Psychiatric evidence on the ultimate issue.

Federal Rule of Evidence 704(b) prevents psychiatrists and other expert witnesses in federal criminal trials from testifying as to whether a defendant did or did not have a mental state or condition comprising either an element of the crime or an element of the defense. This paper describes the origins of the Rule and its judicial development. The Rule is an exception to a 20th century trend that has seen witnesses increasingly permitted to address the ultimate issue. It has been applied inconsistently, has been criticized in appellate decisions, and has spawned an idiosyncratic legal definition of "helpful." Attempts to circumvent it have included inviting jurors to make inferences, inventing hypothetical cases that mimic the one before the court, and eliciting expert testimony on what is "possible" or "probable." Courts have held that rendering transparent the reasons behind an expert's conclusions can minimize the damage done by ultimate issue testimony.