A jury convicted P. Christopher McCaffrey of “unnatural sexual intercourse” with his daughter, a child under the age of sixteen. We agree with McCaffrey that the testimony of the fresh complaint witness (the child’s mother and McCaffrey’s estranged wife) exceeded the permissible bounds of such testimony, to his prejudice, and accordingly reverse the judgment of conviction.
The child, Erin (a pseudonym), who was five years old at the time of trial, testified on direct examination that, when she was two, McCaffrey had inserted his finger into her vagina. On cross-examination, Erin admitted that this had happened only once. When Erin’s mother was next called as a fresh complaint witness, McCaffrey moved at sidebar for a voir dire of her proposed testimony, on the ground that it would include improper details beyond those testified to by Erin. The trial judge denied his motion. The prosecutor then alerted the judge that Erin had earlier disclosed more information to her mother than she had disclosed in her testimony — including a charge of fellatio — and that the prosecutor was concerned about whether she should ask the mother about such matters.
The judge professed an awareness that fresh complaint evidence is not to be used in any substantive, but only a corroborative manner. See
Commonwealth
v.
Bailey,
*585 Erin’s mother proceeded to testify that in October, 1989, Erin told her that “her father had put his weenie in her mouth and that it tasted yucky.” Immediately after the mother made this statement, the judge asked her to repeat it. After doing so, the mother went on to testify that in March, 1990, Erin had told her that “her daddy had poked” her private parts. Asked by her mother at that time to show what she meant, Erin “took her index finger of her right hand, put it into her underwear and into her vagina.” Following the mother’s direct testimony, the judge instructed the jury specially on fresh complaint evidence, instructions which he later repeated in his general charge.
On appeal the Commonwealth has conceded, as it must, that the mother’s testimony regarding the October, 1989, “weenie” incident exceeded the proper limits of fresh complaint.* 2 The Commonwealth contends, however, that any er *586 ror was harmless because the testimony “did not fill gaps in the prosecution’s case,” and because any risk of prejudice to McCaffrey was “ameliorated” and “minimized” by the judge’s reiterated fresh complaint instructions (the propriety of which McCaffrey does not challenge). We are not persuaded by this “harmless error” argument. The formulaic and unelaborated contention that the mother’s superfluous testimony did not “fill gaps” in the prosecution’s case is subverted by the Commonwealth’s acknowledgement that the testimony introduced new substantive detail and did extend beyond mere corroboration. The prohibition against gap filling simply reflects the fundamental premise that fresh complaint testimony can be utilized for corroborative purposes only and not as substantive evidence of the elements of the crime charged.
Although curative instructions
“usually
render [ ] any error in the introduction of prejudicial evidence harmless,”
Commonwealth
v. Jackson,
Rather than providing “explicit warnings” to the jury,
Commonwealth
v.
Gallego, supra
at 720, that addressed the specific problem created by the mother’s excess testimony, cf.
Commonwealth
v.
Kelly,
We have no doubt that the error (to which McCaffrey strenuously objected) entailed serious potential prejudice to the defense. Erin testified to only one incident of “unnatural sexual intercourse” involving digital penetration. Contrast
Commonwealth
v.
Snow,
The putative actuality of the fellatio accusation was immediately enhanced, at the judge’s insistence, by the mother’s repetition of her testimony. Such testimonial charges of sexual molestation of a child are recognized as being inherently “inflammatory.” See
Commonwealth
v.
Montanino,
Examining the error, as we must, in the context of the whole trial, we find “nothing in the fresh complaint instructions or the charge in its entirety which provides us with a reliable basis for concluding that the risk [that the jury may have relied in its deliberations on the inadmissible reference to the separate act of sexual molestation mentioned by the fresh complaint witness but not the victim] was somehow ameliorated.”
Commonwealth
v.
Tingley,
McCaffrey’s jury, however, were instructed not to disregard but rather to consider carefully the erroneously included evidence, which implicated the central issue in the case and which the prosecutor effectively kept before the jury. Here, the unignorable was more akin to a herd of elephants indelibly trampling that evidence into the jurors’ collective consciousness. In these circumstances, we “cannot say, with fair assurance, after pondering all that happened without stripping the erroneous action from the whole, that the [jury’s] judgment was not substantially swayed by the error . . . [and] that the judge’s error in admitting the improper testimony was not prejudicial.”
Commonwealth
v.
Flebotte,
McCaffrey’s remaining assignments of error require little discussion, except for one that may arise at any retrial. He argues (for the first time on appeal) that Erin was never formally sworn at trial; that her testimony was, therefore, inadmissible; and accordingly, that he was entitled to a required finding of not guilty (even though his motion for such a finding did not mention this supposed defect). His contention falls short of demonstrating a substantial risk of a miscarriage of justice. See
Commonwealth
v.
Freeman,
McCaffrey’s complaint that his cross-examination of the Commonwealth’s expert was prejudicially impaired because of the denial of his request for pretrial disclosure of the expert testimony is also belied by the record. McCaffrey’s counsel had over four months to review all of the expert’s office notes and treatment records relating to Erin and was made aware of the expert’s name, address, and status as a potential expert witness at least three weeks prior to trial. Nothing has been presented, beyond conclusory assertion, indicating the inadequacy of that lead time for preparation of effective cross-examination. No continuance to prepare cross-examination was sought. No funds for retainer of a rival expert were requested. No prejudice has been shown. See
Commonwealth
v.
Pope,
McCaffrey’s final claim is that the Commonwealth’s expert was allowed to invade the province of the jury by providing testimony that had the effect of bolstering Erin’s credibil *591 ity. 6 He has raised a troublesome issue. The witness (a psychologist and expert in the area of sexual abuse of children) first testified that she counseled sexually abused children and that such children exhibit certain general behavioral characteristics. She then was permitted to testify, over McCaffrey’s objections, that Erin was currently her patient; that she had been treating Erin for about a year; that at the beginning of the therapy Erin had revealed that she had beén sexually abused by her father; and that Erin had been placed in therapy groups for sexually abused children. The expert further described Erin’s conduct during the early course of treatment in terms that the jury could not have failed to note resembled the behavior the expert had earlier testified to as characteristic of sexually abused children.
It is settled that an expert may not “offer his [or her] opinion on issues that the jury are equally competent to assess, such as credibility of witnesses. On such questions, the influence of an expert’s opinion may threaten the independence of the jury’s decision.”
Simon
v.
Solomon,
We have previously acknowledged this risk by admonishing that:
“In offering an expert who would testify about the ‘general characteristics’ [of child victims of sexual abuse], the Commonwealth must take care not to lead the expert to trespass unduly on the jury’s province to assess the credibility of witnesses, in particular the child as witness .... [T] he prosecution [must] . . . steer the expert clear of the question of the truthfulness of the particular child and in that connection to avoid involvement in the question whether the particular child was in fact abused or whether her behavior did or did not in fact conform to the ‘general characteristics.’ ”
Commonwealth
v.
O’Brien,
See also Commonwealth v. Swain, ante 433, 444-445 (1994). Allowing the psychologist here to testify as both behavioral expert and treating therapist may well have approached too closely the forbidden issue of the victim’s credibility. The fact that she had accepted Erin as a patient to be treated as a sexually abused child could easily give rise to the jury’s inference that she had accepted Erin’s allegations against her father as true and was providing her professional services as a result, thereby endorsing Erin’s credibility as a victim of parental sexual molestation.
Without deciding the issue whether the behavioral expert’s testimony in her dual role 7 as Erin’s therapist was itself error *593 requiring reversal,* *** 8 we urge the Commonwealth in any retrial to pay careful heed to the teachings of Dockham and O’Brien by utilizing sexual abuse experts who have no connection with and make no references to the child victim or *594 her family, lest the very real prospect of prejudicial error invalidate the proceeding. The verdict is set aside, the conviction is reversed, and the case is to stand for a new trial.
So ordered.
Notes
Since the trial of this case, we have declared that when there is forewarning that proposed fresh complaint witnesses may describe alleged sex
*585
ual offenses by the defendant not testified to by the victim, “it is better to conduct a voir dire examination of the witness so that the judge may prune out extraneous matter before the jury hears it.”
Commonwealth
v.
Flebotte,
Hearsay evidence that the victim seasonably made an out-of-court complaint to a witness after a sexual assault has, despite misgivings, been held admissible as part of the prosecution’s case-in-chief, not only regarding the fact that the complaint was made, but also extending to the details of the complaint.
Commonwealth
v.
Licata,
Compare as to the issue of prejudice
Commonwealth
v.
Licata,
The judge twice asked Erin to “agree that when you testify now, you’ll swear to tell us the truth.” The child affirmatively responded both times and indicated that she realized that her failure to tell the truth would result in punishment. We note that, the prosecutor reiterated the judge’s admonitions in questioning Erin.
Had McCaffrey raised this objection at trial in a timely manner, it could have easily been cured by retroactive administration of an oath (or its equivalent), see
State
v.
Munnis,
McCaffrey also asserts that the judge erred regarding this expert in that there was no need for expert testimony because the subject matter was within the common knowledge of the jury and that no adequate foundation was laid for the particular expert utilized. He has, however, shown us nothing that demonstrates any abuse of the trial judge’s wide discretion with respect to the admission of expert testimony generally, including evaluating the qualifications of proposed experts, and the use of expert testimony regarding the behavioral characteristics of sexually abused children specifically. See
Commonwealth
v.
Boyd,
Although the prosecution expert on sexually abused children in
Commonwealth
v.
Hudson, supra,
also treated the victim (a circumstance brought out only in cross-examination and redirect examination of the victim’s mother),
But see
Commonwealth
v.
Ianello,
