Opinion
CHIN, J.
The Legislature has decreed that, when relevant, expert testimony regarding “battered women’s syndrome” is generally admissible in a criminal action. (Evid. Code, § 1107.) We must determine the purposes for which a jury may consider this evidence when offered to support a claim of self-defense to a murder charge.
The trial court instructed that the jury could consider the evidence in deciding whether the defendant actually believed it was necessary to kill in self-defense, but not in deciding whether that belief was reasonable. The instruction was erroneous. Because evidence of battered women’s syndrome may help the jury understand the circumstances in which the defendant found herself at the time of the killing, it is relevant to the reasonableness of her belief. Moreover, because defendant testified, the evidence was relevant to her credibility. The trial court should have allowed the jury to consider [1077]*1077this testimony in deciding the reasonableness as well as the existence of defendant’s belief that killing was necessary.
Finding the error prejudicial, we reverse the judgment of the Court of Appeal.
I. The Facts
A. Prosecution Evidence
During the evening of March 28, 1992, defendant shot and killed Albert Hampton in their Fresno home. Officer Reagan was the first on the scene. A neighbor told Reagan that the couple in the house had been arguing all day. Defendant soon came outside appearing upset and with her hands raised as if surrendering. She told Officer Reagan, “I shot him. That’s right, I shot him. I just couldn’t take him beating on me no more.” She led the officer into the house, showed him a .357 magnum revolver on a table, and said, “There’s the gun.” Hampton was on the kitchen floor, wounded but alive.
A short time later, defendant told Officer Reagan, “He deserved it. I just couldn’t take it anymore. I told him to stop beating on me.” “He was beating on me, so I shot him. I told him I’d shoot him if he ever beat on me again.” A paramedic heard her say that she wanted to teach Hampton “a lesson.” Defendant told another officer at the scene, Officer Terry, “I’m fed up. Yeah, I shot him. I’m just tired of him hitting me. He said, ‘You’re not going to do nothing about it.’ I showed him, didn’t I? I shot him good. He won’t hit anybody else again. Hit me again; I shoot him again. I don’t care if I go to jail. Push come to shove, I guess people gave it to him, and, kept hitting me. I warned him. I warned him not to hit me. He wouldn’t listen.”
Officer Terry took defendant to the police station, where she told the following story. The day before the shooting, Hampton had been drinking. He hit defendant while they were driving home in their truck and continued hitting her when they arrived. He told her, “I’ll kill you,” and shot at her. The bullet went through a bedroom window and struck a tree outside. The day of the shooting, Hampton “got drunk,” swore at her, and started hitting her again. He walked into the kitchen. Defendant saw the gun in the living room and picked it up. Her jaw hurt, and she was in pain. She pointed the gun at Hampton and said, “You’re not going to hit me anymore.” Hampton said, “What are you doing?” Believing that Hampton was about to pick something up to hit her with, she shot him. She then put the gun down and went outside to wait for the police.
Hampton later died of a gunshot wound to his chest. The neighbor who spoke with Officer Reagan testified that shortly before the shooting, she [1078]*1078heard defendant, but not Hampton, shouting. The evening before, the neighbor had heard a gunshot. Defendant’s blood contained no drugs but had a blood-alcohol level of .17 percent. Hampton’s blood contained no drugs or alcohol.
B. Defense Evidence
Defendant claimed she shot Hampton in self-defense. To support the claim, the defense presented first expert testimony and then nonexpert testimony, including that of defendant herself.
1. Expert Testimony
Dr. Lee Bowker testified as an expert on battered women’s syndrome. The syndrome, he testified, “is not just a psychological construction, but it’s a term for a wide variety of controlling mechanisms that the man or it can be a woman, but in general for this syndrome it’s a man, uses against the woman, and for the effect that those control mechanisms have.”
Dr. Bowker had studied about 1,000 battered women and found them often inaccurately portrayed “as cardboard figures, paper-thin punching bags who merely absorb the violence but didn’t do anything about it.” He found that battered women often employ strategies to stop the beatings, including hiding, running away, counterviolence, seeking the help of friends and family, going to a shelter, and contacting police. Nevertheless, many battered women remain in the relationship because of lack of money, social isolation, lack of self-confidence, inadequate police response, and a fear (often justified) of reprisals by the batterer. “The battering man will make the battered woman depend on him and generally succeed at least for a time.” A battered woman often feels responsible for the abusive relationship, and “she just can’t figure out a way to please him better so he’ll stop beating her.” In sum, “It really is the physical control of the woman through economics and through relative social isolation combined with the psychological techniques that make her so dependent.”
Many battered women go from one abusive relationship to another and seek a strong man to protect them from the previous abuser. “[W]ith each successful victimization, the person becomes less able to avoid the next one.” The violence can gradually escalate, as the batterer keeps control using ever more severe actions, including rape, torture, violence against the woman’s loved ones or pets, and death threats. Battered women sense this escalation. In Dr. Bowker’s “experience with battered women who kill in self-defense their abusers, it’s always related to their perceived change of [1079]*1079what’s going on in a relationship. They become very sensitive to what sets off batterers. They watch for this stuff very carefully. [^Q . . . Anybody who is abused over a period of time becomes sensitive to the abuser’s behavior and when she sees a change acceleration begin in that behavior, it tells them something is going to happen . . . .”
Dr. Bowker interviewed defendant for a full day. He believed she suffered not only from battered women’s syndrome, but also from being the child of an alcoholic and an incest victim. He testified that all three of defendant’s partners before Hampton were abusive and significantly older than she.
Dr. Bowker described defendant’s relationship with Hampton. Hampton was a 49-year-old man who weighed almost twice as much as defendant. The two had a battering relationship that Dr. Bowker characterized as a “traditional cycle of violence.” The cycle included phases of tension building, violence, and then forgiveness-seeking in which Hampton would promise not to batter defendant any more and she would believe him. During this period, there would be occasional good times. For example, defendant told Dr. Bowker that Hampton would give her a rose. “That’s one of the things that hooks people in. Intermittent reinforcement is the key.” But after a while, the violence would begin again. The violence would recur because “basically ... the woman doesn’t perfectly obey. That’s the bottom line.” For example, defendant would talk to another man, or fail to clean house “just so.”
The situation worsened over time, especially when Hampton got off parole shortly before his death. He became more physically and emotionally abusive, repeatedly threatened defendant’s life, and even shot at her the night before his death. Hampton often allowed defendant to go out, but she was afraid to flee because she felt he would find her as he had in the past. “He enforced her belief that she can never escape him.” Dr. Bowker testified that unless her injuries were so severe that “something absolutely had to be treated,” he would not expect her to seek medical treatment. “That’s the pattern of her life . . . .”
Dr. Bowker believed defendant’s description of her experiences. In his opinion, she suffered from battered women’s syndrome in “about as extreme a pattern as you could find.”
2. Nonexpert Testimony
Defendant confirmed many of the details of her life and relationship with Hampton underlying Dr. Bowker’s opinion. She testified that her father [1080]*1080forcefully molested her from the time she was seven years old until she was fifteen. She described her relationship with another abusive man as being like “Nightmare on Elm Street.” Regarding Hampton, she testified that they often argued and that he beat her regularly. Both were heavy drinkers. Hampton once threw a can of beer at her face, breaking her nose. Her dental plates hurt because Hampton hit her so often. He often kicked her, but usually hit her in the back of the head because, he told her, it “won’t leave bruises.” Hampton sometimes threatened to kill her, and often said she “would live to regret it.” Matters got worse towards the end.
The evening before the shooting, March 27, 1992, Hampton arrived home “very drunk.” He yelled at her and called her names. At one point when she was standing by the bedroom window, he fired his .357 magnum revolver at her. She testified, “He didn’t miss me by much either.” She was “real scared.”
The next day, the two drove into the mountains. They argued, and Hampton continually hit her. While returning, he said that their location would be a good place to kill her because “they wouldn’t find [her] for a while.” She took it as a joke, although she feared him. When they returned, the arguing continued. He hit her again, then entered the kitchen. He threatened, “This time, bitch, when I shoot at you, I won’t miss.” He came from the kitchen and reached for the gun on the living room table. She grabbed it first, pointed it at him, and told him “that he wasn’t going to hit [her].” She backed Hampton into the kitchen. He was saying something, but she did not know what. He reached for her hand and she shot him. She believed he was reaching for the gun and was going to shoot her.
Several other witnesses testified about defendant’s relationship with Hampton, his abusive conduct in general, and his physical abuse of, and threats to, defendant in particular. This testimony generally corroborated defendant’s. A neighbor testified that the night before the shooting, she heard a gunshot. The next morning, defendant told the neighbor that Hampton had shot at her, and that she was afraid of him. After the shooting, investigators found a bullet hole through the frame of the bedroom window and a bullet embedded in a tree in line with the window. Another neighbor testified that shortly before hearing the shot that killed Hampton, she heard defendant say, “Stop it, Albert. Stop it.”
C. Procedural History
Defendant was charged with murder with personal use of a firearm. At the end of the prosecution’s case-in-chief, the court granted defendant’s motion under Penal Code section 1118.1 for acquittal of first degree murder.
[1081]*1081The court instructed the jury on second degree murder and both voluntary and involuntary manslaughter. It also instructed on self-defense, explaining that an actual and reasonable belief that the killing was necessary was a complete defense; an actual but unreasonable belief was a defense to murder, but not to voluntary manslaughter. In determining reasonableness, the jury was to consider what “would appear to be necessary to a reasonable person in a similar situation and with similar knowledge.”
The court also instructed:
“Evidence regarding Battered Women’s Syndrome has been introduced in this case. Such evidence, if believed, may be considered by you only for the purpose of determining whether or not the defendant held the necessary subjective honest [belief] which is a requirement for both perfect and imperfect self-defense. However, that same evidence regarding Battered Women’s Syndrome may not be considered or used by you in evaluating the objective reasonableness requirement for perfect self-defense.
“Battered Women’s Syndrome seeks to describe and explain common reactions of women to that experience. Thus, you may consider the evidence concerning the syndrome and its effects only for the limited purpose of showing, if it does show, that the defendant’s reactions, as demonstrated by the evidence, are not inconsistent with her having been physically abused or the beliefs, perceptions, or behavior of victims of domestic violence.”
During deliberations, the jury asked for and received clarification of the terms “subjectively honest and objectively unreasonable.” It found defendant guilty of voluntary manslaughter with personal use of a firearm. The court sentenced defendant to prison for eight years, consisting of the lower term of three years for manslaughter, plus the upper term of five years for firearm use. The Court of Appeal remanded for resentencing on the use enhancement, but otherwise affirmed the judgment.
We granted defendant’s petition for review.
II. Discussion
A. Background
With an exception not relevant here, Evidence Code section 1107, subdivision (a), makes admissible in a criminal action expert testimony regarding “battered women’s syndrome, including the physical, emotional, or mental [1082]*1082effects upon the beliefs, perceptions, or behavior of victims of domestic violence . . . .” Under subdivision (b) of that section, the foundation for admission is sufficient “if the proponent of the evidence establishes its relevancy and the proper qualifications of the expert witness.”1 Defendant presented the evidence to support her claim of self-defense. It is undisputed that she established the proper qualifications of the expert witness. The only issue is to what extent defendant established its “relevancy.” To resolve this question we must examine California law regarding self-defense.
For killing to be in self-defense, the defendant must actually and reasonably believe in the need to defend. (People v. Flannel (1979) 25 Cal.3d 668, 674 [160 Cal.Rptr. 84, 603 P.2d 1].) If the belief subjectively exists but is objectively unreasonable, there is “imperfect self-defense,” i.e., “the defendant is deemed to have acted without malice and cannot be convicted of murder,” but can be convicted of manslaughter. (In re Christian S. (1994) 7 Cal.4th 768, 783 [30 Cal.Rptr.2d 33, 872 P.2d 574].)2 To constitute “perfect self-defense,” i.e., to exonerate the person completely, the belief must also be objectively reasonable. (Id. at p. 783; see also People v. Aris (1989) 215 Cal.App.3d 1178, 1186 [264 Cal.Rptr. 167].) As the Legislature has stated, “[T]he circumstances must be sufficient to excite the fears of a reasonable person . . . .” (Pen. Code, § 198; see also § 197, subds. 2, 3.) Moreover, for either perfect or imperfect self-defense, the fear must be of imminent harm. “Fear of future harm—no matter how great the fear and no matter how great the likelihood of the harm—will not suffice. The defendant’s fear must be of imminent danger to life or great bodily injury.” (In re Christian S., supra, 7 Cal.4th at p. 783, italics in original.)
Although the belief in the need to defend must be objectively reasonable, a jury must consider what “would appear to be necessary to a reasonable [1083]*1083person in a similar situation and with similar knowledge . . . (CALJIC No. 5.50.) It judges reasonableness “from the point of view of a reasonable person in the position of defendant . . . .” (People v. McGee (1947) 31 Cal.2d 229, 238 [187 P.2d 706].) To do this, it must consider all the “ ‘ “facts and circumstances ... in determining whether the defendant acted in a manner in which a reasonable man would act in protecting his own life or bodily safety.” ’ ” (People v. Moore (1954) 43 Cal.2d 517, 528 [275 P.2d 485], italics in original.) As we stated long ago, “. . . a defendant is entitled to have a jury take into consideration all the elements in the case which might be expected to operate on his mind . . . .” (People v. Smith (1907) 151 Cal. 619, 628 [91 P. 511].)
We recently discussed this question in a different context. In People v. Ochoa (1993) 6 Cal.4th 1199 [26 Cal.Rptr.2d 23, 864 P.2d 103], the defendant was convicted of gross vehicular manslaughter while intoxicated. The offense requires “gross negligence,” the test for which is “ ‘objective: whether a reasonable person in the defendant’s position would have been aware of the risk involved. [Citation.]’ [Citation.]” (Id. at p. 1204, quoting People v. Bennett (1991) 54 Cal.3d 1032, 1036 [2 Cal.Rptr.2d 8, 819 P.2d 849].) The defendant argued that, “because the test of gross negligence is an objective one .... evidence of his own subjective state of mind was irrelevant and unduly prejudicial.” (People v. Ochoa, supra, at p. 1205, italics in original.) We disagreed. “In determining whether a reasonable person in defendant's position would have been aware of the risks, the jury should be given relevant facts as to what defendant knew, including his actual awareness of those risks.” (Ibid., italics in original.) “[Although the test for gross negligence was an objective one, ‘[t]he jury should therefore consider all relevant circumstances .... [Citations.]’ ” (Ibid., quoting People v. Bennett, supra, 54 Cal.3d at p. 1038.)
What we said in Ochoa about the defendant’s actual awareness applies to this case. Although the ultimate test of reasonableness is objective, in determining whether a reasonable person in defendant’s position would have believed in the need to defend, the jury must consider all of the relevant circumstances in which defendant found herself.
With these principles in mind, we now consider the relevance of evidence of battered women’s syndrome to the elements of self-defense.
B. Battered Women’s Syndrome3
Battered women’s syndrome “has been defined as ‘a series of common characteristics that appear in women who are abused physically and [1084]*1084psychologically over an extended period of time by the dominant male figure in their lives.’ (State v. Kelly (1984) 97 N.J. 178, 193 [478 A.2d 364, 371]; see also People v. Aris (1989) 215 Cal.App.3d 1178, 1194 [264 Cal.Rptr. 167] [‘“a pattern of psychological symptoms that develop after somebody has lived in a battering relationship” ’]; Note, Battered Women Who Kill Their Abusers (1993) 106 Harv.L.Rev. 1574, 1578 [‘a “pattern of responses and perceptions presumed to be characteristic of women who have been subjected to continuous physical abuse by their mate[s]” ’].)” (People v. Romero (1994) 8 Cal.4th 728, 735, fn. 1 [35 Cal.Rptr.2d 270, 883 P.2d 388].)
The trial court allowed the jury to consider the battered women’s syndrome evidence in deciding whether defendant actually believed she needed to kill in self-defense. The question here is whether the evidence was also relevant on the reasonableness of that belief. Two Court of Appeal decisions have considered the relevance of battered women’s syndrome evidence to a claim of self-defense.
People v. Aris, supra, 215 Cal.App.3d at page 1185, applied “the law of self-defense in the context of a battered woman killing the batterer while he slept after he had beaten the killer and threatened serious bodily injury and death when he awoke.” There, unlike here, the trial court refused to instruct the jury on perfect self-defense, but it did instruct on imperfect self-defense. The appellate court upheld the refusal, finding that “defendant presented no substantial evidence that a reasonable person under the same circumstances would have perceived imminent danger and a need to kill in self-defense.” {Id. at p. 1192.)4 The trial court admitted some evidence of battered women’s syndrome, but the defendant argued that it erred “by excluding expert testimony (1) that defendant was a battered woman based on the expert’s psychological evaluation of the defendant and (2) ‘explaining how the psychological impact of being a battered woman affected her perception of danger at the time she shot her husband.’ ” (People v. Aris, supra, 215 Cal.App.3d at p. 1193.)
[1085]*1085Although the trial court did not instruct on perfect self-defense, the appellate court first concluded that battered women’s syndrome evidence is not relevant to the reasonableness element. “[T]he questions of the reasonableness of a defendant’s belief that self-defense is necessary and of the reasonableness of the actions taken in self-defense do not call for an evaluation of the defendant’s subjective state of mind, but for an objective evaluation of the defendant’s assertedly defensive acts. California law expresses the criterion for this evaluation in the objective terms of whether a reasonable person, as opposed to the defendant, would have believed and acted as the defendant did. We hold that expert testimony about a defendant’s state of mind is not relevant to the reasonableness of the defendant’s self-defense.” (People v. Aris, supra, 215 Cal.App.3d at p. 1196, italics in original.)
The court then found the evidence “highly relevant to the first element of self-defense—defendant’s actual, subjective perception that she was in danger and that she had to kill her husband to avoid that danger. ...[']{] The relevance to the defendant’s actual perception lies in the opinion’s explanation of how such a perception would reasonably follow from the defendant’s experience as a battered woman. This relates to the prosecution’s argument that such a perception of imminent danger makes no sense when the victim is asleep and a way of escape open and, therefore, she did not actually have that perception.” (People v. Aris, supra, 215 Cal.App.3d at p. 1197.) The trial court thus erred in not admitting the testimony to show “how the defendant’s particular experiences as a battered woman affected her perceptions of danger, its imminence, and what actions were necessary to protect herself.” (Id. at p. 1198.)
Concerned “that the jury in a particular case may misuse such evidence to establish the reasonableness requirement for perfect self-defense, for which purpose it is irrelevant,” the Aris court stated that, “upon request whenever the jury is instructed on perfect self-defense, trial courts should instruct that such testimony is relevant only to prove the honest belief requirement for both perfect and imperfect self-defense, not to prove the reasonableness requirement for perfect self-defense.” (People v. Aris, supra, 215 Cal.App.3d at p. 1199.) The trial court gave such an instruction here, thus creating the issue before us.
In People v. Day (1992) 2 Cal.App.4th 405 [2 Cal.Rptr.2d 916], the defendant moved for a new trial following her conviction of involuntary manslaughter. Supported by an affidavit by Dr. Bowker, she argued that her attorney should have presented evidence of battered women’s syndrome to aid her claim of self-defense. Relying on Aris, the appellate court first found [1086]*1086that the evidence would not have been relevant to show the objective reasonableness of the defendant’s actions. (People v. Day, supra, at pp. 414- 415.) It also found, however, that the evidence would have been admissible to rehabilitate the defendant’s credibility as a witness. (Id. at pp. 415- 419.) Finding that counsel’s failure to present the evidence was prejudicial, the court reversed the judgment. (Id. at pp. 419-420.)
The Attorney General argues that People v. Aris, supra, 215 Cal.App.3d 1178, and People v. Day, supra, 2 Cal.App.4th 405, were correct that evidence of battered women’s syndrome is irrelevant to reasonableness. We disagree. Those cases too narrowly interpreted the reasonableness element. Aris and Day failed to consider that the jury, in determining objective reasonableness, must view the situation from the defendant’s perspective. Here, for example, Dr. Bowker testified that the violence can escalate and that a battered woman can become increasingly sensitive to the abuser’s behavior, testimony relevant to determining whether defendant reasonably believed when she fired the gun that this time the threat to her life was imminent. Indeed, the prosecutor argued that, “from an objective, reasonable man’s standard, there was no reason for her to go get that gun. This threat that she says he made was like so many threats before. There was no reason for her to react that way.” Dr. Bowker’s testimony supplied a response that the jury might not otherwise receive. As violence increases over time, and threats gain credibility, a battered person might become sensitized and thus able reasonably to discern when danger is real and when it is not. “[T]he expert’s testimony might also enable the jury to find that the battered [woman] ... is particularly able to predict accurately the likely extent of violence in any attack on her. That conclusion could significantly affect the jury’s evaluation of the reasonableness of defendant’s fear for her life.” (State v. Kelly (1984) 97 N.J. 178 [478 A.2d 364, 378], italics added, fn. omitted.)
The Attorney General concedes that Hampton’s behavior towards defendant, including prior threats and violence, was relevant to reasonableness (see People v. Minifie (1996) 13 Cal.4th 1055, 1065 [56 Cal.Rptr.2d 133, 920 P.2d 1337]), but distinguishes between evidence of this behavior— which the trial court fully admitted—and expert testimony about its effects on defendant. The distinction is untenable. “To effectively present the situation as perceived by the defendant, and the reasonableness of her fear, the defense has the option to explain her feelings to enable the jury to overcome stereotyped impressions about women who remain in abusive relationships. It is appropriate that the jury be given a professional explanation of the battering syndrome and its effects on the woman through the use of expert testimony. [Citation.]” (State v. Allery (1984) 101 Wn.2d 591 [682 P.2d 312, 316].)
[1087]*1087The Attorney General also argues that allowing consideration of this testimony would result in an undesirable “battle of the experts” and raises the specter of other battles of experts regarding other syndromes. The Legislature, however, has decided that, if relevant, expert evidence on battered women’s syndrome is admissible. (Evid. Code, § 1107.) We have found it relevant; it is therefore admissible. We express no opinion on the admissibility of expert testimony regarding other possible syndromes in support of a claim of self-defense, but we rest today’s holding on Evidence Code section 1107.
Contrary to the Attorney General’s argument, we are not changing the standard from objective to subjective, or replacing the reasonable “person” standard with a reasonable “battered woman” standard. Our decision would not, in another context, compel adoption of a “ ‘reasonable gang member’ standard.” Evidence Code section 1107 states “a rule of evidence only” and makes “no substantive change.” (Evid. Code, § 1107, subd. (d).) The jury must consider defendant’s situation and knowledge, which makes the evidence relevant, but the ultimate question is whether a reasonable person, not a reasonable battered woman, would believe in the need to kill to prevent imminent harm. Moreover, it is the jury, not the expert, that determines whether defendant’s belief and, ultimately, her actions, were objectively reasonable.
Battered women’s syndrome evidence was also relevant to defendant’s credibility. It “would have assisted the jury in objectively analyzing [defendant’s] claim of self-defense by dispelling many of the commonly held misconceptions about battered women.” (People v. Day, supra, 2 Cal.App.4th at p. 416.) For example, in urging the jury not to believe defendant’s testimony that Hampton shot at her the night before the killing, the prosecutor argued that “if this defendant truly believed that [Hampton] had shot at her, on that night, I mean she would have left. ...[<][] If she really believed that he had tried to shoot her, she would not have stayed.” Dr. Bowker’s testimony “ ‘would help dispel the ordinary lay person’s perception that a woman in a battering relationship is free to leave at any time. The expert evidence would counter any “common sense” conclusions by the jury that if the beatings were really that bad the woman would have left her husband much earlier. Popular misconceptions about battered women would be put to rest. . . .’ ” (People v. Day, supra, 2 Cal.App.4th at p. 417, quoting State v. Hodges (1986) 293 Kan. 63 [716 P.2d 563, 567].) “[I]f the jury had understood [defendant’s] conduct in light of [battered women’s syndrome] evidence, then the jury may well have concluded her version of the events was sufficiently credible to warrant an acquittal on the facts as she related them.” (People v. Day, supra, 2 Cal.App.4th at p. 415.)
[1088]*1088As Day recognizes, People v. McAlpin (1991) 53 Cal.3d 1289 [283 Cal.Rptr. 382, 812 P.2d 563] supports this conclusion. There we held that expert testimony regarding parental reluctance to report child molestation was admissible to bolster a witness’s credibility: “Most jurors, fortunately, have been spared the experience of being the parent of a sexually molested child. Lacking that experience, jurors can rely only on their intuition or on relevant evidence introduced at trial. . . . [Evidence that parents often do not report child molestation] would therefore ‘assist the trier of fact’ (Evid. Code, § 801, subd. (a)) by giving the jurors information they needed to objectively evaluate [the witness’s] credibility.” (Id. at p. 1302, fn. omitted.) (2c) As in McAlpin, the expert testimony in this case was “ ‘needed to disabuse jurors of commonly held misconceptions . . . .’” (Id. at p. 1301.) It was relevant “to explain a behavior pattern that might otherwise appear unreasonable to the average person. Evidence of [battered women’s syndrome] not only explains how a battered woman might think, react, or behave, it places the behavior in an understandable light.” (People v. Day, supra, 2 Cal.App.4th at p. 419.) Thus, it was admissible under Evidence Code sections 801 and 1107.
We do not hold that Dr. Bowker’s entire testimony was relevant to both prongs of perfect self-defense. Just as many types of evidence may be relevant to some disputed issues but not all, some of the expert evidence was no doubt relevant only to the subjective existence of defendant’s belief. Evidence merely showing that a person’s use of deadly force is scientifically explainable or empirically common does not, in itself, show it was objectively reasonable. To dispel any possible confusion, it might be appropriate for the court, on request, to clarify that, in assessing reasonableness, the question is whether a reasonable person in the defendant’s circumstances would have perceived a threat of imminent injury or death, and not whether killing the abuser was reasonable in the sense of being an understandable response to ongoing abuse; and that, therefore, in making that assessment, the jury may not consider evidence merely showing that an abused person’s use of force against the abuser is understandable.5
We also emphasize that, as with any evidence, the jury may give this testimony whatever weight it deems appropriate in light of the evidence as a whole. The ultimate judgment of reasonableness is solely for the jury. We simply hold that evidence of battered women’s syndrome is generally relevant to the reasonableness, as well as the subjective existence, of defendant’s [1089]*1089belief in the need to defend, and, to the extent it is relevant, the jury may consider it in deciding both questions. The court’s contrary instruction was erroneous. We disapprove of People v. Aris, supra, 215 Cal.App.3d 1178, and People v. Day, supra, 2 Cal.App.4th 405, to the extent they are inconsistent with this conclusion.
C. Prejudice
Defendant contends that the instructional error unconstitutionally deprived her of her rights to present a defense and to equal protection of the laws, thus requiring reversal unless the error was harmless beyond a reasonable doubt. (Chapman v. California (1967) 386 U.S. 18, 24 [17 L.Ed.2d 705, 710-711, 87 S.Ct. 824, 24 A.L.R.3d 1065].) We disagree that the Chapman standard applies. The erroneous instruction may have adversely affected the defense, but it did not deprive her of the right to present one or deny her equal protection. In effect, the court excluded some evidence as to one element of the defense. When the reviewing court applying state law finds an erroneous exclusion of defense evidence, the usual standard of review for state law error applies: the court must reverse only if it also finds a reasonable probability the error affected the verdict adversely to defendant. (People v. Fudge (1994) 7 Cal.4th 1075, 1102-1103 [31 Cal.Rptr.2d 321, 875 P.2d 36]; People v. Cudjo (1993) 6 Cal.4th 585, 611 [25 Cal.Rptr.2d 390, 863 P.2d 635] [error in excluding evidence of third party culpability]; People v. Watson (1956) 46 Cal.2d 818, 836 [299 P.2d 243]; Evid. Code, §354.)
Under this standard, however, we conclude the error was prejudicial. The jury found defendant guilty of voluntary manslaughter, not murder. Although the verdict may have been based on a finding of provocation, the arguments to the jury and the jury’s request for clarification of the terms “subjectively honest and objectively unreasonable” suggest the question of unreasonable self-defense was critical. The jury likely concluded that defendant actually believed in the need to defend, but her belief was unreasonable. If so, guilt or innocence hinged on the precise issue—objective reasonableness—on which the court told the jury not to consider the battered women’s syndrome evidence. As stated above, the prosecutor argued that defendant’s actions were unreasonable because the last “threat that she says he made was like so many threats before. There was no reason for her to react that way.” The testimony the court told the jury not to consider was directly responsive to this argument.
Although we do not know what weight the jury would have given the expert testimony in determining reasonableness, the testimony “was not only [1090]*1090relevant, but critical in permitting the jury to evaluate [defendant’s] testimony free of the misperceptions regarding battered women.” (People v. Day, supra, 2 Cal.App.4th at p. 419.) Overall, the evidence, including defendant’s corroborated testimony about the shooting the night before, presented a plausible case for perfect self-defense. The actual verdict was reasonable, but so too would have been a different one. Under all of these circumstances, it is reasonably probable the error affected the verdict adversely to defendant.
III. Disposition
The judgment of the Court of Appeal is reversed.
George, C. J., Mosk, J., Kennard, J., and Werdegar, J., concurred.