Montana is the latest state to reject the prosecution’s attempt to use expert witnesses to vouch for the credibility of accusers in criminal prosecutions. The Montana Supreme Court recently reversed a sexual assault conviction because an expert witness testified about the percentage of accusers who make false accusations.

Facts of the Case

Philip Grimshaw was accused of sexually assaulting his step-cousin, identified in the appellate opinion as “T.G.” Grimshaw invited T.G. to a party where she participated in drinking games. After the party ended, Grimshaw and T.G. drove around and drank more alcohol, as they had done many times before. They then returned to Grimshaw’s home, where T.G. went to sleep on the couch.

Grimshaw and T.G. differed in their account of what happened next. T.G. testified that she woke up to find Grimshaw pulling down her pants. She testified that she said “no” and that he had sex with her anyway. Grimshaw testified that T.G. consented to their sexual contact.

Grimshaw and T.G. exchanged text messages the next day. Grimshaw texted an apology. He said he loved T.G. and knew she would probably never want to talk to him after what happened. T.G. responded “It’s ok, I love you too.” They chatted a bit more before each again texted “I love you” to the other.

Almost two weeks later, T.G. visited a hospital for treatment of a migraine. While she was in the hospital, she told her mother that she had been raped. She then gave a statement to a police detective.

The detective interviewed Grimshaw, who initially denied having a clear memory of events. He then said he remembered cuddling with T.G., remembered that they touched each other sexually, and remembered apologizing to her later for having sex with her.

Grimshaw told the detective that he thought T.G. wanted to have sex with him at the time, although she had earlier told him that they couldn’t have sex because they were step-cousins. According to the detective, Grimshaw also admitted that he raped T.G. by doing something she didn’t want and doing it “forcibly.”

Expert Testimony

The prosecutor called Sheri Vanino as an expert witness. Vanino is a psychologist. She testified about the reasons a sexual assault victim might delay reporting the crime and why she might remain friends with the man who assaulted her.

Vanino did not claim to have met T.G. or to have personal knowledge of her accusation. According to the prosecutor, the only purpose of Vanino’s testimony was to explain that certain “myths” about the behavior of rape victims do not accurately describe the behavior of every victim.

Vanino identified one of those myths as the belief “that women tend to run around and falsely accuse men of rape, or cry rape all the time.” Because that testimony did not go to the behavior of rape victims, but of women who make accusations of rape, Grimshaw’s attorney objected.

Grimshaw’s lawyer argued that statistical evidence about whether false reporting is common or uncommon was barred by Montana precedent that prohibits experts from vouching for an accuser’s credibility. The court overruled the objection on the theory that it was permissible to give testimony that is couched in terms of exposing a myth.

Vanino then testified, over objection, that only 16{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 20{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of sexual assault victims ever tell anyone about the assault and that only 19{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of sexual assault victims fight back. She also testified that only 2{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 8{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of sexual assault accusations are false. According to Vanino, women who make false accusations usually have something to gain or are truly psychotic.

Grimshaw called a psychologist, Bowman Smelko, as a rebuttal expert. Smelko testified that the 2{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 8{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} figure is commonly used in the literature but explained that the studies from which those figures are derived relied on small sample sizes. He also explained the difficulty of verifying false reporting statistics. A false report that is accepted by authorities as true will be statistically recorded as a true report even if it is false.

Appellate Analysis

Montana allows qualified experts to give opinion evidence if the opinions are relevant and beyond the knowledge of a lay juror. However, in Montana and most other states, an expert is not permitted to express an opinion about the credibility of another person. Most states prohibit vouching for the credibility of an accuser.

The Montana Supreme Court concluded that Vanino’s assertion that only 2{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 8{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of sexual assault accusations are false was improper. The testimony necessarily sent the message that there was a 92{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 98{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} probability that T.G. was telling the truth.

The jury likely understood Vanino’s testimony as an opinion that T.G. was telling the truth because, in her expert opinion, most accusers tell the truth. For that reason, the expert testimony improperly vouched for T.G.’s credibility.

The relevance of the testimony is also questionable. The case against Grimshaw depended on whether the jury believed that T.G. was telling the truth. Whether other alleged victims have been truthful in other cases sheds no light on whether T.G.’s accusation against Grimshaw was truthful. Even if it is true that 92{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 98{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of sexual assault accusations are truthful, that fact does not help the jury decide whether T.G. was one of the 2{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} to 8{d61575bddc780c1d4ab39ab904bf25755f3b8d1434703a303cf443ba00f43fa4} of accusers who make false accusations.

Criminal convictions must be based on evidence that the defendant is guilty, not on evidence of statistical probabilities regarding groups of defendants and their accusers. The supreme court concluded that using statistical probabilities to prove guilt would “turn the presumption of innocence on its head.”

The court’s opinion does not engage in a Daubert analysis. It is not clear whether Grimshaw made a Daubert challenge to the testimony. A strong argument can be made that it is impossible to verify whether accusations are true or false and that unverifiable data is, by its nature, unreliable.

An expert in study design might have cast doubt on whether the “two to eight percent” statistic is based on reliable data or a reasonable methodology. However, since experts cannot vouch for the credibility of accusers, the court did not need to address admissibility under Daubert.