OPINION OF THE COURT
FLAHERTY, Justice.
This is an appeal from the order of Superior Court, affirming appellant’s judgment of sentence for involuntary deviate sexual intercourse following a jury trial. The issue is whether the trial court properly permitted expert testimony regarding the victim’s afflication with “rape trauma syndrome” (RTS). We hold that such testimony is inadmissible, and therefore vacate the judgment of sentence and remand for a new trial.
Uncontroverted evidence establishes that on November 26, 1977, an intruder gained entry to the home of the victim by posing as a police officer, then raped her and forced her to submit to other sexual outrages. The victim described the assailant to the police and reported that he identified himself as Gallagher. Two weeks later, on December 10, 1977, the victim examined a photographic display containing the appellant’s picture and also faced him in a voluntary [293]*293one-on-one confrontation; in neither instance did she identify him.
More than four years later, however, the victim identified the appellant from a photographic display. The appellant was then arrested on February 19, 1982, and charged with rape and numerous related offenses. Prior to his arrest, however, the statute of limitations had expired on all the offenses except involuntary deviate sexual intercourse. Accordingly, he was tried only for the latter offense.
At his jury trial, the appellant contested his identification as the rapist. In order to downplay the victim’s repeated failures to identify appellant within weeks of the crimes and bolster her identification after four years, the Commonwealth presented Ann Burgess as an expert witness with respect to RTS. Burgess holds a master’s degree in psychiatric nursing and a doctorate in nursing science. Together with a colleague, Professor Lynda Holmstrom of Boston College, Burgess co-authored an article in the American Journal of Psychiatry which is credited with being the first study of victims of sexual attack to use the term “rape trauma syndrome.”1 Burgess’s qualifications included extensive teaching, research, writing, and forensic experience with the psychological effects found in rape victims. She described the symptomology of the syndrome, now accepted as a standard post-traumatic stress response disorder by the American Psychiatric Association. She then summarized her examination of the victim, stated her diagnosis that the victim suffered from RTS, and related her opinion of how the phenomena of RTS bore upon the identification process.
The appellant was convicted by jury, his post-verdict motions were denied, and he was sentenced to serve ten to twenty years imprisonment. On appeal, a divided panel of the Superior Court affirmed the judgment of sentence. The primary issue is whether Burgess’s testimony regarding RTS was admissible. We hold that the testimony was an [294]*294impermissible encroachment on the jury’s function of determining credibility, and reverse the judgment of sentence.
At the appellant’s trial, there was no question that the victim had been sexually assaulted; the principal question for the jury was the identity of the attacker. The victim’s positive, indeed animated,2 identification of the appellant at trial was seriously undermined by the fact that she had been unable to identify him two weeks after the crime. The testimony of Burgess regarding RTS was introduced for the sole purpose of shoring up the credibility of the victim on the crucial issue of identification.
Determinations of credibility, however, are exclusively the province of the jury. Commonwealth v. Seese, 512 Pa. 439, 443, 517 A.2d 920, 922 (1986). We have consistently rejected expert testimony which encroaches on this vital jury question. Kozak v. Struth, 515 Pa. 554, 531 A.2d 420 (1987) (prohibiting expert testimony on causation and due care because issues of ultimate fact, especially those of [295]*295credibility, are for jury, not expert); Commonwealth v. Seese, supra (prohibiting expert medical testimony regarding the veracity of children who claim to be objects of sexual abuse); Commonwealth v. O’Searo, 466 Pa. 224, 352 A.2d 30 (1976) (prohibiting expert testimony of clinical psychologist to substantiate and corroborate the defendant’s version of the critical events). We must examine the testimony of Burgess in light of this standard.
Burgess first described RTS as occurring in two phases. The first is the acute phase, lasting days or weeks, during which the victim is emotionally overwhelmed and has difficulty performing ordinary functions; this is followed by a long term reorganization phase, lasting months or years, during which the victim deals with symptoms specific to the rape which must be integrated into the victim’s psychological experience to enable her to function at a precrisis level. Burgess then elaborated on how these symptoms could affect the victim’s ability to identify the rapist:
Q And, what about the phobia about repetition of seeing the face? Could you describe that and explain that to us? A The — right. She had an opportunity to see the assailant’s face, and, so, that imprinted, if you will, in her mind, and that is a flashback. That keeps coming back.
In fact, right after the assault, she described how this would happen, where suddenly thinking she saw someone, again a common reaction that she also had, feeling that he is everywhere, because the assault was still so new, still so fresh.
Q How is that integrated, that phobia?
A ....
It’s a gradual process.
Q Is a five-year time period a telling time period for this gradual integration process?
A Our study of our victims showed four to six years later we had still twenty-five percent that were still very symptomatic. We had others, of course, who had recovered, but five years you still have a very — in certain areas you can have specific symptoms in the phobic area, be[296]*296cause that’s more or less the definition of a phobia. It wards off into a particular area of symptoms.
Q And, would that account for her flood of emotions still to this day about the material?
A Yes. What seems to happen in the research we have been looking at is traumatic events are what is called actively stored in the mind, and when a certain, if you will, button is pressed: i.e., seeing someone, or whatever, all of that emotion and everything can just come flooding back, and that’s the phenomenon of a flashback out of the past.
An event comes back, because there has been some triggering in the environment that the person is in, and it just kind of brings it all back.
Q By bringing it all back, does it also bring the phase back of the original assailant? Is that the kind of thing that would flash right back before your eyes?
A Oh, yes.
MR. STANSHINE [Defense counsel]: Objection.
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OPINION OF THE COURT
FLAHERTY, Justice.
This is an appeal from the order of Superior Court, affirming appellant’s judgment of sentence for involuntary deviate sexual intercourse following a jury trial. The issue is whether the trial court properly permitted expert testimony regarding the victim’s afflication with “rape trauma syndrome” (RTS). We hold that such testimony is inadmissible, and therefore vacate the judgment of sentence and remand for a new trial.
Uncontroverted evidence establishes that on November 26, 1977, an intruder gained entry to the home of the victim by posing as a police officer, then raped her and forced her to submit to other sexual outrages. The victim described the assailant to the police and reported that he identified himself as Gallagher. Two weeks later, on December 10, 1977, the victim examined a photographic display containing the appellant’s picture and also faced him in a voluntary [293]*293one-on-one confrontation; in neither instance did she identify him.
More than four years later, however, the victim identified the appellant from a photographic display. The appellant was then arrested on February 19, 1982, and charged with rape and numerous related offenses. Prior to his arrest, however, the statute of limitations had expired on all the offenses except involuntary deviate sexual intercourse. Accordingly, he was tried only for the latter offense.
At his jury trial, the appellant contested his identification as the rapist. In order to downplay the victim’s repeated failures to identify appellant within weeks of the crimes and bolster her identification after four years, the Commonwealth presented Ann Burgess as an expert witness with respect to RTS. Burgess holds a master’s degree in psychiatric nursing and a doctorate in nursing science. Together with a colleague, Professor Lynda Holmstrom of Boston College, Burgess co-authored an article in the American Journal of Psychiatry which is credited with being the first study of victims of sexual attack to use the term “rape trauma syndrome.”1 Burgess’s qualifications included extensive teaching, research, writing, and forensic experience with the psychological effects found in rape victims. She described the symptomology of the syndrome, now accepted as a standard post-traumatic stress response disorder by the American Psychiatric Association. She then summarized her examination of the victim, stated her diagnosis that the victim suffered from RTS, and related her opinion of how the phenomena of RTS bore upon the identification process.
The appellant was convicted by jury, his post-verdict motions were denied, and he was sentenced to serve ten to twenty years imprisonment. On appeal, a divided panel of the Superior Court affirmed the judgment of sentence. The primary issue is whether Burgess’s testimony regarding RTS was admissible. We hold that the testimony was an [294]*294impermissible encroachment on the jury’s function of determining credibility, and reverse the judgment of sentence.
At the appellant’s trial, there was no question that the victim had been sexually assaulted; the principal question for the jury was the identity of the attacker. The victim’s positive, indeed animated,2 identification of the appellant at trial was seriously undermined by the fact that she had been unable to identify him two weeks after the crime. The testimony of Burgess regarding RTS was introduced for the sole purpose of shoring up the credibility of the victim on the crucial issue of identification.
Determinations of credibility, however, are exclusively the province of the jury. Commonwealth v. Seese, 512 Pa. 439, 443, 517 A.2d 920, 922 (1986). We have consistently rejected expert testimony which encroaches on this vital jury question. Kozak v. Struth, 515 Pa. 554, 531 A.2d 420 (1987) (prohibiting expert testimony on causation and due care because issues of ultimate fact, especially those of [295]*295credibility, are for jury, not expert); Commonwealth v. Seese, supra (prohibiting expert medical testimony regarding the veracity of children who claim to be objects of sexual abuse); Commonwealth v. O’Searo, 466 Pa. 224, 352 A.2d 30 (1976) (prohibiting expert testimony of clinical psychologist to substantiate and corroborate the defendant’s version of the critical events). We must examine the testimony of Burgess in light of this standard.
Burgess first described RTS as occurring in two phases. The first is the acute phase, lasting days or weeks, during which the victim is emotionally overwhelmed and has difficulty performing ordinary functions; this is followed by a long term reorganization phase, lasting months or years, during which the victim deals with symptoms specific to the rape which must be integrated into the victim’s psychological experience to enable her to function at a precrisis level. Burgess then elaborated on how these symptoms could affect the victim’s ability to identify the rapist:
Q And, what about the phobia about repetition of seeing the face? Could you describe that and explain that to us? A The — right. She had an opportunity to see the assailant’s face, and, so, that imprinted, if you will, in her mind, and that is a flashback. That keeps coming back.
In fact, right after the assault, she described how this would happen, where suddenly thinking she saw someone, again a common reaction that she also had, feeling that he is everywhere, because the assault was still so new, still so fresh.
Q How is that integrated, that phobia?
A ....
It’s a gradual process.
Q Is a five-year time period a telling time period for this gradual integration process?
A Our study of our victims showed four to six years later we had still twenty-five percent that were still very symptomatic. We had others, of course, who had recovered, but five years you still have a very — in certain areas you can have specific symptoms in the phobic area, be[296]*296cause that’s more or less the definition of a phobia. It wards off into a particular area of symptoms.
Q And, would that account for her flood of emotions still to this day about the material?
A Yes. What seems to happen in the research we have been looking at is traumatic events are what is called actively stored in the mind, and when a certain, if you will, button is pressed: i.e., seeing someone, or whatever, all of that emotion and everything can just come flooding back, and that’s the phenomenon of a flashback out of the past.
An event comes back, because there has been some triggering in the environment that the person is in, and it just kind of brings it all back.
Q By bringing it all back, does it also bring the phase back of the original assailant? Is that the kind of thing that would flash right back before your eyes?
A Oh, yes.
MR. STANSHINE [Defense counsel]: Objection.
THE COURT: Overruled.
BY MS. FLEISHMAN [Prosecutor]:
Q And, could you describe in terms of a heart attack, so that we can understand what that flashback might feel like, could you draw an analogy to a heart attack?
A Yes. We are doing a study now of men who have had heart attacks, and a heart attack is another confrontation with death, and many men feel they are going to die when they have a heart attack.
He had had symptoms not only of pain, but of nausea, so you have two linking, if you will, phenomena that in his mind — if you want to think of the mind like a computer button — was pressed, and the image came back.
Q Did you find that symptom to be true of that flood or flashback symptom in [the victim]?
A Yes, I did.
[297]*297The crux of the testimony appears to be that the victim’s failure to identify the appellant two weeks after the rape is unremarkable, as she was in the acute phase of RTS in which a victim has difficulty performing even normal functions, and the in-court identification five years later is particularly credible, as it results from a flashback, with the mind operating like a computer. It is clear that the only purpose of the expert testimony was to enhance the credibility of the victim.
We stated in Seese, supra:
The question of whether a particular witness is testifying in a truthful manner is one that must be answered in reliance upon inferences drawn from the ordinary experiences of life and common knowledge as to the natural tendencies of human nature, as well as upon observations of the demeanor and character of the witness____ [T]he question of a witness’ credibility has routinely been reserved exclusively for the jury.
512 Pa. at 443, 517 A.2d at 922 (citations omitted; emphasis added). Such testimony would invest the opinions of experts with an unwarranted appearance of authority on the subject of credibility, which is within the facility of the ordinary juror to assess.3
We therefore hold that expert testimony on rape trauma syndrome should not have been admitted in the trial of this case. Accordingly, we reverse the order of Superior Court and remand for a new trial.
Order reversed and case remanded.
LARSEN, J., files a dissenting opinion.
PAPADAKOS, J., files a dissenting opinion which is joined by NIX, C.J.