WILENTZ, C.J.

 The central issue before us is whether expert testimony about the battered-woman's syndrome is admissible to help establish a claim of self-defense in a homicide case.  The question is one of first impression in this state.  We hold, based on the limited record before us (the State not having had a full opportunity to prove the contrary), that the battered-woman's syndrome is an appropriate subject for expert testimony;  that the experts' conclusions, despite the relative newness of the field, are sufficiently reliable under New Jersey's standards for scientific testimony;  and that defendant's expert was sufficiently qualified.  Accordingly, we reverse and remand for a new trial. If on retrial after a full examination of these issues the evidence continues to support these conclusions, the expert's testimony on the battered-woman's syndrome shall be admitted as relevant to the honesty and reasonableness of defendant's belief that deadly force was necessary to protect her against death or serious bodily harm.

I.

 On May 24, 1980, defendant, Gladys Kelly, stabbed her husband, Ernest, with a pair of scissors.  He died shortly thereafter at a nearby hospital.  The couple had been married [p. 188] for seven years, during which time Ernest had periodically attacked Gladys.  According to Ms. Kelly, he assaulted her that afternoon, and she stabbed him in self-defense, fearing that he would kill her if she did not act.

 Ms. Kelly was indicted for murder.  At trial, she did not deny stabbing her husband, but asserted that her action was in self-defense.  To establish the requisite state of mind for her self-defense claim, Ms. Kelly called Dr. Lois Veronen as an expert witness to testify about the battered-woman's syndrome. After hearing a lengthy voir dire examination of Dr. Veronen, the trial court ruled that expert testimony concerning the syndrome was inadmissible on the self-defense issue under State v. Bess, 53 N.J. 10, 247 A.2d 669 (1968). Apparently the court believed that the sole purpose of this testimony was to explain and justify defendant's perception of the danger rather than to show the objective reasonableness of that perception.

 Ms. Kelly was convicted of reckless manslaughter.  In an unreported decision relying in part on Bess, the Appellate Division affirmed the conviction.  We granted certification, 91 N.J. 539, 453 A.2d 859 (1983), and now reverse.

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II.

 The Kellys had a stormy marriage.  Some of the details of their relationship, especially the stabbing, are disputed.  The following is Ms. Kelly's version of what happened--a version that the jury could have accepted and, if they had, a version [p. 189] that would make the proffered expert testimony not only relevant, but critical.

 The day after the marriage, Mr. Kelly got drunk and knocked Ms. Kelly down.  Although a period of calm followed the initial attack, the next seven years were accompanied by periodic and frequent beatings, sometimes as often as once a week.  During the attacks, which generally occurred when Mr. Kelly was drunk, he threatened to kill Ms. Kelly and to cut off parts of her body if she tried to leave him.  Mr. Kelly often moved out of the house after an attack, later returning with a promise that he would change his ways.  Until the day of the homicide, only one of the attacks had taken place in public.

 The day before the stabbing, Gladys and Ernest went shopping.  They did not have enough money to buy food for the entire week, so Ernest said he would give his wife more money the next day.

 The following morning he left for work.  Ms. Kelly next saw her husband late that afternoon at a friend's house.  She had gone there with her daughter, Annette, to ask Ernest for money to buy food.  He told her to wait until they got home, and shortly thereafter the Kellys left.  After walking past several houses, Mr. Kelly, who was drunk, angrily asked "What the hell did you come around here for?"  He then grabbed the collar of her dress, and the two fell to the ground.  He choked her by pushing his fingers against her throat, punched or hit her face, and bit her leg.

 A crowd gathered on the street.  Two men from the crowd separated them, just as Gladys felt that she was "passing out" from being choked.  Fearing that Annette had been pushed around in the crowd, Gladys then left to look for her. Upon finding Annette, defendant noticed that Annette had defendant's pocketbook.  Gladys had dropped it during the fight.  Annette had retrieved it and gave her mother the pocketbook.

 After finding her daughter, Ms. Kelly then observed Mr. Kelly running toward her with his hands raised.  Within seconds [p. 190] he was right next to her. Unsure of whether he had armed himself while she was looking for their daughter, and thinking that he had come back to kill her, she grabbed a pair of scissors from her pocketbook.  She tried to scare him away, but instead stabbed him.

    III.

 The central question in this case is whether the trial court erred in its exclusion of expert testimony on the battered-woman's syndrome.  That testimony was intended to explain defendant's state of mind and bolster her claim of self-defense.  We shall first examine the nature of the battered-woman's syndrome and then consider the expert testimony proffered in this case and its relevancy.

 In the past decade social scientists and the legal community began to examine the forces that generate and perpetuate wife beating and violence in the family.  What  has been revealed is [p. 191] that the problem affects many more people than had been thought and that the victims of the violence are not only the battered family members (almost always either the wife or the children).  There are also many other strangers to the family who feel the devastating impact, often in the form of violence, of the psychological damage suffered by the victims.

 Due to the high incidence of unreported abuse (the FBI and other law enforcement experts believe that wife abuse is the most unreported crime in the United States), estimates vary of the number of American women who are beaten regularly by their husband, boyfriend, or the dominant male figure in their lives.  One recent estimate puts the number of women beaten yearly at over one million.  See California Advisory Comm'n on Family Law, Domestic Violence app. F at 119 (1st report 1978).  The state police statistics show more than 18,000 reported cases of domestic violence in New Jersey during the first nine months of 1983, in 83% of which the victim was female.  It is clear that the American home, once assumed to be the cornerstone of our society, is often a violent place.

 While common law notions that assigned an inferior status to women, and to wives in particular, no longer represent the state [p. 192] of the law as reflected in statutes and cases, many commentators assert that a bias against battered women still exists, institutionalized in the attitudes of law enforcement agencies unwilling to pursue or uninterested in pursuing wife beating cases.  See Comment, The Battered Wife's Dilemma:  Kill or be Killed, 32 Hastings L.J., 895, 897-911 (1981).

 Another problem is the currency enjoyed by stereotypes and myths concerning the characteristics of battered women and their reasons for staying in battering relationships.  Some popular misconceptions about battered women include the beliefs that they are masochistic and actually enjoy their beatings, that they purposely provoke their husbands into violent behavior, and, most critically, as we shall soon see, that women who remain in battering relationships are free to leave their abusers at any time.  See L. Walker, The Battered Woman at 19-31 (1979).

 As these cases so tragically suggest, not only do many women suffer physical abuse at the hands of their mates, but a significant  number of women kill (or are killed by) their husbands.  In 1978, murders between husband and wife or girlfriend and boyfriend constituted 13% of all murders committed in the United States.  Undoubtedly some of these arose from battering incidents. Federal Bureau of Investigation, Crime in the United States 1978 (1978).  Men were the victims in 48% of these killings.  Id.

 As the problem of battered women has begun to receive more attention, sociologists and psychologists have begun to focus on the effects a sustained pattern of physical and psychological [p. 193] abuse can have on a woman.  The effects of such abuse are what some scientific observers have termed "the battered-woman's syndrome," a series of common characteristics that appear in women who are abused physically and psychologically over an extended period of time by the dominant male figure in their lives.  Dr. Lenore Walker, a prominent writer on the battered-woman's syndrome, defines the battered woman as one
who is repeatedly subjected to any forceful physical or psychological behavior by a man in order to coerce her to do something he wants her to do without concern for her rights.  Battered women include wives or women in any form of intimate relationships with men.  Furthermore, in order to be classified as a battered woman, the couple must go through the battering cycle at least twice.  Any woman may find herself in an abusive relationship with a man once.  If it occurs a second time, and she remains in the situation, she is defined as a battered woman.  [L. Walker, supra, at xv].

 According to Dr. Walker, relationships characterized by physical abuse tend to develop battering cycles.  Violent behavior directed at the woman occurs in three distinct and repetitive stages that vary both in duration and intensity depending on the individuals involved.  L. Walker, supra, at 55-70.

 Phase one of the battering cycle is referred to as the "tension-building stage," during which the battering male engages in minor battering incidents and verbal abuse while the woman, beset by fear and tension, attempts to be as placating and passive as possible in order to stave off more serious violence.  Id. at 56-59.

 Phase two of the battering cycle is the "acute battering incident."  At some point during phase one, the tension between the battered woman and the batterer becomes intolerable and more serious violence inevitable.  The triggering event that initiates phase two is most often an internal or external event in the life of the battering male, but provocation for more severe violence is sometimes provided by the woman who can no longer tolerate or control her phase-one anger and anxiety.  Id. at 59-65.

 Phase three of the battering cycle is characterized by extreme contrition and loving behavior on the part of the battering [p. 194] male.  During this period the man will often mix his pleas for forgiveness and protestations of devotion with promises to seek professional help, to stop drinking, and to refrain from further violence.  For some couples, this period of relative calm may last as long as several months, but in a battering relationship the affection and contrition of the man will eventually fade and phase one of the cycle will start anew.  Id. at 65-70.

 The cyclical nature of battering behavior helps explain why more women simply do not leave their abusers.  The loving behavior demonstrated by the batterer during  phase three reinforces whatever hopes these women might have for their mate's reform and keeps them bound to the relationship.  R. Langley & R. Levy, Wife Beating:  The Silent Crisis 112-14 (1977).

 Some women may even perceive the battering cycle as normal, especially if they grew up in a violent household.  Battered Women, A Psychosociological Study of Domestic Violence 60 (M. Roy ed. 1977);  D. Martin, Battered Wives, 60 (1981). Or they may simply not wish to acknowledge the reality of their situation.  T. Davidson, Conjugal Crime, at 50 (1978) ("The middle-class battered wife's response to her situation tends to be withdrawal, silence and denial ...").

 Other women, however, become so demoralized and degraded by the fact that they cannot predict or control the violence that they sink into a state of psychological paralysis and become unable to take any action at all to improve or alter the situation.  There is a tendency in battered women to believe in the omnipotence [p. 195] or strength of their battering husbands and thus to feel that any attempt to resist them is hopeless.  L. Walker, supra, at 75.

 In addition to these psychological impacts, external social and economic factors often make it difficult for some women to extricate themselves from battering relationships.  A woman without independent financial resources who wishes to leave her husband often finds it difficult to do so because of a lack of material and social resources.

 Even with the progress of the last decade, women typically make less money and hold less prestigious jobs than men, and are more responsible for child care. Thus, in a violent confrontation where the first reaction might be to flee, women realize soon that there may be no place to go.  Moreover, the stigma that attaches to a woman who leaves the family unit without her children undoubtedly acts as a further deterrent to moving out.

 In addition, battered women, when they want to leave the relationship, are typically unwilling to reach out and confide in their friends, family, or the police, either out of shame and humiliation, fear of reprisal by their husband, or the feeling they will not be believed.

 Dr. Walker and other commentators have identified several common personality traits of the battered woman:  low self-esteem, traditional beliefs about the home, the family, and the female sex role, tremendous feelings of guilt that their marriages are failing, and the tendency to accept responsibility for the batterer's actions.  L. Walker, supra, at 35-36.

 Finally, battered women are often hesitant to leave a battering relationship because, in addition to their hope of reform on the part of their spouse, they harbor a deep concern about the possible response leaving might provoke in their mates.  They literally become trapped by their own fear.  Case histories are replete with instances in which a battered wife left her husband [p. 196] only to have him pursue her and subject her to an even more brutal attack.  D. Martin, supra, at 76-79.

 The combination of all these symptoms--resulting from sustained psychological and physical trauma compounded by aggravating social and economic factors-- constitutes the battered-woman's syndrome.  Only by understanding these unique pressures that force battered women to remain with their mates, despite their long-standing and reasonable fear of severe bodily harm and the isolation that being a battered woman creates, can a battered woman's state of mind be accurately and fairly understood.

 The voir dire testimony of Dr. Veronen, sought to be introduced by defendant Gladys Kelly, conformed essentially to this outline of the battered-woman's syndrome.  Dr. Vernonen, after establishing her credentials, described in general terms the component parts of the battered-woman's syndrome and its effects on a woman's physical and mental health.  The witness then documented, based on her own considerable  experience in counseling, treating, and studying battered women, and her familiarity with the work of others in the field, the feelings of anxiety, self-blame, isolation, and, above all, fear that plagues these women and leaves them prey to a psychological paralysis that hinders their ability to break free or seek help.

 Dr. Veronen stated that the problems of battered women are aggravated by a lack of understanding among the general public concerning both the prevalence of violence against women and the nature of battering relationships.  She cited several myths concerning battered women that enjoy popular acceptance-- primarily that such women are masochistic and enjoy the abuse they receive and that they are free to leave their husbands but choose not to.

 Dr. Veronen described the various psychological tests and examinations she had performed in connection with her independent research.  These tests and their methodology, including their interpretation, are, according to Dr. Veronen, widely [p. 197] accepted by clinical psychologists.  Applying this methodology to defendant (who was subjected to all of the tests, including a five-hour interview), Dr. Veronen concluded that defendant was a battered woman and subject to the battered-woman's syndrome.

 In addition, Dr. Veronen was prepared to testify as to how, as a battered woman, Gladys Kelly perceived her situation at the time of the stabbing, and why, in her opinion, defendant did not leave her husband despite the constant beatings she endured.

IV.

 Whether expert testimony on the battered-woman's syndrome should be admitted in this case depends on whether it is relevant to defendant's claim of self- defense, and, in any event, on whether the proffer meets the standards for admission of expert testimony in this state.  We examine first the law of self- defense and consider whether the expert testimony is relevant.

 The present rules governing the use of force in self-defense are set out in the justification section of the Code of Criminal Justice.  The use of force against another in self-defense is justifiable "when the actor reasonably believes that such force is immediately necessary for the purpose of protecting himself against the use of unlawful force by such other person on the present occasion."  N.J.S.A. 2C:3-4(a).  Further limitations exist when deadly force is used in self-defense.  The use of such deadly force is not justifiable

unless the actor reasonably believes that such force is necessary to protect himself against death or serious bodily harm ....  [N.J.S.A. 2C:3-4(b)(2) ].

 These principles codify decades of prior case law development of the elements of self-defense.  We focus here on the critical [p. 198] requirement that the actor reasonably believe deadly force to be necessary to prevent death or serious bodily harm, for the proffer of expert testimony was argued to be relevant on this point.

 Self-defense exonerates a person who kills in the reasonable belief that such action was necessary to prevent his or her death or serious injury, even though this belief was later proven mistaken.  "Detached reflection cannot be demanded in the presence of an uplifted knife," Justice Holmes aptly said, Brown v. United States, 256 U.S. 335, 343, 41 S.Ct. 501, 502, 65 L.Ed. 961, 963 (1921);  and the law accordingly requires only a reasonable, not necessarily a correct, judgment.  See State v. Hipplewith, 33 N.J. 300, 316- 17, 164 A.2d 481 (1960);  State v. Mount, 73 N.J.L. 582, 583, 64 A.2d 124 (E. & A. 1905);  State  v. Lionetti, 93 N.J.L. 24, 107 A. 47 (Sup.Ct.1919).

 While it is not imperative that actual necessity exist, a valid plea of self-defense will not lie absent an actual (that is, honest) belief on the part of the defendant in the necessity of using force.  While no case in New Jersey has addressed the point directly, the privilege of self-defense does not exist where the defendant's action is not prompted by a belief in its necessity:  "He has no defense when he intentionally kills his enemy in complete ignorance of the fact that his enemy, when killed, was about to launch a deadly attack upon him."  W. LaFave & A. Scott, Criminal Law § 53, at 394 (1972). [n. 7]  The intent of the [p. 199] drafters of the present Code was that a necessity to act should not give rise to a meritorious plea of self-defense where the defendant was unaware of that necessity.  Final Report of the New Jersey Criminal Law Revision Commission, Vol. II:  Commentary, at 83 (1971) [hereinafter cited as Commission Report ].  Ultimately, of course, it is for the jury to determine if the defendant actually did believe in the necessity of acting with deadly force to prevent an imminent, grave attack.  See, e.g., State v. Fair, 45 N.J. 77, 93, 211 A.2d 359 (1965).
 

7. See also Restatement of Torts 2d § 63 (1965) at 101.  Under principles of self-defense as a justification for the torts of assault and battery--which closely parallel criminal self-defense principles--no privilege of self-defense exists for one acting in ignorance of another's intent to inflict harm on him.  Cf. Perkins, "Self-Defense Re-examined," 1 U.C.L.A.L.Rev. 133, 134 (1954).

8. The rejected form of § 2C:3-4 was patterned after § 3.04 of the Model Penal Code.  The purpose of the proposed Code and M.P.C. provisions was to prevent one who killed in the honest but mistaken and unreasonable belief in the necessity of the action from being convicted of a crime like murder, which is premised on an act motivated by unlawful purpose.  See Model Penal Code § 3.04 commentary at 14-15 (Tent. Draft No. 8 1958); Commission Report, supra, Vol. II:  Commentary, at 83-84.

 With the foregoing standards in mind, we turn to an examination of the relevance of the proffered expert testimony to Gladys Kelly's claim of self- defense.

V.

 Gladys Kelly claims that she stabbed her husband in self-defense, believing he was about to kill her.  The gist of the State's case was that Gladys Kelly was the aggressor, that she consciously intended to kill her husband, and that she certainly was not acting in self-defense.

 The credibility of Gladys Kelly is a critical issue in this case.  If the jury does not believe Gladys Kelly's account, it [p. 201] cannot find she acted in self-defense.  The expert testimony offered was directly relevant to one of the critical elements of that account, namely, what Gladys Kelly believed at the time of the stabbing, and was thus material to establish the honesty of her stated belief that she was in imminent danger of death. [n. 10]
 

10. The factual contentions of the parties eliminated any issue concerning the duty to retreat.  If the State's version is accepted, defendant is the aggressor;  if defendant's version is accepted, the possibility of retreat is excluded by virtue of the nature of the attack  that defendant claims took place.  We do not understand that the State claims defendant breached that duty under any version of the facts.  If, however, the duty becomes an issue on retrial, the trial court will have to determine the relevancy of the battered-woman's syndrome to that issue. Without passing on that question, it appears to us to be a different question from whether the syndrome is relevant to defendant's failure to leave her husband in the past.

 Moreover, defendant's credibility was also attacked in other ways.  Gladys Kelly's prior conviction for conspiracy to commit robbery was admitted into evidence for the express purpose of impeachment, even though this conviction had occurred nine years before the stabbing.  Other questions, about Gladys Kelly's use of alcohol and drugs and about her premarital sexual conduct, were clearly efforts to impeach credibility.

 As can be seen from our discussion of the expert testimony, Dr. Veronen would have bolstered Gladys Kelly's credibility.  [p. 202] Specifically, by showing that her experience, although concededly difficult to comprehend, was common to that of other women who had been in similarly abusive relationships, Dr. Veronen would have helped the jury understand that Gladys Kelly could have honestly feared that she would suffer serious bodily harm from her husband's attacks, yet still remain with him.  This, in turn, would support Ms. Kelly's testimony about her state of mind (that is, that she honestly feared serious bodily harm) at the time of the stabbing.

 On the facts in this case, we find that the expert testimony was relevant to Gladys Kelly's state of mind, namely, it was admissible to show she honestly believed she was in imminent danger of  death.  Ibn-Tamas v. United States, 407 A.2d 626 (D.C.1979) (on remand, trial court excluded expert testimony on battered-woman's syndrome;  the Court of Appeals affirmed the exclusion of the testimony, holding that the trial court was not compelled to admit the evidence;  455 A.2d 893 (D.C.1983));  Hawthorne v. State, 408 So.2d 801 (Fla.Dist.Ct.App.1982), petition for review denied, 415 So.2d 1361 (Fla.1982);  Smith v. State, 247 Ga. 612, 277 S.E.2d 678 (1981); State v. Anaya, 438 A.2d 892 (Me.1981);  State v. Allery, 101 Wash.2d 591, 682 P.2d 312 (Wash.Sup.Ct.1984);  see also People v. Minnis, 118 Ill.App.3d 345, 74 Ill.Dec. 179, 455 N.E.2d 209 (1983) (expert testimony on battered-woman's syndrome admissible to explain reasons why defendant dismembered body of victim/husband where prosecution introduced fact of dismemberment as substantive evidence of guilt).  But see State v. Thomas, 66 Ohio St.2d 518, 423 N.E.2d 137 (1981). Moreover, we [p. 203] find that because this testimony was central to the defendant's claim of self- defense, its exclusion, if otherwise admissible, cannot be held to be harmless error.

  [p. 204] We also find the expert testimony relevant to the reasonableness of defendant's belief that she was in imminent danger of death or serious injury.  We do not mean that the expert's testimony could be used to show that it was understandable that a battered woman might believe that her life was in danger when indeed it was not and when a reasonable person would not have so believed, for admission for that purpose would clearly violate the rule set forth in State v. Bess, supra, 53 N.J. 10, 247 A.2d 669.  Expert testimony in that direction would be relevant solely to the honesty of defendant's belief, not its objective reasonableness.  Rather, our conclusion is that the expert's testimony, if accepted by the jury, would have aided it in determining whether, under the circumstances, a reasonable person would have believed there was imminent danger to her life.

 At the heart of the claim of self-defense was defendant's story that she had been repeatedly subjected to "beatings" over the course of her marriage.  While defendant's testimony was somewhat lacking in detail, a juror could infer from the use of the word "beatings," as well as the detail given concerning some of these events (the choking, the biting, the use of fists), [p. 205] that these physical assaults posed a risk of serious injury or death.  When that regular pattern of serious physical abuse is combined with defendant's claim that the decedent sometimes threatened to kill her, defendant's statement that on this occasion she thought she might be killed when she saw Mr. Kelly running toward her could be found to reflect a reasonable fear;  that is, it could so be found if the jury believed Gladys Kelly's story of the prior beatings, if it believed her story of the prior threats, and, of course, if it believed her story of the events of that particular day.

 The crucial issue of fact on which this expert's testimony would bear is why, given such allegedly severe and constant beatings, combined with threats to kill, defendant had not long ago left decedent.  Whether raised by the prosecutor as a factual issue or not, our own common knowledge tells us that most of us, including the ordinary juror, would ask himself or herself just such a question.  And our knowledge is bolstered by the experts' knowledge, for the experts point out that one of the common myths, apparently believed by most people, is that battered wives are free to leave.  To some, this misconception is followed by the observation that the battered wife is masochistic, proven by her refusal to leave despite the severe beatings;  to others, however, the fact that the battered wife stays on unquestionably suggests that the "beatings" could not have been too bad for if they had been, she certainly would have left.  The expert could clear up these myths, by explaining that one of the common characteristics of a battered wife is her inability to leave despite such constant beatings;  her "learned helplessness";  her lack of anywhere to go;  her feeling that if she tried to leave, she would be subjected to even more merciless treatment;  her belief in the omnipotence of her battering husband;  and sometimes her hope that her husband will change his ways.

 Unfortunately, in this case the State reinforced the myths about battered women.  On cross-examination, when discussing an occasion when Mr. Kelly temporarily moved out of the [p. 206] house, the State repeatedly asked Ms. Kelly:  "You wanted him back, didn't you?"  The implication was clear: domestic life could not have been too bad if she wanted him back.  In its closing argument, the State trivialized the severity of the beatings, saying:

 I'm not going to say they happened or they didn't happen, but life isn't pretty.  Life is not a bowl of cherries.  We each and every person who takes a breath has problems.  Defense counsel says bruised and battered.  Is there any one of us who hasn't been battered by life in some manner or means?

 The difficulty with the expert's testimony is that it sounds as if an expert is giving knowledge to a jury about something the jury knows as well as anyone else, namely, the reasonableness of a person's fear of imminent serious danger.  That is not at all, however, what this testimony is directly aimed at.  It is aimed at an area where the purported common knowledge of the jury may be very much mistaken, an area where jurors' logic, drawn from their own experience, may lead to a wholly incorrect conclusion, an area where expert knowledge would enable the jurors to disregard their prior conclusions as being common myths rather than common knowledge.  After hearing the expert, instead of saying Gladys Kelly could not have been beaten up so badly for if she had, she certainly would have left, the jury could conclude that her failure to leave was very much part and parcel of her life as a battered wife.  The jury could conclude that instead of casting doubt on the accuracy of her testimony about the severity and frequency of prior beatings, her failure to leave actually reinforced her credibility.

 Since a retrial is necessary, we think it advisable to indicate the limit of the expert's testimony on this issue of reasonableness.  It would not be proper for the expert to express the opinion that defendant's belief on that day was reasonable, not because this is the ultimate issue, but because the area of expert knowledge relates, in this regard, to the reasons for [p. 207] defendant's failure to leave her husband.  Either the jury accepts or rejects that explanation and, based on that, credits defendant's stories about the beatings she suffered.  No expert is needed, however, once the jury has made up its mind on those issues, to tell the jury the logical conclusion, namely, that a person who has in fact been severely and continuously beaten might very well reasonably fear that the imminent beating she was about to suffer could be either life-threatening or pose a risk of serious injury.  What the expert could state was that defendant had the battered-woman's syndrome, and could explain that syndrome in detail, relating its characteristics to defendant, but only to enable the jury better to determine the honesty and reasonableness of defendant's belief.  Depending on its content, the expert's testimony might also enable the jury to find that the battered wife, because of the prior beatings, numerous beatings, as often as once a week, for seven years, from the day they were married to the day he died, 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.

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