Expert Recklessness, Imperfect Self-Defense, and Plain Error: United States v. Maryboy

Introduction

United States v. Maryboy, 23-4117 (10th Cir. May 29, 2025), presents two intertwined questions about criminal procedure and proof: (1) the permissible scope of expert testimony on a defendant’s mental state under Federal Rule of Evidence 704(b), and (2) the necessity of a jury instruction placing on the government the burden to disprove imperfect self-defense beyond a reasonable doubt. Perry Maryboy, an enrolled Indian charged with second-degree murder (18 U.S.C. § 1111) and a related § 924(c)(1)(A) firearms offense, was convicted by a jury in the District of Utah. On appeal under plain-error review, the Tenth Circuit reversed his murder conviction and remanded for a new trial, finding two errors that could have led the jury to convict him of second-degree murder rather than the lesser offense of involuntary manslaughter.

At trial, Maryboy claimed he intended only to fire “warning shots” after a heated roadside confrontation, but one discharge struck Antonio Montowine in the back of the head, killing him. The government’s firearms expert testified repeatedly that such conduct was “extremely reckless” and “the ultimate expression of recklessness,” language substantially identical to the court’s own instruction defining malice aforethought. Separately, although the district court instructed on perfect self-defense and included imperfect self-defense in the involuntary-manslaughter instruction, it failed to tell the jury that the government must disprove imperfect self-defense beyond a reasonable doubt before convicting on the murder count.

Summary of the Judgment

The Tenth Circuit, in an opinion by Judge Phillips (joined by Judges Hartz and Federico), applied the four-prong plain-error test and held:

Rule 704(b) Violation: The government’s firearms expert (FBI Special Agent Olson) was asked how “reckless” the hypothetical conduct would be, and he answered that it was “the ultimate expression of recklessness” because “it could take another human life.” That opinion, identical in substance to the jury’s definition of malice aforethought (“reckless and wanton disregard for human life . . . extreme in nature”), impermissibly commented on the element of malice, in violation of Rule 704(b).
Jury Instruction Error: Although the court instructed on perfect self-defense and described imperfect self-defense as part of the involuntary-manslaughter instruction, it never told the jury that the government had to disprove imperfect self-defense beyond a reasonable doubt to convict on second-degree murder. Because imperfect self-defense negates malice aforethought, due process required such an instruction.

Both errors satisfied plain-error review: they were clear under existing Tenth Circuit precedent; they struck at the single disputed element—malice aforethought—creating a reasonable probability of a different verdict (guilty of involuntary manslaughter rather than murder); and they undermined the fairness and integrity of the trial. The conviction was reversed and remanded for a new trial on both charges.

Analysis

Precedents Cited

Fed. R. Evid. 704(b) – prohibits expert opinion on whether the defendant had the mental state constituting an element of the crime.
United States v. Wood, 207 F.3d 1222 (10th Cir. 2000) – expert testimony “substantively indistinguishable” from mens rea violates Rule 704(b).
United States v. Dennison, 937 F.2d 559 (10th Cir. 1991) – hypothetical-based expert testimony dictating a defendant’s capacity to form intent.
Diaz v. United States, 602 U.S. 526 (2024) – expert testimony about “most people” in a group does not violate Rule 704(b) absent direct opinion on the defendant’s own mental state.
United States v. Lofton, 776 F.2d 918 (10th Cir. 1985) – due process requires the government to prove absence of a mitigating defense (heat-of-passion or imperfect self-defense) beyond a reasonable doubt.
United States v. Sago, 74 F.4th 1152 (10th Cir. 2023) – imperfect self-defense mitigates murder to involuntary manslaughter; absence of such defense is an element of murder.

Legal Reasoning

1. Rule 704(b) Error. Expert testimony is barred from stating an opinion “about whether the defendant did or did not have a mental state . . . that constitutes an element of the crime charged.” Fed. R. Evid. 704(b). Second-degree murder’s mens rea—malice aforethought—may be shown by “reckless and wanton disregard for human life . . . extreme in nature.” The government elicited from its firearms expert the opinion that the hypothetical warning-shot scenario was “the ultimate expression of recklessness” because it “could take another human life.” That opinion was “substantively indistinguishable” from the jury instruction itself, and thus “necessarily dictate[d]” the malice element, violating Rule 704(b).

The government’s counterarguments—that the testimony addressed only a “hypothetical person,” that “recklessness” was colloquial or only objective, or that any error was undeniable harmless given overwhelming evidence—were all rejected. Under Wood and Dennison, an expert may describe the mechanics of the conduct or precipitating facts, but may not overlay the conduct with the precise legal conclusion that the defendant satisfied the requisite mens rea.

2. Jury Instruction Error. Maryboy requested (and the court granted) perfect self-defense instructions and included imperfect self-defense when defining involuntary manslaughter. But nowhere did the court instruct that to convict of second-degree murder the government had to disprove imperfect self-defense beyond a reasonable doubt. Due process—and Lofton—mandate that a jury be told of the government’s burden to negate any mitigating defense it has credited. Here, the instructions effectively allowed the jury to convict of murder even if it found that the shooting was an unreasonable (imperfect) self-defense, undermining any presumption of innocence as to malice aforethought.

Impact

United States v. Maryboy clarifies two critical trial safeguards:

Expert Witness Limits: Government experts must carefully avoid couching testimony in expressly legal terms that mirror the jury’s definition of a mental-state element. Their role is to provide factual or mechanical context, not to testify that “this conduct was malice.”
Jury Instructions on Mitigating Defenses: When a defendant offers evidence of imperfect self-defense (or any partial exculpatory theory), the court must instruct that the government bears the burden of disproving that defense beyond a reasonable doubt before a murder conviction can stand.

These refinements will influence future § 1111 prosecutions and any criminal trials where expert testimony or self-defense hinging on reasonableness of force comes into play. They also underscore that close distinctions between homicide grades must be respected to protect the presumption of innocence.

Complex Concepts Simplified

Plain-Error Review: Courts may correct an unpreserved error on appeal if (1) there was an error; (2) it was “plain” (clear under existing law); (3) it affected substantial rights (reasonable probability of a different outcome); and (4) it seriously undermines the trial’s fairness or integrity.
Federal Rule of Evidence 704(b): In criminal cases, experts may not state opinions on whether the defendant actually had the mental state required for the crime or a defense. Those determinations are jury functions.
Malice Aforethought: The mens rea for second-degree murder. It may be shown by intent or by a “reckless and wanton disregard for human life that is extreme in nature.”
Perfect vs. Imperfect Self-Defense:
- Perfect Self-Defense is a complete defense to murder: the defendant must have subjectively believed lethal force was necessary and that belief must have been objectively reasonable.
- Imperfect Self-Defense mitigates murder to involuntary manslaughter: the defendant must have subjectively believed lethal force was necessary, but that belief may have been unreasonable.
Involuntary Manslaughter: An unintentional killing resulting from reckless or grossly negligent conduct that “is not extreme in nature.”

Conclusion

United States v. Maryboy reinforces strict boundaries around expert testimony on ultimate issues and reaffirms the defendant’s right to a jury instruction placing on the government the burden to disprove partial defenses. By reversing and remanding for a new trial, the Tenth Circuit ensures that jurors decide malice vs. mere negligence or imperfect self-defense based on properly stated law and evidence, preserving the integrity of verdicts in closely contested homicide cases.