One problem with relying only on subject matter experts for course development is that experts can only articulate about 30 percent of their knowledge.
Ken Koedinger, Professor of Carnegie Mellon University, as cited.
This phenomenon is called the “curse of expertise,” and it shows up in all sorts of settings—not just the instructor who can’t communicate what she knows to her students, but also the parent helping with homework who can’t get a concept across to his child, the marketer or salesperson who misjudges what customers knows, and the manager who’s frustrated that his employees don’t “get it” more quickly.
Annie Murphy Paul, The Brilliant Report, as cited.