28 U.S.C. App. (FEDERAL RULES OF EVIDENCE) Rule 704

(a) In General—Not Automatically Objectionable. An opinion is not objectionable just because it embraces an ultimate issue. (b) Exception. In a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense. Those matters are for the trier of fact alone.