Rule 704: Opinion on an Ultimate Issue

Rule 704(a) abolishes the old common law rule against opinions on ultimate issues. Under the common law, witnesses could not give opinions that went to the 'ultimate issue' — the very question the jury had to decide. The modern rule rejects this prohibition, recognizing that it was too restrictive and difficult to apply. Now, both lay and expert witnesses can give opinions that embrace an ultimate issue, such as 'the defendant was negligent' or 'the contract was breached.'

Rule 704(b) creates one important exception: in criminal cases, an expert witness cannot testify about whether the defendant had the mental state required for the crime or a defense. This exception was added in 1984 in direct response to the John Hinckley insanity acquittal, where expert witnesses testified that Hinckley was insane when he attempted to assassinate President Reagan. Congress decided that psychiatrists and psychologists should not be allowed to tell the jury whether the defendant was legally insane or had the requisite criminal intent.

Under 704(b), an expert can describe a defendant's mental condition, diagnose disorders, and explain how those conditions affect behavior. What the expert cannot do is take the final step of concluding that the defendant therefore did or did not have the specific mental state required by law. That ultimate conclusion must be left to the jury.