In criminal law an ultimate issue is a vital legal issue at stake in the trial of a crime on which an expert witness is giving testimony. For example, if the controversy is over the defendant’s mental state at the time of the offence, the ultimate issue would be the defendant’s sanity or insanity at the material time of commission of the offence.
Till the amendment of the Federal Rules of Evidence in 1984 expert witnesses were permitted to give testimony on ultimate issues such as, whether the insanity defense could be invoked in case of a particular defendant.
Thereafter in the wake of the trial of John Hinckley, Jr. on charges of attempted assassination of President Regan changes were made to the law due to the public discontent over Hinckley’s successful exploitation of the insanity defense. Rule 704 (b) was added to the Federal Rules of Evidence which put bounds or fetters on expert witness testimony.
The expert witness testimony is limited to giving an opinion on whether the defendant had a serious mental disorder at the time of commission of the offence and explaining the symptoms of any diagnosis made, including other testimony on the defendant’s mental status and instincts. The expert witness is however not allowed to make a statement on the issue whether the legal test of insanity has been met. Deciding such ultimate issue is the domain of the judge and jury on which the expert witness cannot tread.
