Trial court does not have a sua sponte duty to instruct the jury on the limited purpose of expert testimony on child sexual abuse accommodation syndrome (CSAAS). When I.T. was 18 years old she reported that Mateo had sexually abused her multiple times when she was between the ages of 9 and 12. At Mateo’s retrial for continuous sexual abuse of a child under age 14 (Pen. Code, § 288.5, subd. (a)), the prosecution admitted expert testimony on CSAAS, which is a theory that explains why it is not unusual for a minor who has been sexually abused to delay disclosure. The jury found him guilty. Mateo appealed, arguing that the trial court had a sua sponte duty to instruct the jury with CALCRIM No. 1193 to explain the limited purpose of expert testimony on CSAAS, and alternatively, that his trial counsel rendered ineffective assistance for failing to request such an instruction. Held: Affirmed. Only one instruction need be given sua sponte on expert testimony: an instruction that informs the jury that they are not bound to accept the opinion of any expert as conclusive, but should give to it the weight to which they shall find it to be entitled. (See Pen. Code, § 1127b.) The Court of Appeal disagreed with People v. Housley (1992) 6 Cal.App.4th 947, 957, which imposed a sua sponte duty on the trial court to instruct the jury that an expert’s testimony on CSAAS is not evidence that the defendant committed the crime and should only be considered in deciding whether the victim’s conduct was not inconsistent with the conduct of someone who has been molested (now embodied in CALCRIM No. 1193). Even if the trial court did err in failing to instruct the jury, it was harmless because the prosecution’s expert testified that CSAAS was not intended to predict whether a child had actually suffered abuse, Mateo had confessed, and the victim’s testimony was detailed and consistent. The court also rejected Mateo’s ineffective assistance of counsel claim.

The full opinion is available on the court’s website here: http://www.courts.ca.gov/opinions/documents/B259997M.PDF