State v. P. Grimshaw, 469 P.3d 702

Defendant Philip Grimshaw (step‑cousin of the complainant T.G.) was convicted of felony sexual intercourse without consent after conflicting accounts: T.G. testified she was raped; Grimshaw testified the sex was consensual and made inconsistent statements to police.
The State disclosed Dr. Sheri Vanino as a "blind" expert to testify about rape myths and victim behavior; Grimshaw retained Dr. Bowman Smelko as a rebuttal expert.
Over Grimshaw's objections, the district court allowed Dr. Vanino to give statistical testimony that only 16–20% of sexual assaults are reported and that 2–8% of reports are false. Dr. Smelko testified in rebuttal, acknowledging the 2–8% figure but noting study limitations.
The prosecutor emphasized the experts’ false‑report statistics during closing; the jury convicted Grimshaw. He was sentenced to 40 years, with 20 suspended, and appealed.
The sole contested legal issue on appeal was whether admitting expert testimony about false‑report statistics was an abuse of discretion that denied Grimshaw a fair trial; the Montana Supreme Court reversed and remanded for a new trial.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Admissibility of expert testimony about statistical rates of false sexual‑assault reports	Expert testimony on rape myths and false‑report statistics is relevant and permissible to rebut defense and assist the jury	Statistical testimony about false reports improperly comments on victim credibility and is barred by precedent	Admission of testimony that 2–8% of reports are false was an abuse of discretion and prejudicial; conviction reversed and remanded for a new trial

State v. Brodniak, 221 Mont. 212 (expert testimony on malingering and statistical false‑report rates improperly commented on victim credibility)
State v. Walker, 394 Mont. 1 (expert testimony on battered‑woman syndrome can help juries understand victim behavior)
State v. Van Kirk, 306 Mont. 215 (distinguishes harmless error from prejudicial trial error)
State v. St. Germain, 336 Mont. 17 (standard of review for admissibility of expert testimony)
State v. Guill, 355 Mont. 490 ("opening the door" permits limited rebuttal evidence)
Rogers v. State, 360 Mont. 334 (expert may not comment on a victim's credibility)