OPINION OF THE COURT
NEWMAN, Justice.Appellant Betty Legg appeals from a Superior Court Order, reversing an Order of the Court of Common Pleas of Allegheny County (PCRA Court), in which the PCRA Court granted Appellant’s petition for collateral relief pursuant to the Post-Conviction Relief Act (PCRA), 42 Pa.C.S. § 9541 et seq. The issue before us is whether Appellant’s trial counsel was ineffective for failing to investigate and present the diminished capacity defense at Appellant’s murder trial. We conclude that trial counsel was ineffective where available expert testimony indicated that Appellant was suffering from major depression and anxiety that would have prevented her from rationally forming the specific intent to kill. Therefore, we reverse the Order of the Superior Court and reinstate the PCRA Court’s Order, granting Appellant a new trial.
FACTUAL AND PROCEDURAL HISTORY
The record establishes that Appellant and the victim, Ernest Legg (Mr. Legg), were married in 1956. In 1960, Appellant learned that her husband was having an affair with another woman, Amy Stephenson. Nonetheless, the Leggs reconciled and stayed together until 1980 when Mr. Legg suddenly left. The Leggs obtained a divorce in 1985. They, however, continued to have sexual relations. In early 1986, Appellant learned that Mr. Legg had rekindled his relationship with Stephenson in 1985.
Appellant repeatedly called Stephenson demanding that she leave Mr. Legg alone. Sometimes she would call and then hang up the telephone. She also followed Mr. Legg and *441Stephenson by car. Due to her mounting anger toward Stephenson and her fear that she might harm Mr. Legg, Appellant voluntarily sought psychiatric assistance on June 11, 1986. She went to the Northern Communities Mental Health/Mental Retardation Center (Northern Communities MHMR), which recommended immediate in-patient hospitalization. That same day, Appellant was admitted to St. John’s Hospital psychiatric ward because she was actively suicidal and was talking about killing her husband. St. John’s diagnosed her with recurrent major depression and treated her with tranquilizers. After two days, Appellant left St. John’s, against medical advice and discontinued her medications. Appellant then saw a clinical psychologist, Lois Dabney-Smith, Ph.D., several days later for one session but did not return for further treatment.
On her birthday, July 27,1986, Mr. Legg took Appellant out for dinner. They later engaged in sexual relations, which led Appellant to believe that they were going to be able to resolve their differences. Then, on August 6,1986, due to Appellant’s repeated telephone calls, Stephenson filed charges of harassment against Appellant. The criminal complaint stunned Appellant. Shortly before 4:00 p.m., on August 8, 1986, she drove across town to meet Mr. Legg at his place of employment. She had a gun that she always kept under the front seat of her car for protection.1 As Mr. Legg exited his place of employment, she approached him, carrying the gun in a bag. Mr. Legg allowed her to join him in the front seat of his car. Appellant begged him to tell her the truth about Stephenson, but Mr. Legg denied any romantic involvement with her. The couple began arguing. She pulled the gun from the bag.
According to Appellant, she had only intended to scare Mr. Legg into telling her the truth. She claims that when Mr. Legg saw the weapon, he grabbed her wrist and the gun went *442off; as she tried to remove her finger from the trigger, it fired a second time. Mr. Legg exited the car and collapsed. He died of gun shot wounds in the arm and back. After the shooting, Appellant claims that she was in “shock.” She drove to the Public Safety Building, where she worked as a dispatcher, to report the incident. She informed the desk sergeant, “I shot my husband” and handed him the gun.
She was then arrested and charged with criminal homicide, 18 Pa.C.S. § 2501, and a violation of the Uniform Firearms Act, 18 Pa.C.S. § 6101. On March 24, 1987, Appellant waived her right to a jury trial and proceeded to a bench trial before the Honorable Alan S. Penkower in the Court of Common Pleas of Allegheny County (trial court). An attorney of the Office of the Allegheny County Public Defender represented Appellant. At trial, defense counsel primarily relied on the defense of accident or homicide by misadventure.2 Alternatively, counsel asserted a heat of passion defense.3 Trial counsel did not raise a diminished capacity defense.4 The trial *443court convicted Appellant of first degree murder and sentenced her to life imprisonment. The Superior Court affirmed the judgment of sentence and we denied her Petition for Allowance of Appeal.
On February 27, 1991, Appellant filed a PCRA petition requesting a new trial, asserting, inter alia, that trial counsel was ineffective for failing to raise the diminished capacity defense. Judge Penkower also presided at the PCRA hearing. At the hearing, Appellant, her trial counsel, and psychologist Dr. Herbert I. Levit, testified concerning her diminished capacity, after which the PCRA Court granted her a new trial. The Superior Court reversed the PCRA Court, holding that trial counsel reasonably relied on the defense of accident or homicide by misadventure, which could have produced an acquittal, and avoided the defense of diminished capacity, that would have admitted guilt and resulted in a sentence for criminal homicide. We granted allocatur on the issue of whether the Superior Court erred in reversing the PCRA Court’s determination that trial counsel was ineffective for failing to pursue a diminished capacity defense.
DISCUSSION
To establish an ineffective assistance of counsel claim pursuant to the PCRA, 42 Pa.C.S. § 9543, an appellant must prove that (1) the underlying claim is of arguable merit; (2) counsel’s action or inaction was not grounded on any reasonable basis designed to effectuate his client’s interest; and (3) counsel’s omission or commission so undermined the trial that the verdict is unreliable. Commonwealth v. Szuchon, 534 Pa. 483, 633 A.2d 1098 (1993).5
Here, Appellant argues that, as found by the PCRA Court, the diminished capacity defense would have been a critical issue at trial and defense counsel’s failure to explore, investigate, and present available psychiatric evidence constó*444tuted ineffective assistance of counsel. Initially, we note that “[i]n asserting a diminished capacity defense, a defendant is attempting to prove that he was incapable of forming the specific intent to kill; if the defendant is successful, first degree murder is mitigated to third degree.” Commonwealth v. Travaglia, 541 Pa. 108, 124, 661 A.2d 352, 359 (1995), cert. denied, 516 U.S. 1121, 116 S.Ct. 931, 133 L.Ed.2d 858 (1996). Thus, the defendant admits general criminal culpability, but seeks to reduce the degree of guilt to third degree murder. Commonwealth v. Zettlemoyer, 500 Pa. 16, 454 A.2d 937 (1982), cert. denied, 461 U.S. 970, 103 S.Ct. 2444, 77 L.Ed.2d 1327, rehearing denied, 463 U.S. 1236, 104 S.Ct. 31, 77 L;Ed.2d 1452 (1983). Diminished capacity, however, is an extremely limited defense. Travaglia. Psychiatric testimony that addresses “mental disorders affecting the cognitive functions [of deliberation and premeditation] necessary to formulate a specific intent” is admissible. Zettlemoyer, 500 Pa. at 28, 454 A.2d at 943. However, psychiatric evidence that a defendant lacked the ability to control his actions or that he acted impulsively is irrelevant and inadmissible on the issue of the defendant’s specific intent to kill. Id.
At the PCRA hearing, Appellant’s expert, Dr. Levit, testified that she was actively homicidal and suicidal at the time of her hospitalization two months before the shooting. He stated that she sought help because she was reaching a point where she could no longer control her behavior. R. 340a. According to Dr. Levit, Appellant suffered from major depression with anxiety at the time of the shooting. Such a disorder, he explained, would have distorted her perception of the situation with her ex-husband and impaired her judgment. R. 334a-35a. Dr. Levit opined that the shooting resulted from her depression and anxiety associated with her feelings of anger and betrayal. R. 344a. He clarified that due to her depression, anxiety, and the intensity of her anger and feelings of betrayal, she lacked the ability to rationally formulate the intent to kill Mr. Legg. R. 349a, 367a-68a. Dr. Levit’s testimony spoke to more than an inability of Appellant to control herself, and directly related Appellant’s underlying *445mental defect to her inability to formulate a specific intent to kill. Based on this testimony, the PCRA Court held that Appellant’s claim has arguable merit. The Commonwealth concedes that she satisfied the first prong of an ineffectiveness claim.
The second prong of Appellant’s ineffectiveness claim, whether trial counsel had a reasonable basis for not presenting a diminished capacity defense, depends on whether trial counsel knew or, through reasonable investigation, should have known of Appellant’s psychiatric history. See Commonwealth v. Walker, 540 Pa. 80, 656 A.2d 90 (1995), cert. denied, 516 U.S. 854, 116 S.Ct. 156, 133 L.Ed.2d 100 (1995)(counsel not ineffective for failing to call witnesses he had no way of knowing were available); Commonwealth v. Peterkin, 511 Pa. 299, 513 A.2d 373 (1986), cert. denied, 479 U.S. 1070, 107 S.Ct. 962, 93 L.Ed.2d 1010 (1987)(counsel has duty to make reasonable investigations). Trial counsel testified that, although he knew that Appellant was experiencing anxiety and depression because of the impending trial, he did not know that she had been diagnosed with major depression and hospitalized before she shot Mr. Legg. R. 387a-89a. However, the record shows that the Public Defender Investigator, who interviewed Appellant after her arrest, noted in his report that Appellant had been to St. John’s Hospital and had seen “Dr. Dabney.” Trial counsel, a Public Defender, inexplicably testified that he never reviewed the investigator’s report. R. 420a. Trial counsel also could not recall whether he interviewed the psychologists or counselors from the Allegheny County Behavior Clinic and Northern Communities MHMR who examined Appellant post-arrest to determine her competency to stand trial. R. 407a. Counsel acknowledged at the PCRA hearing, however, that the Behavior Clinic’s report referenced a previous mental disorder and hospitalization. R. 388a. Counsel, thus, had available to him sufficient indicia of Appellant’s pre-arrest mental disorder to warrant further investigation. A reasonable investigation would have revealed Appellant’s diagnosis and treatment history.
*446With respect to counsel’s strategy, the Superior Court held that counsel had rationally chosen to pursue the defense of an accident, which could have resulted in an acquittal or a finding of guilt less than first degree murder, and to avoid the defense of diminished capacity.6 Because counsel did not investigate or could not recall investigating Appellant’s mental history, we cannot agree with the Superior Court that he made a rational decision to avoid a diminished capacity defense. In fact, counsel’s testimony indicates otherwise. When questioned whether he would have proceeded differently if he had known that Appellant suffered from major depression and had homicidal and suicidal ideations, he replied, “Absolutely. First [sic] comes to mind is the defense of diminished capacity, if that can be developed.” R. 389a-90a.
*447Further, the Superior Court determined that counsel had reasonably relied on the defense of accident instead of diminished capacity, which would have admitted guilt and required a sentence for criminal homicide. We disagree with the Superior Court’s assessment of counsel’s strategy. Although counsel may have hoped for an acquittal, proof of an accidental killing coupled with evidence of malice, implied from Appellant’s use of the gun, may have produced a verdict of third degree murder. See Commonwealth v. Hobson, 484 Pa. 250, 398 A.2d 1364 (1979). Therefore, the evidence presented actually offered the trial court five possible verdicts concerning the homicide charge: first degree murder; third degree murder; voluntary manslaughter; involuntary manslaughter; and acquittal. Particularly when the facts supported a finding of first degree murder, we find no reasonable justification for counsel’s failure to also advance a diminished capacity defense that may have produced a verdict of third degree murder. Therefore, we conclude that the record supports the trial court’s finding that counsel had no reasonable basis for failing to investigate and present a diminished capacity defense.
The cases relied upon by the Commonwealth, Commonwealth v. Cross, 535 Pa. 38, 634 A.2d 173 (1993), cert. denied, 513 U.S. 833, 115 S.Ct. 109, 130 L.Ed.2d 56 (1994), and Commonwealth v. Mizell, 493 Pa. 161, 425 A.2d 424 (1981), are distinguishable. In each of those cases, this Court held that trial counsel was not ineffective for failing to advance a diminished capacity and/or insanity defense in the defendant’s murder trial. We reasoned that an insanity or diminished capacity defense, which would have admitted the commission of the act, would have directly conflicted with each defendant’s claim of innocence. Each defendant in Cross and Mizell maintained that he had not committed the homicide(s) in question. In contrast, and contrary to the Commonwealth’s assertion, Appellant does not deny shooting her husband. Thus, the sole question and dispositive issue at trial, here, was Appellant’s mental state at the time of the shooting. The Commonwealth sought to prove, inter alia, that Appellant committed an intentional killing with malice. Appellant as*448serted that she did not intend to kill her husband — that the shooting was an accident — or, alternatively, that she shot her husband, but acted in the heat of passion. A diminished capacity defense, which suggests general culpability for the shooting but negates the specific intent to Ml, would not have directly conflicted with Appellant’s position.
Consequently, we turn to the last prong of Appellant’s ineffectiveness claim, whether she was prejudiced as a result of trial counsel’s omissions. The PCRA court held:
Trial counsel’s failure to present expert testimony at trial concerning defendant’s ability to formulate the specific intent to kill such as would tend to reduce the degree of homicide from first degree to third degree murder, deprived the trier of fact of mitigating evidence regarding defendant’s state of mind at the time of the shooting. The Court concludes that trial counsel’s ineffectiveness so undermined the truth-determining process that no reliable adjudication of guilt or innocence of the crime of first degree murder could have taken place under the circumstances.
PCRA Court Findings and Order of Court of March 31, 1994. Although the Superior Court did not decide the prejudice issue, in analyzing the merits of Appellant’s claim, the court appears to have reweighed Dr. Levit’s testimony and concluded that it was speculative. The PCRA court, however, was in the better position to evaluate the doctor’s testimony, and the effect of the omission of that evidence on the deliberative process. Commonwealth v. Lutz, 492 Pa. 500, 424 A.2d 1302 (1981)(appellate court is not to engage in de novo evaluation of testimony; that is task for PCRA court). Although evidence of a specific intent to kill may disprove a defense of diminished capacity, Commonwealth v. Tempest, 496 Pa. 436, 437 A.2d 952 (1981) (superseded by statute on other grounds), that evidence here was not so overwhelming that we can conclude that counsel’s failure to raise a diminished capacity defense resulted in no prejudice to Appellant. Instead, because defense counsel deprived the trial court of its right to weigh the evidence and assess the credibility of the testimony relating to Appellant’s diminished capacity defense, we defer to the *449PCRA court’s conclusion that no reliable adjudication of guilt or innocence could have taken place.
Accordingly, we reverse the Order of the Superior Court and reinstate the PCRA Court’s decision granting Appellant a new trial.
CASTILLE, J., files a dissenting opinion in which NIGRO and SAYLOR, JJ., join.. According to Appellant, she received the gun from her grandmother’s estate several years earlier. Appellant had no formal weapons training and had never test fired the gun, but knew it was loaded. She did not have a license to cariy it.
. Homicide by misadventure, which is excusable, is defined as:
the accidental killing of another, where the slayer is doing a lawful act, unaccompanied by any criminally careless or reckless conduct. "Three elements enter into the defense of excusable homicide by misadventure: [1] The act resulting in death must be a lawful one; [2] It must be done with reasonable care and due regard for the lives and persons of others; and [3] the killing must be accidental and not intentional, or without unlawful intent, or without evil design or intention on the part of the slayer.... Even though the homicide is unintentional, it is not excusable where it is the result or incident of an unlawful act, such as pointing or presenting a gun, pistol or other firearm at another person in such a manner as to constitute an offense under the laws of the state....”
Commonwealth v. Hobson, 484 Pa. 250, 258-59, 398 A.2d 1364, 1368 (1979) (citations omitted).
. The heat of passion defense, a partial defense to murder, is defined as: a homicide intentionally committed under the influence of passion. ... The term "passion” as here used includes both anger and terror provided they reach a degree of intensity sufficient to obscure temporarily the reason of the person affected.
Commonwealth v. McCusker, 448 Pa. 382, 388 n. 4, 292 A.2d 286, 289 n. 4 (1972). A killing committed in the heat of passion constitutes voluntary manslaughter. 18 Pa.C.S. § 2503; McCusker.
. As discussed below, a diminished capacity defense alleges an inability to form the specific intent to kill. Commonwealth v. Travaglia, 541 Pa. *443108, 124, 661 A.2d 352, 359 (1995), cert. denied, Travaglia v. Pennsylvania, 516 U.S. 1121, 116 S.Ct. 931, 133 L.Ed.2d 858 (1996).
. Appellant’s PCRA hearing pre-dated the 1995 amendments to Section 9543 of the PCRA.
. Underlying Appellant's ineffectiveness claim is her contention that trial counsel had abandoned the accident defense during trial; therefore, he was ineffective in failing to present a diminished capacity defense. The Superior Court referred to the following excerpt of defense counsel’s closing argument in holding that counsel had not abandoned the accident defense:
I would argue, Your Honor, certainly the first shot is an accidental shooting when he reaches for the gun. I can't argue, Your Honor, accidental shooting or homicide by misadventure, because I understand the law is when you are involved in an unlawful act, i.e. possession of a firearm in close proximity of someone, with the purse, et cetera, and she has testified it was her intention to pull it out to scare him, that that unlawful act would bar Your Honor from rendering a verdict of not guilty of criminal homicide by virtue of that defense.
That does not preclude Your Honor from finding that verdict out of your mercy dispensing powers, so I’m not arguing that on the basis of the facts alone. At most, you have what I suggest is an unintentional killing based upon exactly the facts you heard here, and that woman’s testimony, Judge.
R. 303a. The focus on whether trial counsel abandoned or actively pursued the accident defense is misplaced. Although counsel stated his belief that Appellant’s unlawful possession of a firearm would preclude the court from rendering a verdict of not guilty by virtue of an accident or homicide by misadventure, R. 303a, he nonetheless called upon the court to exercise its mercy dispensing powers to find Appellant not guilty or guilty of involuntary manslaughter by virtue of an accidental shooting or homicide by misadventure, R. 303a-305a. Alternatively, counsel suggested that, at most, Appellant may have committed voluntary manslaughter as a result of the heat of passion. R. 304a. As discussed below, nothing precluded trial counsel from presenting the alternative argument of diminished capacity.