The defendant, Kenneth Joseph Hudson, appeals from his convictions of four counts of a seven-count indictment charging rape of a child under the age of sixteen *537years.1 We granted the defendant’s application for direct appellate review.
We summarize the evidence before the jury. At the time of trial the victim was sixteen years old and in the eleventh grade. The victim’s mother married the defendant when the victim was approximately five years old, and from the age of five until nine, she was repeatedly raped by the defendant. She claimed that over twenty incidents of sexual abuse occurred during that four and one-half year period. On occasion the defendant threatened to kill the victim and her mother if the victim told of his actions.
The victim did not tell her mother about any of the incidents until just before the defendant and the victim’s mother separated. During the time of the incidents, the victim went to the hospital “a lot” because of stomach pain. In addition, she suffered from recurring nightmares where she imagined that the defendant would kill her mother then force the victim to have sexual intercourse with him and kill her. The stomach pain and nightmares persisted until 1989.
On cross-examination, the defendant attacked the victim’s credibility. The tenor of the questioning suggested that the victim’s alleged abdominal pain was not evident until she began menstruating and intimated that the victim’s abdominal pain was caused by irregularities in her menstrual cycle or other medical reasons. The defendant suggested further that the victim’s difficulties with her mother’s current boy friend caused her to run away from home.
The mother stated that the authorities at the schools the victim attended constantly informed her that the victim complained of stomachaches. According to her mother, doctors believed that the victim’s stomachaches were “in her head.” She said that, during the years that these incidents occurred, she noticed changes in the victim’s behavior. At some point the victim ran away from home for five or six hours. The *538victim left a note stating that “she was tired of her nightmares and she was leaving.”
Dr. Mark N. Webber, the victim’s physician, stated that the victim’s internal examination was similar to that of a sexually active adolescent. He observed no evidence of trauma, which would be as expected if the sexual activity occurred five or six years earlier. During a May, 1989, visit, Dr. Webber noted that the victim looked “very sad,” was quiet, and would not make significant eye contact. Because it was the victim’s fourth visit and he was unable to find any physical problem with the victim, Dr. Webber suggested she consult with a psychiatrist.
Dr. Karen Schwartz, a psychologist who qualified as an expert on sexually abused children, described posttraumatic stress disorder (PTSD). Dr. Schwartz stated that PTSD, as described in the Diagnostic and Statistical Manual of Mental Disorders (3d ed. rev. 1987), is a disorder primarily developed to describe situations where veterans would experience symptoms years after a traumatic episode. According to Dr. Schwartz the diagnosis of PTSD rests on four symptom categories. At the outset, there must be a traumatic event. Dr. Schwartz testified that sexual abuse could be a traumatic event. She stated that the degree of trauma has some correlation to how closely related the offending person is to the victim. Dr. Schwartz then described the other types of PTSD symptoms. First, there is a reexperiencing phenomenon where individuals may have memories that intrude into their daily lives. Reexperiencing phenomena could take the form of nightmares. The second category is a form of psychic numbing or avoidance of stimuli associated with the trauma. An example of psychic numbing includes a sexually abused adolescent who, years later, reveals the abuse but shows no emotion. The third category involves hypervigilance or hyper-arousal. In children and adolescents this could take the form of headaches or stomachaches. Dr. Schwartz noted that symptoms of the disorder are dependent on the developmental level of the person traumatized. At .different ages, one could expect to see a different constellation of symptoms. In *539response to a hypothetical question, Dr. Schwartz explained that abdominal pain is “not syndrome specific,” because “it does [not] point to one specific syndrome more than another.” She testified that more information on a patient’s history and level of functioning would be needed. Dr. Schwartz did note that it would be a “red flag” of a number of different things including trauma. In taking a patient’s history, Dr. Schwartz said that she would ask the patient whether she had been sexually abused. She further stated that the symptoms manifest themselves in different ways within the preschool population, school-age population, and the adolescent population. According to Dr. Schwartz people with PTSD sometimes will not exhibit symptoms until months or years after they experience the trauma. Dr. Schwartz noted that it is fairly typical that children delay reporting that they were sexually abused. On cross-examination, Dr. Schwartz admitted that, if a patient reports sexual abuse and exhibits various symptoms, her goal is to treat the patient and not to determine whether the patient was telling the truth. In her view, whether the victim was truthful was an appropriate inquiry to the legal arena but not for her purposes. She further testified that all symptoms are subjective. Dr. Schwartz also admitted that, because an individual exhibits certain symptoms, such as stomachaches, that does not automatically prove that she was the victim of sexual abuse. She testified that the symptoms could be caused by a variety of things and that, often times, the symptoms occur as part of a child’s normal development. Dr. Schwartz did not connect her testimony to the victim or the defendant.2
The defendant argues that the judge erred in admitting expert testimony concerning the typical characteristics of sexually abused children. On appeal, the defendant raises three issues, concerning the expert testimony that: (1) the syndrome evidence describing the typical sexually abused child should not be admissible as affirmative evidence of the de*540fendant’s guilt; (2) the expert testimony was improper because it constituted opinion evidence which bolstered the victim’s credibility; and (3) the admission of the syndrome evidence was prejudicial. We conclude that the judge did not abuse his discretion by admitting the expert testimony and affirm the convictions.
Prior to trial, a voir dire was conducted to determine whether an expert could testify on PTSD in a child rape case. Dr. Schwartz and Dr. Douglas Gallaway testified. At the conclusion of the voir dire, the judge noted that, although he had not made a final decision, he was leaning toward admitting the expert testimony. Another judge was assigned to the trial. The defendant filed a motion in limine concerning the expert testimony. Before ruling on the motion, the trial judge informed the parties he wished a voir dire of the expert. He instructed the prosecutor not to discuss the expert during his opening statement. After a voir dire of Dr. Schwartz on the third day of trial, she was permitted to testify as an expert.
1. The admission of expert testimony lies “largely in the discretion of the trial judge.” Commonwealth v. Maltais, 387 Mass. 79, 93 (1982). We have held that expert testimony “is admissible if, in the judge’s discretion, the subject is not within the common knowledge or common experience of the jury.” Commonwealth v. Francis, 390 Mass. 89, 98 (1983).
This court has recently opined that courts have uniformly allowed expert testimony on the typical symptoms of sexually abused children because the information is beyond the common knowledge of jurors and of assistance in assessing a victim witness’s testimony and credibility. Commonwealth v. Dockham, 405 Mass. 618, 629 (1989). In Dockham, the expert explained that sexually abused children “often delay disclosure of sexual abuse or make gradual disclosures, retract their statements, and repress the abuse.” Id. at 628. The expert described behavioral signs and symptoms, such as, “sexualized play, knowledge of adult sexual functions, fears and anxieties related to body parts, people, and places involved in *541the sexual abuse.” In addition, the expert stated that “sexually abused children exhibit impaired trust, withdrawal, depression, guilt, shame, anxiety, and hypervigilance.” Id. In Commonwealth v. Mamay, 407 Mass. 412, 421 (1990), we held that it was within the trial judge’s discretion to admit expert testimony concerning rape trauma syndrome to explain the conduct of victims who returned to the office of the defendant physician after having been sexually assaulted by him.
There is a similar solidarity of judicial precedent in excluding the evidence to prove that the victim was in fact sexually abused. State v. J.Q., 130 N.J. 554, 563-564 (1993), and cases cited.3 “Courts rarely permit the testimony for the purpose of establishing substantive evidence of abuse, but allow it to rehabilitate the victim’s testimony.” Id. at 566. The critical factor, therefore, in determining admissibility of such evidence is its relevance to the issues in the case.
We do not accept the defendant’s contention that Dr. Schwartz’s testimony was used affirmatively to prove guilt. Dr. Schwartz did not directly refer to the victim or her symptoms. She testified generally about PTSD. The testimony was admissible to rebut the defendant’s suggestions made during cross-examination that the victim was a troubled teenager, whose medical problems were related to menstruation, and whose unhappy home life could lead to false claims of rape. Dr. Schwartz did not state that the victim had been sexually abused nor did she ever posit that the de*542fendant was the abuser. Furthermore, on cross-examination, Dr. Schwartz admitted that merely because an individual exhibits certain symptoms of the disorder does not automatically prove that she was the victim of sexual abuse.
This case presents us with a fact pattern similar to Commonwealth v. Dockham, supra. There the expert testified to the typical symptoms of one sexually abused to explain one victim’s behavior as well as his delayed disclosure of abuse. Similar to Dockham, the expert testimony here is permissible to explain the victim’s postassault behavior and symptoms including her stomachaches and nightmares. Because there was no physical evidence of trauma, this case hinged on the weight the jury assigned to the testimony of the victim and the defendant. Although the victim’s symptoms are not as striking as those of the victim in Dockham, the knowledge that they are frequently found among those suffering from PTSD was not information within the common understanding of jurors. As in Dockham, this information is relevant to assist the jury in assessing the victim’s testimony and credibility. Although, as the dissent points out, the judge did not charge the jury that the expert testimony was not affirmative evidence of sexual abuse, no request was made for a limiting instruction and no objection was made to the charge on this ground.4
However, the judge did instruct the jury:
“You the jurors are not bound by the opinion of an expert. The experts cannot decide the case for you the jury. They cannot [usurp]; take away your fact finding function. They can, however, by their opinion assist you to find facts upon which you will decide the case. You must weigh the testimony of the expert as you would that of any other witness. You may reject it, you may reject the opinion if in your judgment the reasons given for it are unsound. You may accept it in whole. Or you *543may accept it in part and reject it in part. Depending upon the reasonableness or unreasonableness of the testimony of that individual.”
We conclude that the judge did not abuse his discretion by admitting Dr. Schwartz’s testimony because that information was “beyond the common knowledge of jurors and of assistance in assessing a victim witness’s testimony and credibility.” Commonwealth v. Dockham, supra at 629.
2. “It is a fundamental principle that ‘a witness cannot be asked to assess the credibility of his testimony or that of other witnesses.’ ” Commonwealth v. Triplett, 398 Mass. 561, 567 (1986), quoting Commonwealth v. Dickinson, 394 Mass. 702, 706 (1985). In the defendant’s view, Dr. Schwartz’s testimony, especially relating to stomachaches, served to endorse the credibility of the victim. See Commonwealth v. Montanino, 409 Mass. 500, 504 (1991). Dr. Schwartz never explicitly or impliedly rendered an opinion as to the truthfulness of the victim or to the general veracity of sexually abused children. In fact, she testified that whether a child was truthful was not for her determination. We conclude, therefore, that Dr. Schwartz’s testimony did not improperly bolster the victim’s credibility.
3. Because we hold that the expert testimony was properly admissible to explain aspects of the victim’s behavior, we reject the defendant’s final contention that its admission unfairly prejudiced the defendant and denied him a fair trial.
Judgments affirmed.
The defendant was acquitted on three counts charging rape of a child under the age of sixteen.
The fact that Dr. Schwartz treated the victim was mentioned during cross-examination and redirect examination of the victim’s mother.
But see State v. Alberico, 861 P.2d 192, 210 (N.M. 1993) (expert testimony on PTSD is probative of sexual abuse; reasoning that, because PTSD is valid scientific technique for identifying certain symptoms of abuse, “[allowing an expert to testify that PTSD symptoms are a common reaction to sexual assault for the purpose of rebutting the defense that the victim’s reactions to the alleged incident are inconsistent with sexual assault is no different from allowing the expert to testify that the alleged victim’s symptoms are consistent with sexual abuse”). Contrast Commonwealth v. Dunkle, 529 Pa. 168, 173 (1992) (expert testimony concerning typical behavior of sexually abused children not admissible because such testimony is neither generally accepted as diagnostic tool nor relevant evidence).
The dissent also refers to the reference of the expert testimony by the prosecutor in his closing argument. The defendant did not object at trial to the argument on this basis.