This trial concerned events occurring between seven and eight years earlier, as to which no physical evidence could be produced. Defendant did not admit to any unlawful conduct; nor was there any other direct evidence linking him to the crimes. The case, therefore, was essentially a credibility contest between complainant and defendant. The prosecutor thereupon took several steps to improperly and prejudicially bolster the credibility of complainant, as a result of which defendant was deprived of a fair trial. Accordingly, I would reverse and remit to County Court for a new trial.
After complainant reported the alleged sexual abuse to his mother, his physical examination was conducted by a nurse-*468practitioner at the Erie County Child Advocacy Center. Defense counsel sought to preclude the nurse-practitioner’s testimony for several reasons, including that it would constitute improper bolstering of the complainant’s testimony and that it would be prejudicial to defendant. In response to an inquiry by the court, the prosecutor indicated that the testimony was being offered for several reasons. Notably, the prosecutor stated that, among other things, the nurse-practitioner would testify to:
“her observations of the child’s demeanor, his accelerated heart rate, his flushed face, his downcast eyes, everything that would relate to his credibility to rebut a potential defense of fabrication here.
“So in other words, when the nurse is asking him she’s documenting things in the chart that are certainly relevant not only to her diagnosis but in the broader sense to his credibility as a witness because she’s showing that he’s embarrassed, he[’s] flustered, et cetera, and that she’s actually looking for lesions.”
I agree with the majority that those portions of the nurse-practitioner’s testimony that were relevant to diagnosis and treatment were properly admitted. For example, testimony concerning complainant’s account of what had happened to him, the extent of the ensuing physical examination, the absence of any lesions or other visible signs of sexual abuse on complainant’s body and the significance of such absence were all properly admitted as relevant to diagnosis and treatment (see People v Ortega, 15 NY3d 610, 617 [2010]).
The same cannot be said about the nurse-practitioner’s testimony concerning complainant’s demeanor during the examination. The nurse-practitioner testified that when complainant related what had happened to him, “[h]e was embarrassed, downcast eyes, flushed face.” She indicated that she had recorded these details “[b]ecause they were definitely significant when I saw them. He was embarrassed.” The nurse-practitioner further testified that complainant’s elevated heart rate during the examination indicated to her that “[h]e was nervous.”
Complainant’s embarrassment or nervousness attending the examination had no medical significance whatsoever. The majority’s purported justification for the elicitation of this testimony—that it was relevant to whether some type of counseling or therapy would be required (majority op at 451-*469452)—is pure invention. To the contrary, the prosecution was perfectly blunt about why the testimony as to complainant’s demeanor was being offered: it was for credibility purposes. The nurse-practitioner’s testimony on this point was not relevant to diagnosis and treatment and its admission on that basis was error.
Furthermore, in this situation the error cannot be considered harmless. “[U]nless the proof of the defendant’s guilt, without reference to the error, is overwhelming, there is no occasion for consideration of any doctrine of harmless error” (People v Crimmins, 36 NY2d 230, 241 [1975]). As noted, the People’s case rested almost entirely on the credibility of the alleged victim. It was therefore blatantly improper and prejudicial to use the testimony of a medical professional to bolster complainant’s testimony in this manner.
Moreover, the error was egregiously compounded by the scope of the expert testimony on child sexual abuse accommodation syndrome (CSAAS). Prior to trial, defense counsel sought to preclude the CSAAS expert’s testimony for two reasons. Counsel argued, first, that the subject matter of the expert’s testimony— that children often delay reporting abuse—was not outside the ken of the average juror. In addition, counsel asserted that the expert’s testimony would be “overwhelmingly prejudicial and unfair” because the jury would inevitably draw the conclusion that complainant had been abused because he fit within the pattern of behavior recognized by CSAAS. The request was denied and the prosecutor proceeded to ask the expert, in hypothetical terms, about virtually every detail in the case.
Several aspects of complainant’s testimony were later raised with the CSAAS expert. Specifically, during his direct testimony, complainant detailed his sexual abuse by a family member—his mother’s second cousin. Complainant also related two incidents where he and his young friends had touched each other’s penises, which complainant explained was, at least in part, because he was repeating behavior that had been done to him by defendant. Complainant further testified that what ultimately convinced him to tell his mother about the sexual abuse was a video about catching on-line predators that he had watched with his eighth grade computer tech class. Additionally, on cross-examination, complainant testified that his memory had improved as the years passed. Complainant also testified that he was not afraid to return to defendant’s house after the alleged abuse, but rather that he wanted to go to the house and had fun when he was there.
*470The expert testified after the jury had heard complainant’s version of events. In response to the prosecution’s questions, the expert testified that abuse by a stranger was rare; a child was more likely to be abused by a person he knows and the child’s feelings of helplessness would be enhanced if the abuser were a trusted adult who was close to the child’s mother. The expert also testified that he had seen children become hypersexualized and use sexuality as a coping mechanism. In response to a question about a child’s willingness to return to the scene of the abuse, the expert testified that the child could convince himself that the abuse would not happen again and that the child could be willing to spend time with the abuser because he would want to repeat any positive experiences he had with that person. The expert also testified that boys were “far more likely to delay” reporting sexual abuse and that any period of delay was likely to be far longer than for girls—specifically, that a delay of six or seven years would not be unusual. The expert also responded in the affirmative to the prosecutor’s query as to whether “an educational component about the awareness of sexual abuse [could] be a triggering event” for the disclosure of abuse. Finally, the expert testified that a child’s memory could actually improve and that more details could come to mind after the child disclosed the abuse.*
“[E]xpert opinion is proper when it would help to clarify an issue calling for professional or technical knowledge, possessed by the expert and beyond the ken of the typical juror” (People v Taylor, 75 NY2d 277, 288 [1990] [citation omitted]). However, where “the sole reason for questioning the ‘expert’ witness is to bolster the testimony of [the complainant] by explaining that his version of the events is more believable than the defendant’s, the ‘expert’s’ testimony is equivalent to an opinion that the defendant is guilty, and the receipt of such testimony may not be condoned” (People v Ciaccio, 47 NY2d 431, 439 [1979]). Although we have recognized that CSAAS can be used for the purpose of explaining behavior by a complainant that might appear unusual to the average juror—such as why a child might not immediately report sexual abuse—we have contrasted the permissible use of such testimony with testimony that opines that complainant’s “behavior [was] consistent with such abuse” (People v Carroll, 95 NY2d 375, 387 [2000]; see also People v *471Mercado, 188 AD2d 941, 942 [3d Dept 1992] [expert’s testimony “constitute(d) an impermissible comparison of the complainants’ behavior with that commonly associated with victims of these crimes”]).
Even though the expert did not expressly render an opinion as to whether or not complainant was a victim of sexual abuse, the expert’s confirmation of nearly every detail of the case and of complainant’s behavior as consistent with that of a victim of sexual abuse was the functional equivalent of rendering an opinion as to complainant’s truthfulness (see Ciaccio, 47 NY2d at 439). The expert’s testimony had the effect of improperly bolstering complainant’s testimony and, in the context of this case, was extremely prejudicial.
As noted above, this was a case where the credibility of the parties was the key issue facing the jury. Each of the errors, in bolstering complainant’s testimony with the nurse-practitioner’s perception of his demeanor and the CSAAS expert’s validation of his behavior as consistent with that of a victim of sexual abuse, and certainly their cumulative effect, deprived defendant of a fair trial.
Judges Graffeo, Smith and Pigott concur with Judge Read; Chief Judge Lippman dissents and votes to reverse in a separate opinion in which Judges Ciparick and Jones concur.
Order affirmed.
On cross and redirect, the expert even managed to inform the jury that in his experience of 154 cases, he had seen only four instances of false allegations, three of them in the context of divorce battles.