Can expert testimony about cognitive capacity violate Utah's credibility rules? State v. Adams Explained

In State v. Adams, the Utah Supreme Court addressed whether expert testimony about a witness’s cognitive capacity to fabricate testimony violates evidentiary rules prohibiting credibility bolstering. The case provides important guidance for practitioners handling cases involving witnesses with intellectual disabilities.

Background and Facts

Adams was convicted of forcible sexual abuse against Carleen, a 34-year-old woman with Down syndrome who had the cognitive abilities of a three-and-a-half-year-old. Adams’s defense was that Carleen’s mother coached her to fabricate the allegations. At trial, Dr. Hawks testified about Carleen’s intellectual disabilities and opined that she likely lacked the cognitive ability to be coached or to invent such a story. Detective DeHart also testified that Carleen did not appear coached and that her account remained consistent.

Key Legal Issues

The case presented three issues: (1) whether Dr. Hawks’s testimony about Carleen’s cognitive capacity violated Utah Rule of Evidence 608(a), which prohibits opinion testimony on truthfulness; (2) whether the expert testimony satisfied foundational requirements under Rule 702; and (3) whether Detective DeHart’s improper credibility vouching constituted reversible error.

Court’s Analysis and Holding

The Supreme Court affirmed on all issues. Regarding Dr. Hawks’s testimony, the court distinguished between testimony about mental capacity to fabricate and prohibited direct opinions on truthfulness. Dr. Hawks did not offer a subjective credibility determination but testified only about Carleen’s cognitive limitations. The court found this testimony admissible under Rule 702 as specialized knowledge assisting the fact finder. For Detective DeHart’s testimony, while improper under Rule 608(a), the court applied plain error review and found the error harmless given other persuasive evidence.

Practice Implications

This decision establishes that expert testimony about a witness’s cognitive capacity to fabricate or learn false stories does not automatically violate Rule 608(a). However, practitioners must carefully frame such testimony to address mental capacity rather than specific truthfulness. The case also demonstrates the importance of preserving objections to avoid plain error review’s higher burden.