We report two experimental studies on the potentially detrimental effects of nonshared, external representations in an instructional setting. Domain experts viewing an external representation responded to E-mail inquiries for medical advice written by people who were unable to see this external representation. Based on research on expert-layperson communication, we predicted that the experts' extensive and highly integrated knowledge of their own domain would make it very difficult for them to comprehend the completely different perspective of a layperson. We assumed that such a one-sided immersion in one's own privileged knowledge would be exacerbated by the availability of external representations (e.g., diagrams of body functions). Results confirmed that visualizations making the subject matter immediately evident for the expert did have such a detrimental effect.