Consider the difference between the expert testimony of an orthopedic surgeon in a personal injury suit and the testimony of a psychiatrist in a murder trial in which some elements of the mens rea are at issue. In both instances an expert opinion is received in evidence, providing the trier of fact with technical, specialized information which must, or should, be available in order to permit a rational decision-making process. Well-established rules govern the nature of expert evidence and its mode of presentation. In legal theory, the orthopedic surgeon and the psychiatrist are both experts-physicians-who perform comparable functions in the court proceedings. Presumably, they are governed by the same rules of evidence, they are subject to the same restrictions, and their testimony bears an essential, though perhaps fragmentary, relationship to the chief issues at trial.
Bernard L. Diamond & David W. Louisell,
The Psychiatrist as an Expert Witness: Some Ruminations and Speculations,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol63/iss8/3