It’s well settled that it’s the jury’s critical and exclusive function to make witness-credibility determinations. Hence the Tenth Circuit’s unequivocal finding this last week in United States v. Magnan that error occurs when an expert witness usurps the role of the jury by placing a mathematical estimate on the rate of false accusations by victims.
In Magnan, the defendant was facing numerous counts of various sex crimes, including multiple counts of sexual abuse of a minor. At trial, the government’s expert witness cited to studies regarding false accusations by children, and summarized the studies’ conclusions as follows:
[What the studies determined was that] the rate of false disclosures by or false allegations exclusively by a child was in the 2 to 4 percent range. Some literature goes as high as 5, maybe a little higher. But the standard studies that are frequently quoted are 2 to 4 percent.
The Tenth Circuit’s conclusion? The expert’s citation to the studies that only 2-4% of children lie about being sexually abused was the equivalent of vouching for the alleged victims. And by impermissibly bolstering the credibility of the witnesses, the government’s expert usurped the jury’s exclusive function.
Magnan is a valuable precedent to have in your pocket, especially when moving pretrial to exclude such impermissible evidence from being introduced in the first place.