In the past, expert witnesses were allowed to give testimony on ultimate issues, such as the applicability of the insanity defense to a particular defendant.
In 1984, Federal Rule of Evidence 704(b) was added following the trial of John Hinckley, Jr. for the attempted assassination of U.S. President Ronald Reagan.
[6][7] The rationale for this restriction was stated in the legislative history of the rule as the following:[2] The purpose of this amendment is to eliminate the confusing spectacle of competing expert witnesses testifying to directly contradictory conclusions as to the ultimate legal issue to be found by the trier of fact.The result is that large gray areas remain regarding exactly what testimony is allowed.
[8] For example, the Third Circuit Court of Appeals in United States v. Rutland ruled that testimony from "an extraordinarily qualified handwriting expert" was admissible on the "ultimate issue of authorship of key documents".
Expert testimony that the defendant had a "personality configuration inconsistent with the outrageous and senseless murders of [his] family"[2] was not allowed under the rules of evidence in effect at the time because it was considered confusing and misleading.
Also, an expert witness would not be in violation of 704(b) in use today if he gave testimony regarding the defendant's positive behaviors, such as acting like a loving father and husband, which might create the impression that he was not capable of committing such a crime, but is an opinion unrelated to guilt.
[2] The position under English law is different from that in the United States as there is no rule preventing an expert from giving an opinion on the 'ultimate issue' in England and Wales.