Errors in medicine result in over 44,000 preventable deaths annually. Some of these errors are made by specialized physicians at the time of diagnosis. Building on error frameworks proposed in the literature, we tested the experimental hypothesis that physicians within a given specialty have a bias in diagnosing cases outside their own domain as being within that domain. Thirty-two board-certified physicians from four internal medicine subspecialties worked four patient cases each. Verbal protocol analysis and general linear modeling of the numerical data seem to confirm the experimental hypothesis, indicating that specialists try to "pull" cases toward their specialty. Specialists generate more diagnostic hypotheses within their domain than outside, and assign higher probabilities to diagnoses within that domain.