Rule 702 prohibits admission of any opinion not based on specialized knowledge—a prohibition which presumably can include ultimate-issue opinions. Indeed, Rule 704(b) (an amendment to the original Rule 704 that was inspired by John Hinckley’s acquittal on insanity grounds) makes this point concretely with respect to mental state testimony in criminal cases. The position we take is that the same evidentiary prohibition should apply to all types of cases.
