Rule 705

Disclosure of Facts or Data Underlying Expert Opinion

The expert may testify in terms of opinion or inference and give reasons therefor without first testifying to the underlying facts or data, unless the court requires otherwise. The expert may in any event be required to disclose the underlying facts or data on cross-examination.

Note (Federal Judicial Center)

The rule enacted by the Congress is the rule prescribed by the Supreme Court, amended only by substituting "court" in place of "judge."


Gender-neutralizing amendments were made to this rule in 1987; a further clarifying amendment was made April 22, 1993, effective Dec. 1, 1993.

Advisory Committee’s Note

The hypothetical question has been the target of a great deal of criticism as encouraging partisan bias, affording an opportunity for summing up the middle of the case, and as complex and time consuming. Ladd, Expert Testimony, 5 Vand. L. Rev. 414, 426-427 (1952). While the rule allows counsel to make disclosure of the underlying facts or data as a preliminary to the giving of an expert opinion, if he chooses, the instances in which he is required to do so are reduced. This is true whether the expert based his opinion on data furnished him at secondhand or observed by him at firsthand.

The elimination of the requirement of preliminary disclosure at the trial of underlying facts or data has a long background of support. In 1937 the Commissioners on Uniform State Laws incorporated a provision to this effect in their Model Expert Testimony Act, which furnished the basis for Uniform Rules 57 and 58. Rule 4515, N.Y. CPLR (McKinney 1963), provides:

Unless the court orders otherwise, questions calling for the opinion of an expert witness need not be hypothetical in form, and the witness may state his opinion and reasons without first specifying the data upon which it is based. Upon cross-examination, he may be required to specify the data. . . .

See also California Evidence Code 802; Kansas Code of Civil Procedure 60-456, 60-457; New Jersey Evidence Rules 57, 58.

If the objection is made that leaving it to the cross-examiner to bring out the supporting data is essentially unfair, the answer is that he is under no compulsion to bring out any facts or data except those unfavorable to the opinion. The answer assumes that the cross-examiner has the advance knowledge which is essential for effective cross-examination. This advance knowledge has been afforded, though imperfectly, by the traditional foundation requirement. Rule 26(b)(4) of the Rules of Civil Procedure, as revised, provides for substantial discovery in this area, obviating in large measure the obstacles which have been raised in some instances to discovery of findings, underlying data, and even the identity of the experts. Friedenthal, Discovery and Use of an Adverse Party’s Expert Information, 14 Stan. L. Rev. 455 (1962).

These safeguards are reinforced by the discretionary power of the judge to require preliminary disclosure in any event.

Senate Judiciary Committee Report, 1984 Amendment

The purpose of this amendment is to eliminate the confusing spectacle of competing expert witnesses testifying to directly contradictory conclusions as to the ultimate legal issue to be found by the trier of fact. Under this proposal, expert psychiatric testimony would be limited to presenting and explaining their diagnoses, such as whether the defendant had a severe mental disease or defect and what the characteristics of such a disease or defect, if any, may have been. The basis for this limitation on expert testimony in insanity cases is ably stated by the American Psychiatric Association:

[I]t is clear that psychiatrists are experts in medicine, not the law. As such, it is clear that the psychiatrist’s first obligation and expertise in the courtroom is to "do psychiatry," i.e., to present medical information and opinion about the defendant’s mental state and motivation and to explain in detail the reason for his medical-psychiatric conclusions. When, however, "ultimate issue" questions are formulated by the law and put to the expert witness who must then say "yea" and "nay," then the expert witness is required to make a leap in logic. He no longer addresses himself to medical concepts but instead must infer or intuit what is in fact unspeakable, namely, the probable relationship between medical concepts and legal or moral constructs such as free will. These impermissible leaps in logic made by expert witnesses confuse the jury. [Foot Note omitted.] Juries thus find themselves listening to conclusory and seemingly contradictory psychiatric testimony that defendants are either "sane" or "insane" or that they do or do not meet the relevant legal test for insanity. This state of affairs does considerable injustice to psychiatry and, we believe, possibly to criminal defendants. In fact, in many criminal insanity trials both prosecution and defense psychiatrists do agree about the nature and even the extent of mental disorder exhibited by the defendant at the time of the act.

Psychiatrists, of course, must be permitted to testify fully about the defendant’s diagnosis, mental state and motivation (in clinical and commonsense terms) at the time of the alleged act so as to permit the jury or judge to reach the ultimate conclusion about which they and only they are expert. Determining whether a criminal defendant was legally insane is a matter for legal fact-finders, not for experts.

Moreover, the rationale for precluding ultimate opinion psychiatric testimony extends beyond the insanity defense to any ultimate mental state of the defendant that is relevant to the legal conclusion sought to be proven. The Committee has fashioned its Rule 704 provision to reach all such "ultimate" issues, e.g., premeditation in a homicide case or lack of predisposition in entrapment.

Note to 1993 Amendment

This rule, which relates to the manner of presenting testimony at trial, is revised to avoid an arguable conflict with revised Rules 26(a)(2)(B) and 26(e)(1) of the Federal Rules of Civil Procedure or with revised Rule 16 of the Federal Rules of Criminal Procedure, which require disclosure in advance of trial of the basis and reasons for an expert’s opinions.

If a serious question is raised under Rule 702 or 703 as to the admissibility of expert testimony, disclosure of the underlying facts or data on which opinions are based may, of course, be needed by the court before deciding whether, and to what extent, the person should be allowed to testify. This rule does not preclude such an inquiry.

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