Loading
Federal Rules of Evidence
6 min read
Published Date:
Last Modified Date:
Federal Rule of Evidence 704 allows expert opinions to address ultimate issues, but it does not permit experts to replace the judge or jury by stating legal conclusions or otherwise bypassing Rules 702, 703, and 403. This article explains the distinction between permissible ultimate-issue testimony and impermissible legal conclusions, the criminal-case restriction in Rule 704(b), and practical considerations for framing expert opinions.
In federal proceedings governed by the Federal Rules of Evidence, Rule 704 addresses whether an expert may give an opinion that reaches an issue the judge or jury must ultimately decide. Rule 704(a) generally allows an opinion to embrace an ultimate issue, but it does not make every such opinion admissible. Other evidence rules still apply, and courts may restrict opinions that state the law or tell the factfinder what result to reach.
This guide explains the difference between an ultimate-issue opinion and a legal conclusion, the criminal-case limitation in Rule 704(b), and practical ways attorneys can frame expert testimony.
Rule 704(a) states that an opinion "is not objectionable just because it embraces an ultimate issue." The rule eliminated the ultimate-issue rule as a separate ground for objection. An otherwise admissible opinion is therefore not excluded simply because it addresses an issue the judge or jury must ultimately decide.
Rule 704(a) is not an independent basis for admission. The Advisory Committee notes explain that Rules 701, 702, and 403 continue to guard against opinions that are unhelpful, merely tell the jury what result to reach, or are phrased in terms of inadequately explored legal criteria.
For example, a mechanical engineer may be permitted to testify that a component fractured because of fatigue and contributed to an incident if the opinion falls within the expert's qualifications and satisfies Rule 702 and other applicable evidentiary requirements. Whether the defendant is legally liable remains a separate question.
Courts generally distinguish specialized factual or technical analysis from testimony that tells the jury what the law means or effectively resolves a legal question. The boundary can depend on the jurisdiction, context, and controlling precedent.
In United States v. Bilzerian, the Second Circuit explained that experts may help jurors understand unfamiliar industry concepts but generally may not give opinions on issues of law. In Hygh v. Jacobs, the court held that characterizing police conduct as "not justified under the circumstances," "not warranted under the circumstances," and "totally improper" crossed into a legal conclusion entrusted to the jury and should have been excluded, although the court found the error harmless. In Specht v. Jensen, the Tenth Circuit, addressing the issue under Rule 702, rejected attorney-expert testimony that effectively instructed the jury that the defendants had violated the plaintiffs' constitutional rights.
Experts can instead explain the facts, methods, professional standards, measurements, and technical findings supporting their opinions without substituting a legal label.
In federal proceedings governed by the Federal Rules of Evidence, Rule 704 addresses whether an expert may give an opinion that reaches an issue the judge or jury must ultimately decide. Rule 704(a) generally allows an opinion to embrace an ultimate issue, but it does not make every such opinion admissible. Other evidence rules still apply, and courts may restrict opinions that state the law or tell the factfinder what result to reach.
This guide explains the difference between an ultimate-issue opinion and a legal conclusion, the criminal-case limitation in Rule 704(b), and practical ways attorneys can frame expert testimony.
Rule 704(a) states that an opinion "is not objectionable just because it embraces an ultimate issue." The rule eliminated the ultimate-issue rule as a separate ground for objection. An otherwise admissible opinion is therefore not excluded simply because it addresses an issue the judge or jury must ultimately decide.
Rule 704(a) is not an independent basis for admission. The Advisory Committee notes explain that Rules 701, 702, and 403 continue to guard against opinions that are unhelpful, merely tell the jury what result to reach, or are phrased in terms of inadequately explored legal criteria.
For example, a mechanical engineer may be permitted to testify that a component fractured because of fatigue and contributed to an incident if the opinion falls within the expert's qualifications and satisfies Rule 702 and other applicable evidentiary requirements. Whether the defendant is legally liable remains a separate question.
Courts generally distinguish specialized factual or technical analysis from testimony that tells the jury what the law means or effectively resolves a legal question. The boundary can depend on the jurisdiction, context, and controlling precedent.
In United States v. Bilzerian, the Second Circuit explained that experts may help jurors understand unfamiliar industry concepts but generally may not give opinions on issues of law. In Hygh v. Jacobs, the court held that characterizing police conduct as "not justified under the circumstances," "not warranted under the circumstances," and "totally improper" crossed into a legal conclusion entrusted to the jury and should have been excluded, although the court found the error harmless. In Specht v. Jensen, the Tenth Circuit, addressing the issue under Rule 702, rejected attorney-expert testimony that effectively instructed the jury that the defendants had violated the plaintiffs' constitutional rights.
Experts can instead explain the facts, methods, professional standards, measurements, and technical findings supporting their opinions without substituting a legal label.
Rule 704(b) creates a specific exception in criminal cases. An expert must not state an opinion about whether the defendant had a mental state or condition that constitutes an element of the charged crime or a defense. Congress added this limitation in 1984 as part of the Insanity Defense Reform Act.
Rule 704(b) does not prohibit all expert evidence bearing on mental state, although any such testimony must remain within the rule's limits and satisfy other applicable evidentiary requirements. The restriction may also apply to an opinion framed in broader terms if it necessarily includes the defendant.
In Diaz v. United States, 602 U.S. 526 (2024), the Supreme Court held that testimony about the mental state of most members of a group does not, by itself, state an opinion about the defendant's own mental state. Accordingly, testimony about what most drug couriers know did not violate Rule 704(b).
The way an expert opinion is framed can affect whether it offers specialized analysis or crosses into a legal or prohibited mental-state conclusion.
Examples include:
Both framing and substance can affect the Rule 704 analysis, along with foundation, controlling precedent, and other evidence rules.
Rule 704 does not replace the ordinary admissibility analysis. Rule 702 permits testimony from a witness qualified by knowledge, skill, experience, training, or education if the proponent demonstrates to the court, more likely than not, that the requirements of Rule 702(a) through (d) are satisfied. Those requirements include helpful specialized knowledge, sufficient facts or data, reliable principles and methods, and a reliable application of those principles and methods to the facts of the case.
Rule 703 addresses the facts or data on which an expert may base an opinion. Otherwise inadmissible facts or data may support the opinion if experts in the particular field would reasonably rely on those kinds of information. That does not mean the information may automatically be disclosed to the jury; Rule 703 imposes a separate balancing requirement for disclosure by the proponent. Rule 403 permits exclusion when probative value is substantially outweighed by risks such as unfair prejudice, confusing the issues, or misleading the jury.
An opinion allowed under Rule 704 can therefore still be excluded under other evidence rules.
Attorneys evaluating expert testimony under Rule 704 should:
Can an Expert Witness Testify on an Ultimate Issue?
Yes. Rule 704(a) says an opinion is not automatically objectionable merely because it embraces an ultimate issue. Other admissibility rules still apply.
Can an Expert Say That a Defendant Was Negligent or Liable?
Such testimony may be challenged because terms such as "negligent" and "liable" can state legal conclusions rather than technical or factual opinions. Whether the testimony is permitted depends on the wording, governing law, purpose, and controlling precedent.
What Does Rule 704(b) Prohibit After Diaz v. United States?
Rule 704(b) prohibits an expert in a criminal case from stating an opinion about whether the defendant did or did not have the mental state or condition that constitutes an element of the crime or a defense. Diaz makes clear that testimony about the mental state of most members of a group does not necessarily state an opinion about the defendant's own mental state.
Does Rule 704 Make Otherwise Unreliable Expert Testimony Admissible?
No. Rule 704 addresses whether opinion testimony may embrace an ultimate issue. Expert testimony must still satisfy Rule 702 and other applicable evidence rules.
Rule 704(b) creates a specific exception in criminal cases. An expert must not state an opinion about whether the defendant had a mental state or condition that constitutes an element of the charged crime or a defense. Congress added this limitation in 1984 as part of the Insanity Defense Reform Act.
Rule 704(b) does not prohibit all expert evidence bearing on mental state, although any such testimony must remain within the rule's limits and satisfy other applicable evidentiary requirements. The restriction may also apply to an opinion framed in broader terms if it necessarily includes the defendant.
In Diaz v. United States, 602 U.S. 526 (2024), the Supreme Court held that testimony about the mental state of most members of a group does not, by itself, state an opinion about the defendant's own mental state. Accordingly, testimony about what most drug couriers know did not violate Rule 704(b).
The way an expert opinion is framed can affect whether it offers specialized analysis or crosses into a legal or prohibited mental-state conclusion.
Examples include:
Both framing and substance can affect the Rule 704 analysis, along with foundation, controlling precedent, and other evidence rules.
Rule 704 does not replace the ordinary admissibility analysis. Rule 702 permits testimony from a witness qualified by knowledge, skill, experience, training, or education if the proponent demonstrates to the court, more likely than not, that the requirements of Rule 702(a) through (d) are satisfied. Those requirements include helpful specialized knowledge, sufficient facts or data, reliable principles and methods, and a reliable application of those principles and methods to the facts of the case.
Rule 703 addresses the facts or data on which an expert may base an opinion. Otherwise inadmissible facts or data may support the opinion if experts in the particular field would reasonably rely on those kinds of information. That does not mean the information may automatically be disclosed to the jury; Rule 703 imposes a separate balancing requirement for disclosure by the proponent. Rule 403 permits exclusion when probative value is substantially outweighed by risks such as unfair prejudice, confusing the issues, or misleading the jury.
An opinion allowed under Rule 704 can therefore still be excluded under other evidence rules.
Attorneys evaluating expert testimony under Rule 704 should:
Can an Expert Witness Testify on an Ultimate Issue?
Yes. Rule 704(a) says an opinion is not automatically objectionable merely because it embraces an ultimate issue. Other admissibility rules still apply.
Can an Expert Say That a Defendant Was Negligent or Liable?
Such testimony may be challenged because terms such as "negligent" and "liable" can state legal conclusions rather than technical or factual opinions. Whether the testimony is permitted depends on the wording, governing law, purpose, and controlling precedent.
What Does Rule 704(b) Prohibit After Diaz v. United States?
Rule 704(b) prohibits an expert in a criminal case from stating an opinion about whether the defendant did or did not have the mental state or condition that constitutes an element of the crime or a defense. Diaz makes clear that testimony about the mental state of most members of a group does not necessarily state an opinion about the defendant's own mental state.
Does Rule 704 Make Otherwise Unreliable Expert Testimony Admissible?
No. Rule 704 addresses whether opinion testimony may embrace an ultimate issue. Expert testimony must still satisfy Rule 702 and other applicable evidence rules.
Federal Rules of Evidence
Federal Rules of Evidence