In this case, petitioner’s two attorneys offered no evidence on his behalf at the penalty phase of his capital trial, leaving the jury’s judgment of death a foregone conclusion. Following petitioner’s application for a writ of habeas corpus, this court issued an order to show cause and an order of reference.
The evidence at the reference hearing disclosed only too clearly why petitioner’s trial counsel had failed to present evidence at the penalty phase: counsel’s investigative efforts, which were wholly inadequate, left counsel with no mitigating evidence to offer. At the reference hearing, petitioner presented a host of witnesses, all of whom stated that they would have been willing to testify on petitioner’s behalf at trial, but that they were never asked to do so. An investigator for petitioner’s appellate counsel was able to locate all of these witnesses in less than two weeks, yet trial counsel spoke to almost none of them. Based on the evidence presented at the reference hearing, the referee, an experienced trial judge, found that trial counsel had ineffectively represented petitioner. In the referee’s words, “nothing, absolutely nothing of a competent nature was done by way of penalty phase preparation by defense counsel in this case.” The referee concluded that the ineffective representation had been prejudicial to petitioner.
The majority sidesteps the question whether the representation that petitioner received at the penalty phase was inadequate, because it concludes *217that any possible inadequacy was harmless. In the majority’s view, had the evidence in mitigation that petitioner offered at the reference hearing been presented at the penalty phase of his capital trial, it would have been seriously weakened by the prosecution’s evidence in rebuttal. Thus, the majority asserts, defense counsel’s failure to offer any mitigating evidence at trial could not have affected the outcome of the case.
I cannot agree. In my view, the prosecution’s rebuttal evidence would have done little to weaken petitioner’s substantial evidence in mitigation. I cannot say with confidence that if the jury at the penalty phase had been given such evidence, it would have returned a verdict of death. I would therefore grant the petition for writ of habeas corpus.
I
The evidence at the guilt phase of petitioner’s trial showed that petitioner and three companions burglarized the home of Bobby Hassan, a marijuana dealer, and ransacked the house. They shot Bobby and his son, Eric Hassan, in the head while the victims were lying on a bed, killing both; Bobby’s hands were tied behind his back.
The evidence also showed that petitioner and two other young men entered the home of Michael, Cora, and Mary Taylor and robbed the occupants, while a fourth accomplice waited outside. Petitioner forced Mary Taylor to enter the bathroom, where he raped her. The robbers then murdered Michael Taylor by shooting him in the head. The prosecution offered no evidence showing who fired the shots that killed either the Hassans or Michael Taylor.
The jury convicted petitioner of three counts of murder (Pen. Code, § 187),1 five counts of robbery (§ 211), two counts of burglary (§ 459) and one count of rape in concert (§§261, subd. (2), 264.1), and found that he was armed with a firearm in the course of each offense (§ 12022, subd. (a)). With respect to the three counts of murder, the jury found these special circumstances: robbery murder (§ 190.2, subd. (a)(17)(i)), burglary murder (§ 190.2, subd. (a)(17)(vii)), and multiple murder (§ 190.2, subd. (a)(3)). In addition, as to one of the counts of murder, the jury found the existence of a rape-murder special circumstance (§ 190.2, subd. (a)(17)(iii)).
At the penalty phase of the trial, the prosecution presented evidence, in aggravation of penalty, that at the age of 18 petitioner had shot a former gang member, Mark Howard, in an incident unrelated to the murders in this *218case. These were the circumstances that led to the shooting: Howard was in Helen Keller Park in the Los Angeles area, when Walter Gregory approached and said that petitioner wanted to talk to him. Howard then spoke to petitioner, who was with a group of people. Petitioner demanded that Howard return a radio that Howard had taken from Gregory. Howard said he took the radio because Gregory owed him money. Petitioner produced a revolver and said that if Howard did not return the radio he would blow Howard’s head off. Howard slapped petitioner, whereupon petitioner shot Howard six times in the stomach and the chest. Howard recovered, but a bullet remains lodged close to his spine, and his ability to use his left leg is seriously impaired. As a result of this incident, petitioner entered a plea of guilty to a charge of assault with a deadly weapon, and was sentenced to three years in prison. The parties stipulated that Howard was an ex-gang member who was once associated with the Denver Lanes and Athens Park Boys gangs.
Petitioner offered no evidence at the penalty phase of his capital trial. At petitioner’s request, the court took judicial notice that Jerome Evan Malett, an accomplice in the murder of Michael Taylor, had been convicted of eight offenses arising out of the Taylor killing, one of which was first degree murder, and that the trial court had sentenced Malett to a term of twenty-seven years to life on the charge of murder, to run consecutively to a term of twenty-one years, four months on the other crimes. The parties also stipulated that petitioner was born on February 1, 1959 (and thus was 21 years old in December 1980, when the murders occurred).
II
At the reference hearing, the referee found that 15 witnesses would have been available to testify for petitioner at the penalty phase of his capital trial, but that none was called by the defense. The witnesses included petitioner’s mother, his brothers, and his sisters. Other witnesses were petitioner’s aunts, Murdie Faye Washington (a legislative analyst for the City of Los Angeles), Gwendolyn Frazier (an optician), and Geraldine Gray (an analyst in the workers’ compensation department of a law firm); and petitioner’s uncle, Alvin Williams, who at the time of petitioner’s trial was a detective with the Los Angeles Police Department. The majority discusses in detail the nature of these witnesses’ testimony. (Maj. opn., ante, pp. 190-194.)
In brief, as established at the reference hearing, the witnesses would have given this testimony at trial if they had been called: petitioner began life with a kindly disposition and in his early years was protective of his younger siblings, nieces, and nephews, but he was victimized by an angry, alcoholic, *219abusive stepfather, who beat petitioner with a thick belt, and often administered those beatings for reasons relating more to his own inebriation than to petitioner’s behavior. Petitioner’s stepfather also mistreated petitioner’s mother; this abuse, which on several occasions required police intervention, was administered in front of petitioner and his siblings.
The referee found the witnesses credible.
All of the witnesses could have been located with little difficulty: Casey Cohen, an investigator retained by petitioner’s appellate counsel, testified that he was able to interview all of them in less than 2 weeks, and that he devoted only 15 or 16 hours to the task. Two of petitioner’s sisters (Shelene Hearting and Denise Ross) and two of his aunts (Murdie Faye Washington and Gwendolyn Frazier) testified that they attempted to speak to petitioner’s counsel during the trial, but that counsel showed no interest in talking to them.
Also testifying at the reference hearing were Charles Watson, an investigator retained by trial counsel, and Elizabeth Harris, who had served as the second attorney for the defense at petitioner’s trial. Watson stated that he had interviewed only two penalty phase witnesses, both friends of petitioner. Attorney Harris testified that she had spoken to approximately three members of petitioner’s family; all of the conversations occurred in the hallway of the courthouse or on the telephone. Harris also spoke to an expert on gang psychology and to the police officer who, at the guilt phase of petitioner’s trial, had testified for the prosecution about petitioner’s gang activities. Although Harris’s primary responsibility was to prepare for the penalty phase of petitioner’s trial, she obtained none of petitioner’s school, juvenile, or prison records.
At the reference hearing, the prosecution offered no evidence to explain counsel’s failure to interview petitioner’s relatives. Petitioner’s chief trial counsel, Gerald Lenoir, testified he could not recall whether there had been any tactical constraints that weighed against the presentation of testimony by petitioner’s family members on petitioner’s behalf. Lenoir’s assistant, Attorney Harris, testified she was unaware of any financial or tactical constraints that would have weighed against presenting such evidence. When the prosecutor asked Harris at the reference hearing whether she believed she had ineffectively represented petitioner at the penalty phase of petitioner’s capital trial, Harris responded: “I absolutely feel I did not do a competent job at the penalty phase, Mr. Semow, and I’m not happy to say that, but that’s true.”
*220III
Under the Sixth Amendment to the United States Constitution, as well as under article I, section 15 of the California Constitution, a criminal defendant has the right to competent counsel. (In re Marquez (1992) 1 Cal.4th 584, 602 [3 Cal.Rptr.2d 727, 822 P.2d 435].) This means, “ ‘the reasonably competent assistance of an attorney acting as [a] diligent conscientious advocate.’ ” (People v. Ledesma (1987) 43 Cal.3d 171, 215 [233 Cal.Rptr. 404, 729 P.2d 839].)
A capital defendant who asserts ineffective representation at the penalty phase of the trial must overcome two hurdles to obtain relief. First, the defendant must show that counsel’s performance was deficient; i.e., that “counsel’s representation fell below an objective standard of reasonableness.” (Strickland v. Washington (1984) 466 U.S. 668, 688 [80 L.Ed.2d 674, 693, 104 S.Ct. 2052].) Second, the defendant must demonstrate that counsel’s ineffectiveness resulted in prejudice. Ineffective assistance of counsel is prejudicial when “there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different. A reasonable probability is a probability sufficient to undermine confidence in the outcome.” (Id. at p. 694 [80 L.Ed.2d at p. 698]; In re Marquez, supra, 1 Cal.4th at p. 603.) The defendant need not show that it is more likely than not that the outcome of the trial would have been different had the inadequacy not occurred. (Strickland v. Washington, supra, 466 U.S. at p. 693 [80 L.Ed.2d at p. 697]; In re Wilson (1992) 3 Cal.4th 945, 956 [13 Cal.Rptr.2d 269, 838 P.2d 1222].)
The majority declines to decide whether defense counsel’s representation at the penalty phase of petitioner’s capital trial was inadequate, because it concludes that any possible inadequacy was harmless. I do not share that conclusion, as I shall explain in part V. Thus, under the test set forth in Strickland v. Washington, supra, 466 U.S. 668, I must first determine whether defense counsel’s investigation and preparation of the penalty phase was ineffective. As shown below, the evidence presented at the reference hearing supports the referee’s findings that defense counsel’s penalty phase preparation was “totally inadequate,” and that “absolutely nothing of a competent nature was done by way of penalty phase preparation by defense counsel in this case.”
IV. Counsel’s Deficient Performance
The record at the reference hearing shows that petitioner’s trial counsel made little or no effort to determine the existence of witnesses who could *221testify on petitioner’s behalf at the penalty phase of his capital trial. At the reference hearing, the investigator for the defense testified that the only penalty phase investigation he did was to interview two friends of petitioner, and to talk to petitioner’s mother on the telephone. Elizabeth Harris, who assisted Attorney Gerald Lenoir in representing petitioner at the trial, testified that she spoke to three members of petitioner’s family over the telephone or in the hallway at court. She never made any systematic effort to gather evidence in mitigation for presentation at the penalty phase. Members of petitioner’s family were unanimous in their testimony that defense counsel made no attempt to interview them or to learn what evidence they might be able to offer on petitioner’s behalf.
I recognize that sometimes family members may be reluctant or unwilling to testify on behalf of a defendant who is facing the death penalty. Not so here. At the reference hearing, petitioner’s appellate counsel presented numerous family members who mentioned their willingness to testify on petitioner’s behalf at his trial had they been asked to do so. It did not take too great an effort to locate these individuals; appellate counsel’s investigator located and interviewed them in less than two weeks. The record provides no basis to conclude that trial counsel would have encountered greater difficulty in contacting these relatives; indeed, several family members testified that they attempted to speak to trial counsel but were rebuffed.
Nor does the record show that trial counsel attempted to gather any other mitigating evidence to offer on petitioner’s behalf, aside from consulting an expert on gang psychology. As a result, defense counsel had no mitigating evidence to offer at the penalty phase of petitioner’s trial.
To render effective assistance, counsel must perform effectively not only at trial but also in preparing for trial. To enable counsel to make intelligent decisions regarding the presentation of evidence, “counsel must make a rational and informed decision on strategy and tactics founded upon adequate investigation and preparation.” (In re Marquez, supra, 1 Cal.4th at p. 602.) In this case, the failure to conduct an adequate investigation left the defense with no recourse other than to submit the issue of penalty to the jury solely on the prosecution’s evidence.
The Attorney General insists that the penalty phase investigation by the defense team was adequate. He asserts that Attorney Harris interviewed “six or seven of petitioner’s relatives, plus petitioner, Deputy Sheriff Ronnie Williams and a psychiatrist.” But at the reference hearing petitioner’s relatives testified that defense counsel had not interviewed them, and the referee found this testimony to be credible. Even if Attorney Harris did talk to these *222individuals, the record supports the referee’s conclusion that she never actually interviewed them. Moreover, Harris apparently made no written reports of the conversations in question; thus lead Attorney Lenoir had no adequate basis on which to make his decision whether to call any of petitioner’s relatives to testify on petitioner’s behalf at the penalty phase of his capital trial.
In the view of the Attorney General, petitioner’s relatives who testified at the reference hearing were either lying or having a “universal failure of recollection” about their contacts with defense counsel. The Attorney General points to the statement by Gloria Brown (petitioner’s mother) that no one ever asked her to testify on petitioner’s behalf as being inconsistent with the testimony of (1) investigator Watson, who testified that he asked Brown to come to court and to bring another relative with her, and (2) Geraldine Gray (Brown’s sister and petitioner’s aunt), who testified that Brown asked her to come to court because defense counsel wanted character witnesses.2 Even if one were to assume that Gloria Brown did not accurately recall the conversations she had with petitioner’s attorneys and investigator, the Attorney General offers no reason for distrusting the testimony of petitioner’s other relatives, all of whom stated at the reference hearing that neither of petitioner’s two attorneys made any effort to call them as witnesses. Because the referee had the opportunity to observe the witnesses’ demeanor and their manner of testifying, the referee’s credibility determination must be given “great weight.” (In re Hitchings (1993) 6 Cal.4th 97, 109 [24 Cal.Rptr.2d 74, 860 P.2d 466].) I see no reason to disturb that determination.
The Attorney General further contends that, at the time of petitioner’s trial, evidence that petitioner’s stepfather was an alcoholic who physically abused his wife and children was not available to the defense. Therefore, the Attorney General argues, trial counsel cannot be faulted for failing to discover such evidence. The Attorney General asserts that at the time of petitioner’s trial no family member would have been willing to discuss these matters because they were in a state of “denial.” The Attorney General points out that no one in petitioner’s family disclosed the abuse to trial counsel, and that in an interview with a California Youth Authority psychiatrist several years before the trial in this case, petitioner denied he had been physically abused. Additionally, the Attorney General notes that according *223to a psychiatrist who testified at the reference hearing on petitioner’s behalf, victims of child and spousal abuse are frequently reluctant to admit the occurrence of abuse.
I agree with the referee that competent trial counsel would have unearthed information pertaining to the abuse to which petitioner’s alcoholic stepfather subjected his wife and children. Although, as the Attorney General points out, victims of abuse are frequently reluctant to disclose such abuse, this reluctance would not have been shared by petitioner’s aunts and uncle, nonvictims who had observed Henry Brown’s abusive behavior towards his wife and children and who offered some of the strongest testimony in this regard at the reference hearing. Moreover, all of the family members testified at the reference hearing that, had they been asked, they would have been willing to testify at trial, and the referee found them to be credible witnesses.
The Attorney General argues that defense counsel “was not required to speak to every sibling and all other relatives in order to be considered a ‘competent’ attorney.” True. But counsel’s inadequacy here was not a failure to interview all members of petitioner’s family, but the failure to interview any family member. As the referee found, “nothing, absolutely nothing of a competent nature was done by way of penalty phase preparation by defense counsel in this case.”
Petitioner does not challenge the adequacy of defense counsel’s representation at the guilt phase of the capital trial; indeed, given the strong evidence of guilt, it is unlikely that he could show that any inadequacy at the guilt phase was prejudicial. Nevertheless, in evaluating the adequacy of counsel’s representation at the penalty phase, it is significant that counsel’s representation of petitioner at the guilt phase was minimal at best. For instance, defense counsel filed no written motions on petitioner’s behalf before, during, or after the trial. Defense counsel’s cross-examination of prosecution witnesses was generally brief; the defense called no witnesses at the guilt phase. On several occasions, the defense either failed to object or made inadequate objections to arguably inadmissible evidence offered by the prosecutor at the guilt phase. (See People v. Champion (1995) 9 Cal.4th 879, 914, 918-919, 923 [39 Cal.Rptr.2d. 547, 891 P.2d 93].)
It may well be that a more strenuous defense effort at the guilt phase would have been futile. When the evidence leaves little doubt of a defendant’s guilt in a particular case, it is not unreasonable for a defense attorney to offer little evidence at the guilt phase and to focus instead on the presentation of evidence in mitigation at the penalty phase. But in that situation, the *224need to offer significant evidence in mitigation is particularly acute.3 Here, defense counsel did little at either the guilt or the penalty phase of petitioner’s capital trial.
The Attorney General asserts that trial counsel had sound tactical reasons for presenting no mitigating evidence at the penalty phase. This argument is flawed at its outset, because it assumes that trial counsel had conducted an investigation that provided an adequate basis on which to make an informed tactical decision whether to present such evidence. As the referee found, and as the evidence at the reference hearing showed, trial counsel undertook no such investigation; thus counsel was in no position to make the tactical decision that the Attorney General seeks to attribute to counsel. As this court has explained: “ ‘Unless a minimally adequate investigation is undertaken, it is impossible to make a tactical decision about whether to present or withhold mitigating evidence at the penalty phase.’ [¶] ... In some cases, counsel may reasonably decide not to put on mitigating evidence, but to make that decision counsel must understand what mitigating evidence is available .... [Counsel’s] decision not to find out, and to offer nothing in mitigation, cannot be supported as a tactical choice.” (In re Marquez, supra, 1 Cal.4th at p. 606.)
Even if trial counsel’s investigation could somehow be characterized as adequate, counsel could not, on the facts of this case, have reasonably decided to offer no evidence at the penalty phase.
California’s death penalty law requires that the jury at the penalty phase —in choosing between the alternative penalties of death and life imprisonment without possibility of parole—“be guided by the aggravating and mitigating circumstances” and “impose a sentence of death if [it] concludes that the aggravating circumstances outweigh the mitigating circumstances.” (§ 190.3, final par.) Under this system, therefore, if the evidence submitted to the penalty jury shows only aggravating circumstances, with no counterbalancing mitigating circumstances, the jury essentially has no option other than to return a verdict of death. There can be no tactical advantage to the defense in putting the penalty jury in that situation, as petitioner’s trial counsel did here.
At the guilt phase of petitioner’s capital trial, the jury convicted petitioner of three counts of first degree murder, five counts of robbery, two counts of *225burglary, and one count of rape in concert. There was nothing mitigating about the circumstances of these crimes, with the possible exception that no evidence identified petitioner as the person who fired the fatal bullets. At the start of the penalty phase, the jury knew nothing about petitioner other than that he had committed these crimes and that he was a member of a juvenile gang. At the close of the penalty phase, the only additional information presented to the jury was that petitioner had committed another criminal offense that was the subject of an earlier, unrelated proceeding. With this massive accumulation of evidence in aggravation, it was essential that defense counsel give the penalty jury a reason to spare petitioner’s life.
In general, the objective of defense counsel at the penalty phase must be to “portray the defendant as a human being with positive qualities” and to “attempt to convince the sentencer that the defendant has redeeming qualities.” (Goodpaster, The Trial for Life: Effective Assistance of Counsel in Death Penalty Cases (1983) 58 N.Y.U. L.Rev. 299, 335, fn. omitted.) “A true advocate cannot permit a capital case to go to the sentencer on the prosecution’s one-sided portrayal alone and claim to be rendering effective assistance.” (Ibid.) The task of defense counsel is “to gather and present at least some evidence of the defendant’s background which might serve to explain the defendant’s crimes and elicit a compassionate response from the sentencer.” (Id. at p. 336.)
What to offer as mitigating evidence may depend on what the prosecution can be expected to present in rebuttal of that evidence. Thus, as the Attorney General in this case points out, there may be tactical reasons for a defense attorney not to present certain witnesses, or not to offer evidence of particular incidents in the defendant’s life or particular traits of the defendant’s character. But the availability of rebuttal evidence can never justify a defense attorney’s decision to present no mitigating evidence at all, when the result is to leave the penalty jury with only one possible penalty verdict— death.4
In short, here the evidence at the reference hearing amply supports the referee’s conclusion that trial counsel’s investigation and presentation of evidence at the penalty phase of petitioner’s capital case was totally inadequate, and was unsupported by any tactical justification.
*226V. Prejudice to Petitioner
The majority concludes that any inadequacy by defense counsel at the penalty phase of petitioner’s capital trial did not prejudice petitioner. This conclusion appears to be based on three grounds: (1) that the effectiveness of petitioner’s evidence in mitigation would, if presented at trial, have been substantially weakened by the prosecutor’s evidence in rebuttal; (2) that defense counsel presented a persuasive argument in mitigation to the jury, and that the mitigating evidence not presented would thus be relatively unimportant; and (3) the prosecution’s case in aggravation was so strong that petitioner’s evidence in mitigation would have been insufficient to overcome it. As I shall explain, none of these arguments is convincing.
A. Impeachment and Rebuttal
The majority asserts that the mitigating evidence petitioner offered at the reference hearing could not have altered the outcome of petitioner’s capital trial because at trial the presentation of that evidence would have been subject to “substantial impeachment and potentially devastating rebuttal.” (Maj. opn., ante, p. 205.) Not so.
According to the majority, the force of the mitigating evidence that petitioner’s stepfather subjected him to abusive treatment would have been weakened considerably by a psychiatric report that was prepared when petitioner was 15 years old, and included a statement by petitioner that his stepfather had never physically abused him. Petitioner’s proof of the abuse, the majority states, would have been further impaired by the fact that his mother, who left petitioner’s abusive stepfather when petitioner was a teenager, later reconciled with petitioner’s stepfather. In my view, this “impeachment” is so weak as to be virtually meaningless. The psychiatric report was prepared for a juvenile court judge, following petitioner’s confinement at a juvenile camp where he had been placed for approximately six months. It is hardly surprising that after being unable to go home for many months, a 15-year-old teenager would deny the existence of any problems that might cause a judge to deny him the opportunity to return to his family. Nor should special significance be attached to the reconciliation of petitioner’s mother with her abusive husband; tragically, such reconciliations are commonplace in our society.
The majority’s reliance on this “impeachment” might have some merit if proof of the abuse depended solely on the testimony of petitioner and his mother. But at the reference hearing there were numerous other witnesses— *227petitioner’s brothers, sisters, aunts, and uncle—who testified to the physical and mental abuse to which petitioner was subjected as a child. The referee found these relatives to be credible witnesses. Their testimony is in no way impeached by petitioner’s statements to the psychiatrist or by the reconciliation of petitioner’s mother with his stepfather.
The majority asserts that if, at the penalty phase of his capital trial, petitioner had presented the mitigating testimony offered at the reference hearing, the prosecution would have been able to rebut that testimony with evidence that petitioner had engaged in criminal acts as a teenager. This rebuttal, the majority states, would have included three sustained petitions alleging burglary involving theft of guns, one sustained petition alleging four counts of robbery, and one sustained petition alleging that petitioner brandished a cooking fork. Assuming for the sake of argument that evidence of petitioner’s juvenile delinquency would have been admissible,5 it would have had little weight. The jury already knew (from evidence presented at the guilt phase of petitioner’s capital trial) that in his teenage years petitioner was a gang member and that at the age of 18 he had shot another gang member, Mark Howard. Thus, evidence that petitioner had engaged in additional delinquent acts as a teenager would not have added significantly to the jury’s understanding of petitioner’s character. Petitioner’s presentation at the reference hearing suggested that the brutal physical abuse he suffered as a child at the hands of his stepfather had a profound adverse affect on his initial kindly disposition. Evidence of petitioner’s criminal conduct as a teenager would have done little to rebut this argument.
B. Counsel’s Penalty Phase Argument
The majority examines at some length “the record of what counsel did do at trial.” Petitioner’s trial attorneys, the majority asserts, did not “give up”; *228rather, they “had a specific tactical approach,” and “gave the penalty jury a reason to spare defendant’s life.” (Maj. opn., ante, p. 209.) The majority reasons that Attorney Lenoir did such a good job at the penalty phase that his performance “would have justified a decision not to use the additional evidence presented at the reference hearing even after full investigation” and thus that counsel’s performance “must be considered in deciding whether it is reasonably probable the result would have been different had the evidence been presented.” (Maj. opn., ante, p. 211.) I cannot agree with the majority that trial counsel made an effective presentation at the penalty phase of petitioner’s capital case, or that counsel gave the jury a reason to spare petitioner’s life.
The majority points to evidence that defense counsel presented (through a stipulation with the prosecutor) that Mark Howard, who was shot by petitioner when petitioner was 18 years old (see pt. I, ante), was a “former gang member.” I do not see how Howard’s gang membership would play a significant role in persuading the jury to spare petitioner’s life: unless one regards an assault on a former gang member as commendable behavior, this evidence did not mitigate-, that is, the jury would not consider it as an affirmative reason not to impose the death sentence. At best, it slightly weakened a portion of the prosecution’s case in aggravation.
According to the majority, defense counsel gave the jury a reason to spare petitioner’s life when the defense, through stipulation, revealed that codefendant Malett received a life sentence for murdering Michael Taylor, and that Malett’s jury found that Malett used a firearm in the commission of that crime. In his closing argument to the jury, petitioner’s trial counsel argued that this finding by Malett’s jury showed that Malett, not petitioner, was the actual killer of Michael Taylor and that it would be unjust for the jury to sentence petitioner to death when Malett had not received a similar sentence. But as the prosecutor effectively emphasized in his closing argument, Malett’s jury found not true an allegation that Malett “personally” inflicted great bodily injury on the victim. That finding indicated that Malett’s jury was not convinced that Malett was the actual killer. Moreover, Malett was convicted only of the murder of Michael Taylor, while petitioner was convicted of murdering not only Taylor but also Bobby and Eric Hassan. Under these circumstances, the jury deciding petitioner’s penalty may well have found no unfairness in Malett’s receiving a lighter sentence than petitioner. Therefore, contrary to the majority’s assertion, it cannot be said that codefendant Malett’s sentence provided the jury with a reason to spare petitioner’s life.
In any event, as the majority acknowledges, this court has held that evidence about the fate of codefendants in a capital case is irrelevant, *229because it has no bearing on “the character and record of the defendant” that form the basis for determining whether a person convicted of capital murder should be sentenced to death. (People v. Belmontes (1988) 45 Cal.3d 744, 811 [248 Cal.Rptr. 126, 755 P.2d 310].) And here, in closing argument to the jury, the prosecutor pointed out that codefendant Malett’s sentence was irrelevant to the penalty determination: “The fact is that it’s not the test for your purposes as to whether or not this case is more or less aggravated than some other case.”
The trial court in this case properly instructed the jury at the penalty phase on the factors it was entitled to consider in deciding whether to sentence petitioner to death.6 Because codefendant Malett’s life sentence had no bearing on any of those factors, we must assume that the jury gave it little or no weight.
Finally, the majority notes that defense counsel mentioned to the jury petitioner’s age and that counsel made a closing argument at the penalty phase. But neither petitioner’s age (21 when he committed the murders) nor defense counsel’s closing argument gave the jury a plausible reason to spare petitioner’s life. Counsel’s argument was short (it encompasses only 10 pages of transcript, 4 of which are mostly blank) and, in my view, not particularly persuasive.7
C. Evidence in Aggravation
The majority considers any ineffectiveness by petitioner’s trial counsel at the penalty phase harmless in light of the strong aggravating evidence presented by the prosecution. I disagree. In my view, the majority’s position is inconsistent with previous decisions by this court and by the federal courts.
*230I do not wish to downplay the seriousness of petitioner’s crimes; as the majority has pointed out, the circumstances of the offenses constituted strong aggravating evidence. It may well be that even if trial counsel had conducted an adequate investigation and presented to the jury at the penalty phase the evidence in mitigation heard by the referee in this case, the jury would nevertheless have returned a verdict of death. But that should not be the focus of our inquiry. The United States Supreme Court has said that to obtain relief on the ground that trial counsel was ineffective, a defendant need not show that it was more likely than not that a different result would have occurred if counsel had given the defendant effective representation. (Strickland v. Washington, supra, 466 U.S. at p. 693 [80 L.Ed.2d at p. 697].) Rather, the defendant need only show a “reasonable probability” that counsel’s ineffective representation affected the result in the case; that is, a probability “sufficient to undermine confidence in the outcome.” (Id. at p. 694 [80 L.Ed.2d at p. 698].)
This case is strikingly similar to In re Marquez, supra, 1 Cal.4th 584, in which this court recently reversed a death judgment because of defense counsel’s failure to present any mitigating evidence at the penalty phase. There, the jury convicted the defendant of multiple counts of murder and robbery, arising out of two unrelated killings. At the penalty phase of the trial, the defense counsel, as a result of inadequate investigation, presented no mitigating evidence on the defendant’s behalf. The defendant filed a petition for writ of habeas corpus; we issued an order to show cause and an order of reference. At the reference hearing, the defendant presented the testimony of family members who would have been willing to appear as witnesses on the defendant’s behalf at the penalty phase of his criminal trial. These witnesses testified to the defendant’s good character as a child; they also mentioned that the defendant had assisted his mother and sisters with “women’s work,” saved a calf from slaughter, and at the age of eight worked to contribute income to his family. In reversing the judgment of death, this court pointed out that the mitigating evidence “was substantial, and not cumulative to any evidence offered at trial” and would have permitted the jury “to make an ‘individualized’ decision, one based not only on the facts of the crime, but on the whole life of the defendant.” (Id. at p. 609.)
Here, as in Marquez, supra, 1 Cal.4th 584, petitioner committed multiple acts of robbery and murder. Here, as in Marquez, defense counsel, as a result of his inadequate investigation, offered no relevant evidence in mitigation at the penalty phase of the trial. Here, as in Marquez, the mitigating evidence that petitioner offered at the reference hearing pertained to his character as a child—he was kind and helpful, and assisted in caring for younger family members. But here, unlike Marquez, there was also additional mitigating *231evidence available: at the reference hearing, numerous witnesses testified that petitioner suffered childhood abuse at the hands of his alcoholic stepfather, testimony that the referee found credible.
I note that similar evidence presented at reference hearings has led to the reversal of death judgments for ineffective assistance of counsel in several federal cases that were discussed in this court’s decision in In re Marquez, supra, 1 Cal.4th 584. For instance, in Thomas v. Kemp (11th Cir. 1986) 796 F.2d 1322, 1325, two of the defendant’s high school teachers testified at the reference hearing that he had a “difficult home environment,” had suffered “mental and physical abuse,” and that his mother had a “drinking problem.” The federal reviewing court held that trial counsel's failure to offer this mitigating evidence together with evidence of the defendant’s good character eliminated “the key aspect of the penalty trial,” thus preventing the jury from taking into consideration the personal characteristics of the defendant as is necessary for an “individualized” penalty determination. (Ibid.) And in Pickens v. Lockhart (8th Cir. 1983) 714 F.2d 1455, the federal court of appeals reversed the defendant’s death sentence, based on the defense attorney’s failure to offer evidence of the defendant’s “turbulent family background, beatings by a harsh father, and emotional instability.” (Id. at p. 1466.) Additionally, in Armstrong v. Dugger (11th Cir. 1987) 833 F.2d 1430, and in Blake v. Kemp (11th Cir. 1985) 758 F.2d 523, the federal reviewing court held that the defense counsel’s failure to offer any mitigating evidence at the penalty phase of a capital trial established ineffective representation and required reversal of the judgment of death.
More recent federal decisions are in accord. Where defense counsel offered some mitigating evidence at either the guilt or the penalty phase of a capital trial, federal courts have sometimes treated the failure to offer additional mitigating evidence as either harmless or as tactically justified. But where defense counsel presented no evidence in mitigation, and the defendant later showed in habeas corpus proceedings that trial counsel could have presented a plausible case in mitigation, federal courts have generally held such failure to be ineffective representation requiring reversal of the death judgment. (Jackson v. Herring (11th Cir. 1995) 42 F.3d 1350; Cave v. Singletary (11th Cir. 1992) 971 F.2d 1513; Deutscher v. Whitley (9th Cir. 1991) 946 F.2d 1443; Blanco v. Singletary (11th Cir. 1991) 943 F.2d 1477; Horton v. Zant (11th Cir. 1991) 941 F.2d 1449; Kenley v. Armontrout (8th Cir. 1991) 937 F.2d 1298; Harris v. Dugger (11th Cir. 1989) 874 F.2d 756; Middleton v. Dugger (11th Cir. 1988) 849 F.2d 491; Ford v. Lockhart (E.D.Ark. 1994) 861 F.Supp. 1447; Eutzy v. Dugger (N.D.Fla. 1989) 746 F.Supp. 1492.)
*232The majority states that in People v. Mayfield (1993) 5 Cal.4th 142 [19 Cal.Rptr.2d 836, 852 P.2d 331], this court found trial counsel’s ineffectiveness to be harmless under facts similar to those in this case. I dissented from the majority’s conclusion in Mayfield that counsel’s ineffectiveness was harmless. In any event, Mayfield is readily distinguishable. In Mayfield, defense counsel called a psychologist who “gave evidence of [the defendant’s] background, medical difficulties, character and remorse.” (Id. at p. 202.) Here, by contrast, trial counsel did not call a single witness to testify on petitioner’s behalf at the penalty phase. As a consequence, the jury was given no information to assist in its penalty determination. To borrow a phrase from Tyler v. Kemp (11th Cir. 1985) 755 F.2d 741, 745: “The death penalty that resulted in this case was thus robbed of the reliability essential to assure confidence in that decision.”
Conclusion
Recently, in vacating a judgment of death because of ineffective assistance of counsel, based on counsel’s failure to present any mitigating evidence at the penalty phase of the trial, this court stated: “[T]he mitigating evidence uncovered in the habeas corpus investigation . . . was substantial, and not cumulative to any evidence offered at trial. ... It would, in short, permit the jury to make an ‘individualized’ decision, one based not only on the facts of the crime, but on the whole life of the defendant.” (In re Marquez, supra, 1 Cal.4th at p. 609) The same is true here.
As in In re Marquez, supra, 1 Cal.4th 584, defense counsel in this case failed to investigate and to present at the penalty phase of petitioner’s capital trial available mitigating evidence about petitioner’s background and character. As a result, the jurors deciding petitioner’s fate knew him only as a criminal whom they had just convicted of serious crimes, including the murder of three persons. Had petitioner’s trial counsel presented to the jury at the penalty phase a more complete picture of petitioner as a human being —including the physical abuse petitioner had endured and the positive character traits he had shown as a child—the jury might have found sufficient redeeming qualities to elicit a compassionate response and might have returned a verdict of life in prison without parole, rather than death. As this court has explained, a penalty juror may properly conclude that a single mitigating circumstance outweighs all the aggravating evidence. (People v. Berryman (1993) 6 Cal.4th 1048, 1099 [25 Cal.Rptr.2d 867, 864 P.2d 40].)
I do not say that a penalty verdict other than death would have been certain, or even more likely than not. I conclude only that on the facts of this case, in which the jury at the penalty phase of petitioner’s capital trial *233received none of the significant mitigating evidence that existed and that defense counsel could have unearthed without too much effort, my confidence in the verdict of death has been undermined to the point that, in my view, the judgment of death should not be permitted to stand.
Petitioner’s application for a rehearing was denied July 12, 1995. Mosk, J., and Kennard, J., were of the opinion that the petition should be granted.
All statutory references are to the Penal Code.
Gray testified that when she came to court at Brown’s request, a woman (possibly Attorney Harris) took her name and telephone number, saying that the defense would contact her if she was needed. Gray was never called. Brown testified that she attended trial frequently, but she was unsure whether she came to court during the penalty phase. I note that it took only two days to present the penalty phase testimony, which included the prosecution’s aggravating evidence against defendant and codefendant Champion and the mitigating testimony offered by counsel for codefendant Champion.
In cases in which the evidence of guilt is relatively weak, defense counsel may choose not to present marginally mitigating evidence in favor of an argument stressing a “lingering doubt” regarding the defendant’s guilt. Such tactical decisions can, of course, be made only after an adequate investigation to learn whether mitigating evidence exists that may be offered on the defendant’s behalf.
I do not assert that competent defense counsel must always present mitigating evidence at the penalty phase. In some cases, the circumstances of the crime itself may be sufficiently mitigating that counsel may elect not to divert the jury’s focus with other evidence. In other cases, the presentation of mitigating evidence at the guilt phase of trial may obviate the need for additional mitigating evidence at the penalty phase. Here, however, the circumstances of the crimes committed by petitioner were not mitigating, and trial counsel offered no mitigating evidence at the guilt phase.
It is unclear whether the trial court would have admitted this evidence. Evidence of a defendant’s commission of violent criminal acts is generally admissible as part of the prosecution’s case-in-chief at the penalty phase of a capital trial. At petitioner’s trial, however, the trial court excluded the evidence that petitioner had engaged in violent conduct as a juvenile on the ground that the prosecution had failed to give the defense adequate notice of its intent to use this evidence. Notwithstanding the trial court’s conclusion that this evidence was inadmissible as part of the prosecution’s case-in-chief, the court could have permitted the prosecution to use this evidence in rebuttal, but only if the court found that the evidence related directly to a particular incident or character trait that petitioner had offered on his own behalf. (People v. Ramirez (1990) 50 Cal.3d 1158, 1193 [270 Cal.Rptr. 286, 791 P.2d 965].) Here, the evidence of petitioner’s violent criminal acts could not have been used to rebut the most important evidence offered by petitioner at the reference hearing—that he was subjected to physical and mental abuse by his stepfather—but it might have been marginally relevant to rebut the evidence that, as a child, petitioner was loving and protective of his cousins and younger sister.
The trial court read to the jury former CALJIC No. 8.84.1 (4th ed. 1979), which sets forth the applicable aggravating and mitigating factors. The instruction is set forth in full in this court’s opinion affirming petitioner’s conviction on appeal. (People v. Champion, supra, 9 Cal.4th at p. 943, fn. 32.)
In his closing argument, defense counsel argued that petitioner would adjust well to prison if sentenced to life imprisonment without possibility of parole, pointing out that the record contained no evidence that petitioner had ever misbehaved while in confinement. The majority asserts that counsel could not have made this argument if petitioner had presented the mitigating evidence offered at the reference hearing, because the prosecution would, in rebuttal, have presented evidence that as a teenager petitioner had misbehaved in confinement, when he brandished a large serving fork at a cook in a juvenile camp in which he had been ordered placed. I find it difficult to imagine that a jury, having concluded that petitioner would adapt well to a prison setting and that it should spare petitioner’s life, would be dissuaded from this conclusion upon learning of the “fork brandishing” incident.