660.1Use of Force, Self-Defense

Last Updated: 12/01/23

Key Concepts

  • A properly qualified expert may be permitted to testify as an expert in the fields of self-defense or use of force to support the defendant’s claim of self-defense.
  • However, under Daubert, the testimony may be excluded if the trial court finds that the expert lacks the necessary qualifications, the testimony is not relevant or helpful to the jury, or the testimony is not based on reliable scientific principles that were applied appropriately in the present case.


When a defendant charged with an offense such as homicide or assault claims that he or she was acting in self-defense, the defense may seek to offer testimony from an expert witness on the issue of “use of force.” Such an expert might be asked to give an opinion on whether it was reasonable for the defendant to believe that he or she was being threatened or to explain why the amount of force used by the defendant in response to that perceived threat was reasonable.

In exercising their discretion to admit or deny such testimony, North Carolina trial courts have typically focused on: (i) the witness’s qualifications; and (ii) whether the proposed testimony would be genuinely helpful to the jury or whether it would merely express matters of common knowledge and experience “under the guise of an expert.” Compare State v. McDowell, 215 N.C. App. 184 (2011) (defense witness permitted to testify as “an expert in use-of-force science and self-defense tactics”) and State v. Archie, 206 N.C. App. 330 (2010) (unpublished) (defense witness testified “as an expert in the science of the use of force and in crime scene investigation”) with State v. Oakes, 209 N.C. App. 18 (2011) (“defendant was not prejudiced by the trial court's denial of defendant's motion that [witness] be received as an expert witness in the use of force” – defendant’s effort to negate the intent element “through testimony under the guise of an expert” was irrelevant to felony murder conviction); see also State v. Campbell, 149 Conn.App. 405, 88 A.3d 1258 (2014) (noting that the trial court did not abuse its discretion when it excluded the proffered testimony of an expert witness regarding “fight or flight” responses where “the jury would likely be aware of such fight or flight responses as a result of their own experiences”).

Admissibility and Reliability

The North Carolina Supreme Court extensively addressed the admissibility of this type of expert testimony under Daubert and the amended Rule 702 in State v. McGrady, 368 N.C. 880 (2016). In McGrady, the defense sought to introduce testimony by an expert witness on “the science and use of force” to establish that: (i) based on “pre-attack cues” and other variables in the interaction between the victim and the defendant, the defendant’s use of force was a reasonable response to a perceived imminent assault; (ii) defendant’s actions were consistent with a person experiencing a “fight or flight” reaction; and (iii) the science behind human reaction times could explain why some of the defendant’s “defensive” gunshots hit the victim in the back. Id.

The North Carolina Supreme Court held that the trial court did not abuse its discretion in excluding the expert’s testimony about pre-attack cues and threat perception on the grounds that it was not relevant and would not assist the jury because the matters were within the jurors’ common knowledge. Id. Additionally, the trial court did not err by excluding proposed testimony about the human nervous system and reaction times because the expert had no formal medical training and did not know the error rates of the studies (done by others) that he relied on. Furthermore, the trial court did not err in exercising its discretion by finding that the expert’s application of principles to the facts of the case was flawed because he had failed to account for how the defendant’s back injuries and would have altered his reactions. Id.; accord, State v. Thomas, 259 N.C. App. 198 (2018) (affirming trial court’s exclusion of defense expert testimony on “fight or flight” response, even though the witness “held several degrees, including a PhD in psychology, as well as a license to practice psychology in North Carolina,” noting that “these were not medical or scientific degrees”).

Practice Pointer

Common knowledge
Prosecutors seeking to exclude use-of-force or self-defense testimony from a proposed defense expert should ask the trial court to particularly consider the following portion of the McGrady court’s analysis of the defendant’s likely reaction to “pre-attack cues,” perception of a threat, and use of force variables:
The trial court concluded that Mr. Cloutier's testimony about pre-attack cues and use of force variables would not assist the jury because these matters were within the jurors' common knowledge. This ruling was not an abuse of discretion. The factors that Mr. Cloutier cited and relied on to conclude that defendant reasonably responded to an imminent, deadly threat are the same kinds of things that lay jurors would be aware of, and would naturally consider, as they drew their own conclusions. Mr. Cloutier's own expert report stated that, even without formal training, individuals recognize and respond to these cues and variables when assessing a potential threat. And if these cues and variables are logically relevant at all, they are relevant precisely because they are within the understanding of a layperson. Though defendant served in the military, he did not testify that he relied on any specialized training in threat assessment when he evaluated the threat that Mr. Shore posed to his life and the life of his son. Nor was there any evidence that he relied on anything other than common experience and instinct when he did so. Jurors possess this experience and instinct as well, which is exactly why they are tasked with deciding whether a defendant has acted in self-defense. In this instance, therefore, it was reasonable for the trial court to conclude that expert testimony would not assist the jury as required by Rule 702(a).
McGrady, 368 N.C. at 894 (emphasis added). In other words, prosecutors should point out that expert testimony on this topic is only relevant to the extent that it explains how a “normal” person would perceive certain events and how he or she would likely react – and if that’s true, then the jurors are in just as good a position as an expert to evaluate how (or if) such a reaction would have occurred for a normal person on the given facts, and whether that reaction is relevant to deciding the issues in the case.

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