## Facts

- Clyde Vinsonhaler saw Ruben Daniel Diaz, a stranger, enter his cul-de-sac appearing disoriented; Vinsonhaler went inside intending to call police.
- Vinsonhaler later encountered Diaz inside the home; Diaz refused to leave.
- Diaz attacked Vinsonhaler with a knife, stabbing him repeatedly and saying, “I’m going to kill you,” causing severe injuries requiring multiple surgeries.
- Police arrived and subdued Diaz with a taser.
- The State charged Diaz with aggravated battery, use of a deadly weapon in the commission of a crime, and resisting or obstructing a police officer.
- Diaz disclosed proposed expert testimony from Dr. James Davidson that Diaz was delusional and believed Vinsonhaler was an alien, not a person, when Diaz attacked him.
- The State moved in limine to exclude the expert testimony, arguing Idaho Code § 18-207 barred mental-condition evidence and that the testimony was irrelevant and unfairly prejudicial.
- The district court denied the motion (and reconsideration); the Idaho Supreme Court accepted an interlocutory permissive appeal.

## Issues

1. Whether Idaho Code § 18-207 categorically prohibits expert testimony about a defendant’s mental condition when offered to negate the State’s proof of mens rea.
2. Whether § 18-207 prohibits such testimony when offered to support a mistake-of-fact theory.
3. Whether the proposed expert testimony should be excluded pretrial as irrelevant or unfairly prejudicial under Idaho evidentiary rules.

## Decision

- The Idaho Supreme Court affirmed the order denying the State’s motion in limine.
- The court held § 18-207 abolishes insanity as an affirmative defense but does not categorically bar mental-condition evidence offered to rebut proof of a required mental state.
- The court agreed the proffered testimony was relevant to whether Diaz formed the mens rea for aggravated battery and to a potential mistake-of-fact theory.
- The court held Rule 403 concerns did not require categorical pretrial exclusion; the trial court could manage presentation and jury instructions to reduce confusion.

## Legal Principles

- Abolition of the insanity defense by statute does not eliminate the State’s burden to prove every element of the charged offense, including the required mental state, beyond a reasonable doubt.
- Idaho Code § 18-207 bars using “mental condition” as an affirmative excuse-based insanity defense, but it does not bar relevant evidence offered to negate mens rea.
- Expert testimony describing a defendant’s delusional beliefs may be admissible when it bears on what the defendant actually believed or intended at the time of the act.
- A mistake-of-fact theory may negate criminal liability when the claimed factual misapprehension prevents formation of the mental state required by the offense.
- Potential prejudice or jury confusion from mental-condition evidence is addressed through trial-management tools (limits on testimony and proper instructions), not automatic exclusion.

## Conclusion

The Idaho Supreme Court permitted potential admission of expert mental-condition testimony to contest mens rea and support mistake of fact, holding that § 18-207 abolishes insanity as an affirmative defense but does not impose a categorical bar on relevant mental-health evidence.


State v. Diaz, 507 P.3d 1109 (Idaho 2022)