706.5Underlying Facts/Data [Rule 705]

Last Updated: 12/01/23

Key Concepts

  • An expert witness does not have to disclose the underlying facts or data before offering an opinion unless the adverse party requests it.
  • In either case, the underlying facts or data of an expert’s opinion may be inquired about on cross-examination.
  • Expert witnesses do not have to offer opinion testimony phrased as a response to a hypothetical question, but they may do so.
  • If requested, the court must give the jury a limiting instruction that evidence offered solely as the basis of an expert’s opinion is not to be considered as substantive evidence.

The Basic Rule

Rule 705 – Disclosure of Facts or Data Underlying Expert Opinion

The expert may testify in terms of opinion or inference and give his reasons therefor without prior disclosure of the underlying facts or data, unless an adverse party requests otherwise, in which event the expert will be required to disclose such underlying facts or data on direct examination or voir dire before stating the opinion. The expert may in any event be required to disclose the underlying facts or data on cross-examination. There shall be no requirement that expert testimony be in response to a hypothetical question.

G.S. 8C-705.

Legal Overview

Rule 705 sets forth three distinct but related rules regarding the disclosure of the underlying facts and data that formed the basis of an expert witness’s opinion.

Reveal Basis for Opinion in Advance

Unless the adverse party “requests otherwise,” an expert witness is permitted to testify regarding his or her opinion (e.g., “I concluded that the white, powdery substance was cocaine”) without first disclosing the underlying facts or data which support that opinion (e.g., “I conducted the following tests on the powder to determine what it was…”). See G.S. 8C-705; State v. Brown, 101 N.C. App. 71 (1990); State v. Fletcher, 92 N.C. App. 50 (1988); State v. Gary, 78 N.C. App. 29 (1985); State v. Hunt, 297 N.C. 258 (1979); see also State v. Johnson, 105 N.C. App. 390 (1992) (noting that a “general objection” to an expert witness’s testimony does not constitute a specific request for advance disclosure of the underlying facts and data pursuant to Rule 705).

But if the adverse party does make a request, the expert witness must first disclose the underlying facts or data to support the opinion, either on voir dire (if requested) or as part of the direct examination (if voir dire is not specifically requested) before expressing his or her opinion. See G.S. 8C-705; State v. Pretty, 134 N.C. App. 379 (1999); see also 2 Kenneth S. Broun, Brandis & Broun on North Carolina Evidence 738 (2011) (noting that if adverse party specifically requests disclosure through voir dire, but court only allows inquiry on direct and cross-examination, it may be deemed prejudicial error if improper evidence is disclosed to the jury during questioning).

Practice Pointer

Disclose anyway
Even if the defense does not specifically request disclosure of the underlying facts and data behind an expert’s opinion, the prosecutor should always elicit that testimony so that the jury will better understand why the opinion is reliable and credible. For similar reasons, whether the defense requests it or not, in most cases the prosecutor should ask the witness to explain the basis of his or her opinion before stating the conclusion. “Based on my research and testing, the only possible conclusion was X” sounds more convincing than “I concluded it was X, and now here’s the research and testing that shows I was right.”

Even if the defense does not make a specific disclosure request under Rule 705, the state must comply with its pretrial discovery obligations. G.S. 15A-903(a)(2) requires the state to turn over the expert witness’s curriculum vitae, expected opinion, and underlying basis for that opinion. For more information, see the related Pretrial entry on Defendant’s Discovery Rights.

Disclose on Cross-examination

Regardless of whether the adverse party requests advance disclosure of the basis of the expert’s opinion, Rule 705 states that “in any event” the expert may be required to disclose the facts or data underlying his or her opinion on cross-examination. G.S. 8C-705.

Cross-examination of an expert witness is broad and open. The adverse party may not only inquire about the facts and data underlying the opinion, but also may challenge the expert about matters that the expert did not consider but arguably should have, and may point out inconsistent or contrary authority contained in a learned treatise. See State v. Black, 111 N.C. App. 284 (1993) (“wide latitude is generally given to a cross-examiner in his attempts to discredit the expert witness”); State v. Lyons, 343 N.C. 1 (1996); State v. Davis, 340 N.C. 1 (1995); State v. Coffey, 336 N.C. 412 (1994); State v. Allison, 307 N.C. 411 (1983).

The adverse party may not, however, use this rule as “back door” to put its own expert testimony into evidence. Thus, the adverse party may not cross-examine the testifying expert about the opinion of another expert that has not been introduced into evidence, unless the testifying expert actually reviewed and relied upon that other opinion in reaching his or her conclusions. See State v. White, 343 N.C. 378 (1996) (trial court properly allowed state to cross-examine defense psychiatry expert about the contrary opinion of a psychologist, because the defense psychiatrist had relied on that psychologist’s work and conclusions); State v. Golphin, 352 N.C. 364 (2000) (similar ruling).

Hypothetical Questions are Optional

Rule 705 abolished the prior rule that opinion testimony from an expert witness had to be offered in response to a hypothetical question. See G.S. 8C-705; State v. Fearing, 304 N.C. 499 (1981); State v. Morgan, 299 N.C. 191 (1980).

However, Rule 705 does not prohibit the expert from answering hypothetical questions. See, e.g., State v. McCall, 162 N.C. App. 64 (2004) (“The fact that Vaughn's expert testimony took the form of hypothetical questions and was based on information related to her by a third party does not affect the admissibility of her opinion, but instead goes to the weight of the evidence”); State v. Moore, 152 N.C. App. 156 (2002) (detective, qualified as an expert in manufacturing and distributing drugs based on his training and experience, was properly allowed “to testify to a hypothetical question based on the facts of this case” and opine that “I would conclude that that was a drug operation”).

Practice Pointer

Let’s just suppose…
Hypothetical questions can also be useful for asking an expert witness whether and how his or her opinion would change if certain underlying facts or data were changed. This is commonly done on cross-examination in an effort to cast doubt on the degree of certainty of the expert’s conclusion, but it can also be done preemptively on direct examination to help insulate and reinforce the expert’s opinion. 
For example, if the state’s expert has testified that he or she is scientifically certain that the test results show X, the prosecutor may want to ask a few follow-up questions about what kinds of factors could potentially undermine that conclusion (e.g., a tainted sample, testing delays, or alternate causes). Pointing out that those kinds of problematic factors were not present in this case may provide the jury with greater confidence in the expert’s conclusion. 

Admissibility of Basis of Opinion

Whether the basis for an expert’s opinion is disclosed on direct examination or inquired into on cross-examination, the underlying facts and data are only elicited for the purpose of establishing the basis of the expert’s opinion under Rule 703. The facts and data are not admissible as substantive evidence in the case, unless there is an independent basis (e.g., a hearsay exception) for the introduction of the evidence. See State v. Jones, 322 N.C. 406 (1988) (evidence regarding another analyst’s opinion, which expert witness relied on to reach his own opinion, “was admissible as a basis for Navarro's opinion, but not as substantive evidence”). However, the court is only required to give a limiting instruction to the jury about the use of such evidence if it is requested by a party. State v. Goforth, 170 N.C. App. 584 (2005) (quoting Jones, “the admission of evidence, competent for a restricted purpose, will not be held error in the absence of a request by defendant for a limiting instruction.”).

For more information about the admissibility of the facts and data that form the basis of an expert’s opinion, see the related Evidence entry on Basis of Opinion [Rule 703].

For more information about the admissibility of an expert witness’s testimony when it is based on the work or opinion of another expert who is not testifying, see the related Expert Testimony entry on Crawford & Substitute Analysts.

Portions of this entry were excerpted from Jessica Smith, “Criminal Evidence: Expert Testimony,” North Carolina Superior Court Judges’ Benchbook, August 2017.