is not "the application of a theory." Well no, if one has in mind a crudely mechanical, one-size-fits-all application that takes no account of the subject being taught, features of learners, etc. But I feel confident that knowing how people learn, how students study, and so on, makes us better teachers.