I hypothesized that it can be when mastery of the subject comes too easily (making it difficult to relate to those who struggle to master it all) or when the instructor is simply "too close" to the material to be able to conceputalize it through more naive eyes.
Turns out that there's something to this worry: Experts in a given subject often have what some researchers call a "pedagogical blind spot" for that very subject.
These findings are outlined in a paper by Mitchell Nathan and his colleagues. Nathan et al. define expert blind spot as "the inability to perceive the difficulties that novices will experience as they approach a new domain of knowledge." Experts with this blind spot are less effective as educators for an intriguing reason: the “tendency for content area experts to perceive the organization of the domain of study as the central structure for organizing students’ learning experiences, rather than basing instruction on students’ actual developmental processes." In other words, experts sometimes approach the task of teaching in terms of what they know — including, in particular, how they represent to themselves what they know, rather than in terms of the process by which students are likely to master it. Experts have a huge body of domain-specific knowledge, along with the ability to move fluidly across and within that domain (understanding how one claim supports another but speaks against a third, etc.)
Most strikingly, experts are often unable to articulate the cognitive moves they make within their expert domain precisely because this fluidity has become what Aristotle might call " second nature":
Verbal “think aloud” reports also show that experts are less likely than novices to have access to memory traces of their cognitive processes when engaged in tasks within their domain of expertise, because these highly practiced cognitive and perceptual processes have become automatized. This means there is nothing in memory for experts to “replay,” verbalize, and reflect upon. Among novices, these processes tend to be deliberate and stepwise, and so they leave a memory trace that is inspectable and verbalizable.
While Nathan et al that content knowledge is important to pedagogy — that contrary to popular wisdom, those who don't know really can't teach — they conclude that expertise can serve as a barrier to the acquisition of "“pedagogical content knowledge” (PCK), the “blending of content and pedagogy into an understanding of how particular topics, problems, or issues are organized, represented and adapted to the diverse interests and abilities of learners for instruction." Experts may struggle to develop PCK, making "pedagogical decisions that... conflict with the needs of learners." Perhaps unsurprisingly, the best apparent remedy for this conflict is practice at teaching.
As I've emphasized before, we're freaks. Those who succeed in academic philosophy have an atypical and idiosyncratic set of skills and dispositions that, like it or not, few of our students share. As a result, what 'worked' in teaching us often fails when applied to our students. If you're a freak, dang near any teaching technique will help you learn. That's why you're a freak.
So I constantly try to remind myself that my expertise, while important, can stand in the way of being an effective educator — if I blithely assume that philosophy looks to my students the way it looks to me.
Here's a concrete example: Philosophers are good at — and enjoy — thinking abstractly. Most people aren't. As teachers, philosophers often start off with general principles and ask students to test these against examples. That gets things off on the wrong foot. If the aim is to scrutinize a principle, help students extract candidate principles from examples. There's no reason that the order of discovery has to mirror the order of justification — and in many cases, our students will learn more when it doesn't.
That's but one example. I'd be interested to know of instances where you may have realized that your expertise was standing in the way of teaching your students more effectively and how you responded.