RICHARD C. TALLMAN, Circuit Judge:
Demetrie Ladon Mayfield murdered Ora Mae Pope on February 2, 1983, to exact revenge from Ms. Pope and her son, Byron, who had sworn out a complaint against Mayfield for auto theft. He then killed John Moreno to eliminate the only eyewitness to the crime. A San Bernardi-no County jury convicted Mayfield of two counts of first degree murder and found a multiple murder special circumstance to be true. After a separate penalty hearing, [919]*919the jury recommended that Mayfield be put to death.
Every court that has reviewed this case has upheld the conviction and sentence. The California Supreme Court affirmed Mayfield’s death sentence on direct appeal and, after referring several questions to a superior court judge who served as a referee at an extensive evidentiary hearing, it denied Mayfield’s state habeas corpus petition. People v. Mayfield, 5 Cal.4th 142, 19 Cal.Rptr.2d 836, 852 P.2d 331 (1993). The United States Supreme Court denied certiorari. Mayfield v. California, 512 U.S. 1253, 114 S.Ct. 2780, 129 L.Ed.2d 892 (1994). Mayfield then filed a federal habe-as corpus petition in the United States District Court for the Central District of California. The district court denied May-field’s petition, and a three-judge panel of this Court affirmed. Mayfield v. Calderon, 229 F.3d 895 (9th Cir.2000).
We voted to reconsider en banc the claims raised by Mayfield in his federal habeas corpus petition. Mayfield raises seven claims, but ineffective assistance of counsel at the guilt and penalty phases are the predominant ones. He claims that the jury instructions improperly prevented the jury from considering (1) sympathy for Mayfield, (2) the consequences of their verdict, and (3) mitigating evidence not related to the crime. He also asserts that (4) the 1978 California death penalty statute under which he was convicted and sentenced is unconstitutional. Finally, he alleges that his attorney provided ineffective assistance (5) due to conflicts of interest arising out of racial prejudice and concern for reputation, (6) at the guilt phase, and (7) at the penalty phase. With regard to each claim, we must determine whether to grant a certificate of appealability (“COA”). If we grant COAs for any of Mayfield’s claims, we must then address the merits of those claims.
We deny COAs as to claims one through five. We grant COAs as to claims six and seven, alleging ineffective assistance of counsel at the guilt and penalty phases of trial. We agree with the courts that have already reviewed this case that, regardless of whether the performance of Mayfield’s counsel at the guilt phase was deficient, Mayfield suffered no prejudice at the guilt phase. Accordingly, we affirm the district court’s denial of Mayfield’s claim for ineffective assistance at the guilt phase and leave undisturbed the jury’s verdict convicting Mayfield of two counts of first degree murder with special circumstances.
We disagree, however, with those courts’ determinations that Mayfield received effective assistance of counsel at the penalty phase. We hold that the performance of Mayfield’s counsel at the penalty phase was deficient and that Mayfield suffered prejudice as a result. We reverse the district court’s denial of Mayfield’s claim for ineffective assistance at the penalty phase and remand to the district court to grant Mayfield’s habeas corpus petition on this ground. The state court shall conduct a new sentencing proceeding to determine whether Mayfield is to be sentenced to death or to life without parole.
I
To understand the events surrounding Mayfield’s crime and punishment, we provide an overview of the events which led to the murders in 1983.1
[920]*920Demetrie Mayfield lived with Robert Wafer next door to the Pope family in the San Bernardino housing projects during much of 1982. He borrowed Wafer’s 1968 Pontiac so frequently that he had his own key. Wafer left town, abandoning his house and car.
In December 1982, the car was repossessed because of delinquent payments. Ora Mae Pope arranged to take over the payments and to purchase the car for her son, Byron. Byron was in possession of the car for only three days before it disappeared from the Popes’ driveway. Byron reported the car stolen.
Mayfield and two of his friends were arrested in the car a day later. Mayfield pled guilty to one count of unlawful taking of a vehicle and was released pending a sentencing hearing. He was told that he would be sentenced to one year in jail. He did not appear for his sentencing hearing on the scheduled date.
On the evening of February 2, 1983, Mayfield was at Pat Harper’s house, less than a block from the Pope residence. He had stayed with Harper sporadically since his mother had ordered him out of her house. About 11:00 p.m. he walked over to the Pope residence. He stood outside the living room window, where he overheard Ms. Pope and John Moreno talking about him. He returned to Harper’s house and told her that he did not like what they were saying about him and that “he was going to show them.”
Mayfield armed himself with a .12-gauge, sawed-off, single-shot shotgun,2 and two shotgun shells, and went back to the Pope residence. Using a screw driver he also carried from Harper’s house, he removed a screen window from the back of the Pope residence, climbed into the house, and loaded the first shell into the shotgun. He crept down the hall and turned into a small living room, confronting Ms. Pope and Moreno, who were sitting on the couch drinking, smoking, and talking. Mayfield sat on the arm of another couch, approximately five feet from Ms. Pope, with the shotgun leveled at her.
During a fifteen- to twenty-minute verbal confrontation regarding the auto theft charges, Mayfield kept his finger resting on the trigger. According to his audio-taped confession and videotaped re-enactment of the crime, Mayfield claimed that Ms. Pope suddenly stood up from the couch to light a cigarette. Mayfield insisted he “thought she was coming at [him]” and jerked back reflexively, accidentally pulling the shotgun’s “hair” trigger3 and shooting her. Mayfield broke open and re-loaded the shotgun. He then shot Moreno because he had witnessed the crime.
Mayfield retrieved the spent shotgun shells from the floor, took the keys to the Pope residence from a table in the back hall, locked the back door, and returned to Harper’s house. He told Harper, “I did it. I didn’t mean to. It slipped. It was an accident. And then I had to do the second one.” He concealed the shotgun in a torn [921]*921couch cushion in a storage room off the side of her house. Then he returned to the Pope residence.
Mayfield replaced the screen window. He dragged the bodies of Ms. Pope and Moreno from the living room, out the back door, across a short cement walk, and into a storage closet accessible from outside. He used a garden hose to wash their blood from the walk. After retrieving a kitchen knife, which he wrapped hidden in a towel, Mayfield locked the door and waited outside for Byron to come home.
When Byron arrived, Mayfield confronted him, and the two wrestled.4 Mayfield refused to let Byron into his house. He demanded to know why Byron and his mother had pressed charges against him for the car theft and why Byron had been bad-mouthing him around the neighborhood. A mutual friend drove by during the confrontation and stopped to intercede. The friend suggested that Byron simply drop the charges, but Mayfield said it was too late for that. At an impasse and unable to get into his house, Byron and his friend left. Mayfield went to his mother’s home, where he slept in the garage.
The police arrested him there the next morning. After providing blood and urine samples at the hospital, Mayfield was taken to the San Bernardino police station for questioning. After administering Miranda warnings and obtaining a waiver of his constitutional rights, two detectives interviewed Mayfield on audiotape. May-field initially denied any knowledge of, or involvement in, the crime. When confronted with the evidence that police had already gathered against him, Mayfield confessed. Following his confession, Mayfield explained the circumstances of his crime in detail.
After the audiotaped interview, the police asked Mayfield to participate in a videotaped re-enactment of the crime. He agreed. The video shows Mayfield at the crime scene the following day, clad in orange jail coveralls, approaching the house with the shotgun, removing the screen window, confronting Ms. Pope and Moreno (played by plain-clothes police officers), pretending to shoot them, positioning them face down on the sofa, pretending to drag them out the door and across the walk, and placing them atop one another in the storage closet.
On February 10, 1983, Donald S. Ames was appointed to represent Mayfield. After a three-day trial (exclusive of jury selection), the jury deliberated for two and one-half hours before convicting Mayfield of two counts of murder in the first degree with the special circumstance of committing multiple murders. A separate penalty hearing commenced three days later and, after a day and a half, the jury recommended that Mayfield be sentenced to death.
II
Mayfield’s appeal of the district court’s dismissal of his federal habeas corpus petition, initiated after the effective date of the Antiterrorism and Effective Death Penalty Act of 1996, Pub.L. No. 104-122, 100 Stat. 1214 (“AEDPA”), is governed by the COA requirements codified at 28 U.S.C. § 2253.5 We cannot re[922]*922view the merits of Mayfield’s claims unless we first determine with regard to each claim that Mayfield has made “a substantial showing of the denial of a constitutional right” justifying issuance of a COA. 28 U.S.C. § 2253(c) (2000); see also Petrocelii v. Angelone, 248 F.3d 877, 883 (9th Cir.2001) (“[Ejach issue sought to be appealed under AEDPA must be ruled on separately ... on the request for a COA.”); Morns v. Woodford, 229 F.3d 775, 779 (9th Cir.2000) (“Unlike a [Certificate of Probable Cause], which allows a party to appeal an entire petition, a COA is granted on an issue-by-issue basis.”), cert, denied, - U.S. -, 121 S.Ct. 2238, 150 L.Ed.2d 227 (2001).
The Supreme Court set forth the standard for issuance of a COA in Slack v. McDaniel, 529 U.S. 473, 120 S.Ct. 1595, 146 L.Ed.2d 542 (2000). Where, as here, the district court denies a habeas corpus petition on the merits, rather than on procedural grounds, “[t]he petitioner must demonstrate that reasonable jurists would find the district court’s assessment of the constitutional claims debatable or wrong.” Id. at 484, 120 S.Ct. 1595. “[T]he nature of the penalty is a proper consideration in determining whether to issue a certificate of [appealability].” Barefoot v. Estelle, 463 U.S. 880, 893, 103 S.Ct. 3383, 77 L.Ed.2d 1090 (1983); Slack, 529 U.S. at 483, 120 5.Ct. 1595 (“Except for substituting the word ‘constitutional’ for the word ‘federal,’ § 2253 is a codification of the CPC standard announced in Barefoot ... ”). Accordingly, we resolve any doubt regarding whether to issue a COA in favor of May-field. See Petrocelii, 248 F.3d at 884; Lambright v. Stewart, 220 F.3d 1022, 1025 (9th Cir.2000).
Pre-AEDPA standards of review govern our consideration on the merits because Mayfield filed his habeas corpus petition prior to AEDPA’s effective date. Dubria v. Smith, 224 F.3d 995, 1000 n. 1 (9th Cir.2000) (en banc), cert, denied, 531 U.S. 1148, 121 S.Ct. 1089, 148 L.Ed.2d 963 (2001); see also Slack, 529 U.S. at 481, 120 S.Ct. 1595 (noting that the date the habeas corpus petition was filed determines whether the pre-AEDPA or post-AEDPA version of § 2254 applies). We presume that the state court’s findings of historical fact are correct and defer to those findings “in the absence of ‘convincing evidence’ to the contrary” or a demonstrated lack of “fair support in the record.” 28 U.S.C. § 2254(d) (1994); Marshall v. Lonberger, 459 U.S. 422, 432, 103 S.Ct. 843, 74 L.Ed.2d 646 (1983). We review mixed questions of law and fact, such as whether Mayfield received ineffective assistance of counsel, de novo. Dubria, 224 F.3d at 1000. Finally, we review pure questions of law de novo. Schiro v. Farley, 510 U.S. 222, 232, 114 S.Ct. 783, 127 L.Ed.2d 47 (1994).
A
Mayfield argues that by instructing the jury during the penalty phase to consider such guilt phase instructions as it found applicable, the judge created a risk that the jury would apply to its penalty phase deliberations the guilt phase instruction that the jury “not be swayed by ... sympathy.” 6 The risk was exacerbated, he claims, by the prosecutor’s penalty phase [923]*923closing argument that the jury should not determine Mayfield’s penalty “based on any sympathies ... either for defendant or against him.” He urges that the guilt and penalty phase instructions, combined with the prosecutor’s argument, violated the Eighth and Fourteenth Amendments by preventing the jury from considering sympathy for Mayfield in determining his sentence.
The Supreme Court has admonished that state jury instructions must be upheld against constitutional attack unless “there is a reasonable likelihood that the jury has applied the challenged instruction in a way that prevents the consideration of constitutionally relevant evidence.” Boyde v. California, 494 U.S. 370, 380, 110 S.Ct. 1190, 108 L.Ed.2d 316 (1990). At the penalty phase of a capital trial, this directive requires that “the sentencer ... be able to consider and give effect to mitigating evidence in imposing the sentence, so that the sentence imposed reflects a reasoned moral response to the defendant’s background, character, and crime.” Penry v. Johnson, 532 U.S. 782, 121 S.Ct. 1910, 1916, 150 L.Ed.2d 9 (2001) (quotations and citations omitted).
Applying these standards, federal courts have consistently held that jury instructions admonishing the jury to base its penalty determination on mitigating or aggravating evidence, not on sympathy for the defendant, pass constitutional muster. See, e.g., Victor v. Nebraska, 511 U.S. 1, 13, 114 S.Ct. 1239, 127 L.Ed.2d 583 (1994) (holding that an instruction not to be swayed by mere sympathy correctly pointed the jurors’ attention to the evidence before them); Johnson v. Texas, 509 U.S. 350, 371-72, 113 S.Ct. 2658, 125 L.Ed.2d 290 (1993) (holding that the constitution does not require that a capital jury be able to dispense mercy solely on the basis of a “sympathetic response to the defendant”); California v. Brown, 479 U.S. 538, 542-43, 107 S.Ct. 837, 93 L.Ed.2d 934 (1987) (permitting an instruction that the jury could not base its sentencing decision on sympathy); Williams v. Calderon, 52 F.3d 1465, 1481 (9th Cir.1995) (observing that “ ‘no sympath/ instructions have been held by the Supreme Court to be consistent with its mandate ... that the sentencer be permitted to consider all mitigating evidence”) (citations omitted). In light of this precedent, no reasonable jurist could debate or find wrong the district court’s denial of Mayfield’s request for habeas corpus relief on this claim. We decline to grant a COA with regard to this issue.
B
Mayfield argues that by instructing the jury during the penalty phase to consider such guilt phase instructions as it found applicable, the judge created a risk that the jury would apply to its penalty phase deliberations the guilt phase instruction to reach "a just verdict regardless of what the consequences may be." We judge the challenged instruction "not h~ artificial isolation, but ... in the context of the overall charge." Boyde, 494 U.S. at 378, 110 5.Ct. 1190.
The jury was specifically instructed at the penalty phase that it was to reach its decision “guided by the applicable factors of aggravating and mitigating circumstances upon which [it had] been instructed.” The jury was instructed with regard to seven such aggravating or mitigating factors and told that it should also consider “[a]ny other circumstance which extenuates the gravity of the crime.” Finally, the jury was instructed that its objective in weighing the aggravating and mitigating circumstances was to determine whether Mayfield was to be sentenced to death or to life without parole.
[924]*924Viewing these instructions as a whole, it is evident that the jury was fully apprised of the consequences of its deliberations and that a reasonable juror would have construed the instructions as requiring him or her to consider all relevant mitigating evidence. Boyde, 494 U.S. at 378, 110 S.Ct. 1190. Because no reasonable jurist could debate or find wrong the district court’s denial of Mayfield’s request for ha-beas corpus relief on this claim, we decline to grant a COA on this issue.
C
Mayfield argues that, by instructing the jury to consider, in addition to seven defined aggravating or mitigating factors, “[a]ny other circumstance[ ] which extenuates the gravity of the crime even though it is not a legal excuse for the crime,” CALJIC 8.84.1(k), the judge prevented the jury from considering mitigating facts not associated with the crime itself, but associated instead with May-field’s background and character. The Supreme Court squarely rejected this argument in Boyde, holding that “there [was] not a reasonable likelihood that the jurors ... understood the challenged instructions to preclude consideration of relevant mitigating evidence offered by petitioner.” 494 U.S. at 386, 110 S.Ct. 1190. We therefore decline to grant a COA on this claim.
D
Mayfield argues that California’s death penalty scheme is unconstitutional because it does not adequately narrow the class of persons eligible for the death penalty. The 1978 death penalty statute pursuant to which Mayfield was convicted and sentenced narrows the class of persons eligible for the death penalty at both the guilt and the penalty phases.
A defendant is eligible for the death penalty under the 1978 statute only if, at the guilt phase, the jury finds him guilty of first degree murder and finds to be true a statutorily defined special circumstance. See Jurek v. Texas, 428 U.S. 262, 270-71, 96 S.Ct. 2950, 49 L.Ed.2d 929 (1976) (upholding capital punishment statute that required the jury to find at the guilt phase that the defendant’s crime fell within one of five statutory categories of death-eligible murder). At the penalty phase, the class of defendants eligible for death is again narrowed by the jury’s application of a series of statutorily enumerated aggravating or mitigating factors. See Blystone v. Pennsylvania, 494 U.S. 299, 307, 110 S.Ct. 1078, 108 L.Ed.2d 255 (1990) (holding statute sufficiently narrows the class of death-eligible defendants to survive constitutional scrutiny if it “allow[s] the jury to consider all relevant mitigating evidence”); Lowenfield v. Phelps, 484 U.S. 231, 246, 108 S.Ct. 546, 98 L.Ed.2d 568 (1988) (holding that a capital punishment statute is constitutional if it “broadly define[s] capital offenses and provide[s] for narrowing by jury findings of aggravating circumstances at the penalty phase”). A reasonable jurist could not debate, therefore, that the 1978 California statute, which narrowed the class of death-eligible defendants at both the guilt and penalty phases, was constitutional. We decline to grant a COA on this claim.
E
Mayfield argues that Ames’ racism and his concern that he not be perceived by the San Bernardino bar or bench as requesting too much funding prevented Ames from effectively representing Mayfield. Mayfield submitted in support of his federal habeas corpus petition six declarations indicating that Ames was racially prejudiced. Two of the declarations related racial epithets that Ames used in reference [925]*925to minority clients.7 None of the declarations alleged that Ames used racial epithets to describe Mayfield or that Ames’ alleged prejudice affected his representation of Mayfield.8
In order to establish ineffective assistance resulting from a conflict of interest, Mayfield must show “that an actual conflict of interest adversely affected his lawyer’s performance.” Cuyler v. Sullivan, 446 U.S. 335, 348, 100 S.Ct. 1708, 64 L.Ed.2d 333 (1980); Sanders v. Ratelle, 21 F.3d 1446, 1452 (9th Cir.1994). It is by no means clear from precedent that the grounds for conflict alleged by Mayfield are cognizable under ineffective assistance ease law.9 Even assuming that they are, Mayfield has not demonstrated that Ames performed poorly because of the alleged conflicts. Accordingly, we decline to grant a COA on Mayfield’s claim that Ames’ alleged conflicts of interest caused him to provide ineffective assistance of counsel.
F
To prevail on a claim of ineffective assistance at either the guilt or the penalty phase, Mayfield must demonstrate two components. “First, [he] must show that counsel’s performance was deficient.... Second, [he] must show that the deficient performance prejudiced the defense.” Strickland, 466 U.S. at 687, 104 S.Ct. 2052. We hold that reasonable jurists could debate the district court’s assessment of Mayfield’s claim for ineffective assistance of counsel at the guilt phase. Accordingly, we grant a COA on this claim and address the merits.
Under Strickland, “a court need not determine whether counsel’s performance was deficient before examining the prejudice suffered by the defendant as a result of the alleged deficiencies.... If it is easier to dispose of an ineffectiveness claim on the ground of lack of sufficient prejudice, ... that course should be followed.” 466 U.S. at 697, 104 S.Ct. 2052. We heed the Court’s advice and, without resolving the question whether Ames’ performance at the guilt phase was deficient, affirm Mayfield’s conviction of two counts of first degree murder with special circumstances because Mayfield suffered no prejudice as a result of the alleged deficiencies.
To demonstrate that he suffered prejudice as a result of Ames’ allegedly deficient performance, Mayfield must show that the probability that counsel’s deficient performance altered the outcome is “sufficient to undermine confidence in the outcome.” Id. at 694, 104 S.Ct. 2052.10 In light of the [926]*926strong inculpatory evidence presented by the State at trial, we find it unlikely that a competent performance by Ames would have altered the jury’s verdict. The State presented the motive, the murder weapon, and a videotape on which Mayfield first admitted committing the crime and then reenacted it. The State also presented two witnesses who had told police that Mayfield told them that he was going to shoot Ms. Pope days before the crime (although both witnesses recanted their statements at trial). By the conclusion of the State’s case-in-chief, there can have been no reasonable doubt in the jurors’ minds that Mayfield was guilty of two first degree murders. This may well have been one of those cases that was virtually indefensible. The formidable task of presenting a credible defense was compounded by Mayfield’s admission to his lawyer that he had gone to the Pope house that night intending to kill Ora Mae Pope.
We are confident that the evidence presented at the state evidentiary hearing would not have changed the jurors’ minds. Although Mayfield told police he had not used PCP the night of the crime, several witnesses testified that Mayfield chronically abused alcohol and PCP and that he smoked sherm (marijuana laced with PCP) “almost on a daily basis” just prior to the crime. A toxicologist and an endocrinologist testified at the evidentiary hearing that, as a result of Mayfield’s chronic PCP abuse and poorly regulated diabetes,11 he may have been experiencing any of a range of conditions the night of the crime, including “reduced judgment” and “decreased impulse control,” blurred vision, clumsiness, impaired thinking, nausea, and vomiting.
Mayfield also proved at the evidentiary hearing that when Ms. Pope drank she could become erratic and unpredictable. Byron Pope testified that if his mother had been drinking she might “cuss you out ... waive her hands around; you know, get excited” and “she would probably strike you.”
The strength of the State’s evidence of premeditation, the relative dearth of evidence supporting the defense theory of accident, and the brevity of the jury’s deliberations all indicate that Ames’ allegedly deficient performance did not alter the jury’s determination of guilt. The strength and unanimity of the jurors’ belief in Mayfield’s guilt is reflected by the fact that after a three-day trial the jury took scarcely more than two hours to find him guilty of both first degree murder charges and to find true the special circumstance. See Murtishaw v. Woodford, 255 F.3d 926, 974 (9th Cir.2001) (considering the length of jury deliberations as probative of whether improper jury instructions prejudiced the defense). Because Mayfield has not demonstrated that he was prejudiced by Ames’ allegedly deficient performance at the guilt phase, we hold that the district court did not err by denying his claim for ineffective assistance of counsel at that phase.
G
Mayfield’s claim that he received ineffective assistance of counsel at the penalty phase is also governed by the Strickland standard. To prevail, Mayfield must show both that Ames’ performance was deficient and that there is a reasonable probability that the jury would not have sentenced Mayfield to death if Ames had [927]*927performed effectively. See Strickland, 466 U.S. at 695, 104 S.Ct. 2052. We hold that reasonable jurists could debate the district court’s assessment of Mayfield’s claim for ineffective assistance of counsel at the penalty phase. Accordingly, we grant a COA on this claim and address the merits.
1. Ames’ Performance at the Penalty Phase
Ames’ performance at the penalty phase was deficient if, measured against “prevailing professional norms,” it “fell below an objective standard of reasonableness.” Strickland, 466 U.S. at 688, 104 S.Ct. 2052. In making this determination, we must avoid the temptation to second-guess Ames’ performance or to indulge “the distorting effects of hindsight.” Id. at 689, 104 S.Ct. 2052. Every intendment of competence is to be given to counsel. Id. We may find Ames’ performance deficient only if, viewing the situation from his perspective at the time of trial, his decisions cannot be characterized as “sound trial strategy.” Id. (quoting Michel v. Louisiana, 350 U.S. 91, 101, 76 S.Ct. 158, 100 L.Ed. 83 (1955)).
To perform effectively in the penalty phase of a capital case, counsel must conduct sufficient investigation and engage in sufficient preparation to be able to “presentí ] and explain[ ] the significance of all the available [mitigating] evidence.” Williams v. Taylor, 529 U.S. 362, 393, 399, 120 S.Ct. 1495, 146 L.Ed.2d 389 (2000). We hold Ames’ performance deficient because he neither adequately investigated and prepared for the penalty phase nor presented and explained the significance of all the available mitigating evidence to the jury.
In preparing to represent Mayfield at the penalty phase, Ames had an “obligation to conduct a thorough investigation of [Mayfield’s] background.” Williams, 529 U.S. at 396, 120 S.Ct. 1495. Judicial deference to counsel is predicated on counsel’s performance of sufficient investigation and preparation to make reasonably informed, reasonably sound judgments. See Strickland, 466 U.S. at 691, 104 S.Ct. 2052. The evidentiary referee found that it was “apparent ... that [Ames] never gave much thought to a penalty phase.” Ames billed only 40 hours in preparation for both the guilt and penalty phases of trial.12 Ames had only one substantive meeting with his client, the morning trial began, and did not discuss with him possible witnesses or trial strategies. He did not associate co-counsel to assist in May-field’s defense although, under Keenan v. Superior Court, 180 Cal.Rptr. 489, 640 P.2d 108 (1982), Mayfield was entitled to a second attorney.13 He spent less than half the defense investigation budget authorized by San Bernardino County.14 He did not consult experts in endocrinology or toxicology, even though his investigator’s limited efforts revealed evidence of diabetes and substance abuse, nor did he obtain all of Mayfield’s medical records. We agree with the district court’s observation that it was “deplorable that Ames did not [928]*928put forth more of an effort when his client’s life was at stake.”
His performance during the penalty phase reflected his lack of preparation. Ames waived his opening argument, his first opportunity to “explain the significance” of the mitigating evidence to the jury. Although the state evidentiary referee listed no fewer than eighteen potential witnesses, Ames called only one during the penalty phase. Dr. Craig Rath provided mitigating testimony regarding May-field’s family and childhood background, his health history including his diabetes, his work history, his psychiatric profile, and his substance abuse. Dr. Rath also shared with the jury a humanizing anecdote related to him by Mayfield’s Mend, Patricia Harper, that established that Mayfield could be a kind, generous human being. Dr. Rath, who interviewed May-field twice prior to the penalty phase, also told the jury that Mayfield had indicated considerable remorse for what he had done.
Ames did not call an endocrinologist to offer expert testimony regarding May-field’s diabetes or a toxicologist to offer expert testimony regarding his substance abuse. He mistakenly stipulated that Mayfield’s urine tested negative for POP the day after the crime, indicating to the jury both that Mayfield did not have a substance abuse problem and that May-field had lied about it.
Ames did not call Mayfield’s mother because he was afraid that her testimony would do more harm than good, given an incident in which Mayfield may have engaged in inappropriate sexual conduct with his infant sister when he was twelve. He did not call Mayfield’s uncle, a pastor and substance abuse counselor, because he mistakenly believed that he had a recent felony conviction. He did not present any of Mayfield’s other friends or family members simply because he did not expend the effort to locate and interview them. Ames also failed to explain to the jury the significance of the mitigating evidence presented by Dr. Rath during his closing argument, which the California Supreme Court described as “perfunctory.” People v. Mayfield, 19 Cal.Rptr.2d 836, 852 P.2d at 349.
In short, Ames did not, as Williams v. Taylor requires, adequately investigate and prepare for the penalty phase or present and explain to the jury the significance of all the available mitigating evidence.
2. Prejudice at the Penalty Phase
We do not read Williams as establishing a per se rule that we must reverse a death sentence if we find that counsel’s performance at the penalty phase was deficient. Instead, we read the opinion as saying that, applying the Strickland analysis, we must carefully weigh the mitigating evidence (both that which was introduced and that which was omitted or understated) against the aggravating evidence, Williams, 529 U.S. at 397, 120 S.Ct. 1495,15 and determine whether there was “a reasonable probability that, absent the errors, the sentencer — including an appellate court, to the extent it independently reweighs the evidence — would have concluded that the balance of aggravating and mitigating circumstances did not warrant death.” Strickland, 466 U.S. at 695, 104 S.Ct. 2052. After reweighing all the evi[929]*929dence, as required by these Supreme Court cases, we hold that if the jury had considered the mitigating evidence adduced at the state evidentiary hearing, in addition to that presented at trial, there is a reasonable probability that it might not have sentenced Mayfield to death.
The aggravating evidence against May-field was strong. Mayfield’s conduct in planning the commission of the crime evidenced premeditation and malice, motivated by revenge against Ms. Pope and her son, Byron. Mayfield had recently been informed by his probation officer that he could expect to be sentenced to one year in jail for the auto theft conviction. He had failed to appear at his sentencing hearing when he crept over to the Popes’ house under cover of night and, concealed below the living room window, overheard the victims talking about him. He swore he would get them. He armed himself with a .12-gauge, sawed-off shotgun and two shells, strategically placed in the elastic wristband of his jacket where they were readily retrievable. Using a screwdriver, he surreptitiously broke into the house through a bedroom window before confronting the victims.
The State played the videotaped confession and re-enactment for the jury at the guilt phase, depicting the gruesome circumstances of the shootings. After the murders, Mayfield meticulously cleaned up the scene of the crime, attempting to retrieve and dispose of incriminating evidence (picking up and throwing away spent shell casings, attempting to wipe fingerprints from the murder weapon, secreting it in a cushion at the nearby home of a friend, hiding the bodies in a storage area, and hosing blood off of the sidewalk). Then, armed with a knife, he lay in wait for the return of the remaining complainant, Byron Pope.
In addition to presenting the circumstances of the crime, the State presented in aggravation Wanda Griffin’s testimony that Mayfield fired a gun into her house after she separated from him and another ex-girlfriend’s testimony that Mayfield had struck her in the face.
Even in the face of this strong aggravating evidence, however, we must reverse Mayfield’s death sentence if we cannot conclude with confidence that the jury would unanimously have sentenced him to death if Ames had presented and explained all of the available mitigating evidence. See Williams, 529 U.S. at 368-69, 399, 120 S.Ct. 1495 (reversing a death sentence for ineffective assistance at the penalty phase despite strong aggravating evidence). We cannot so conclude on this record.
The mitigation evidence presented at trial through the testimony of Dr. Rath was substantial. Mayfield was born in 1961. He had four siblings. His parents “separated when he was a child.” May-field was diagnosed with diabetes at age nine. He had a hard time “accepting the idea that he was defective and diabetic.” Mayfield was hospitalized 20 to 30 times and never had his diabetes under very good control. Dr. Rath described May-field’s childhood as “all right, although there was growing tension with his mother as he got older to the point where there were disagreements and at least strong verbal altercations when he was in his early teens.”
Dr. Rath testified that Mayfield was “in the low average range of intelligence.” Mayfield had been diagnosed with a child behavioral disorder caused by depression. In his late teens, Mayfield began using POP two or three times a week. May-field’s mental state deteriorated because of drug usage. By the end of 1982, he was “using it basically on a daily basis.”
[930]*930Dr. Rath told the sentencing jury: “At the time of the offense, he apparently was living at home, although he would sporadically live with various girlfriends and so on.” Dr. Rath testified that Mayfield was not under the influence of drugs or alcohol the night of the crimes, which may have been based in part on his mistaken belief that Mayfield’s urine had tested negative for PCP.
Dr. Rath testified that Mayfield’s score was moderately elevated in a “psychopathic deviance” test; that “he has some social problems in terms of his relation to society and the people around him;” that he was “sort of lacking in emotion, emotionally constrictive, emotionally immature, but not psychotic;” and that he had a bad temper. He testified that whether Mayfield is a violent person was “a difficult question,” but that his conduct the night of the crime was “qualitatively different than most of his history.”
Dr. Rath testified that Mayfield “indicated considerable remorse [over the lives he had taken] in different ways at different times.” He further testified that Mayfield sometimes babysat for Pat Harper, who considered him a gentle person for whom the crimes were out of character, and that Mayfield had good rapport with the prison guards. Dr. Rath read for the jury the conclusion of Dr. Hunt, the neurologist, that “[although [Mayfield] has shown rather poor judgment in the past, particularly in caring for himself, the crime of which he is accused is out of character and can be explained only on the basis of definite cerebral impairment due to alcohol and drug abuse.”
At the state evidentiary hearing, May-field presented additional mitigating evidence through the testimony of lay and expert witnesses. A psychiatrist testified that Mayfield was born to a fifteen-year-old, single mother. By the time Mayfield was diagnosed with diabetes at age nine, his mother had three more children by three separate fathers. She testified that Mayfield’s childhood struggle with diabetes was “a real nightmare for him,” punctuated by extended hospitalizations, depression, and refusal to talk or eat.
An endocrinologist explained the hardships of self-monitoring and self-medication borne by diabetics. In particular, Mayfield suffered abdominal and chest pain, dehydration, fatigue, dizziness, nausea, loss of consciousness, and comas. His mother testified that there were periods when Mayfield was hospitalized as often as five times a month and that his symptoms were sometimes so acute she had to call an ambulance.
She further testified that before he was diagnosed with diabetes Mayfield was clean and neat, went to school regularly, did his school work, enjoyed the company of other children, and got along well with his siblings. The endocrinologist testified that a diabetic’s struggle with the psychological and physiological hardships of the disease can contribute to drastic changes in personality. According to the psychiatrist, Mayfield’s mother, who gave him an insulin shot each day, became “the villain in his life.” Mayfield became incorrigible and occasionally had physical altercations with her. Finally, when Mayfield was twelve, his mother sought the help of juvenile authorities.
During his stay with juvenile authorities, Mayfield was diagnosed with childhood behavioral disorder and depression. A psychological evaluation indicated that, although he had low-normal IQ, “he actually performed as though he were mildly retarded.” Mayfield spent a month in Ward B of juvenile hall in 1974, indicating that he had again been diagnosed with a behavioral disorder or was considered a suicide risk.
[931]*931Between the ages of fourteen and seventeen, Mayfield had his diabetes and emotions under better control. His hospitalizations decreased, he progressed in school, and he got a part-time job as a janitor.
When Mayfield was seventeen, his grandmother died and his mother moved the family from Redlands to San Bernardi-no, events the psychiatrist described as “stressors.” Mayfield’s diabetes-related hospitalizations increased. He began drinking and smoking marijuana to fit in with the tough kids of the San Bernardino projects. Marijuana acted as a “gateway drug,” and it was not long before he was using PCP.
Friends and siblings testified that May-field’s personality changed as a result of his drug and alcohol abuse and his poorly controlled diabetes. Mayfield’s relationships with his mother and siblings deteriorated. His mother ordered him out of the house. He divided his nights, from then on, between Harper’s house, Wafer’s house, and a spare mattress in his mother’s garage. In the months leading up to the crime, his alcohol and substance abuse was more frequent.16
The psychiatrist described the end of 1982 as “a growing onslaught of catastrophes, losses, and increased emotional turmoil that were boiling up,” exacerbated by “the continued drug use that makes it more difficult for him to figure out what is the best thing to do and how to control his emotions in this situation.” Mayfield’s girlfriend, Wanda Griffin, three months pregnant with his son, broke up with him and refused to see him. Mayfield again considered suicide. That was when he fired a gun into Griffin’s house. Two weeks later he struck another ex-girlfriend. He was hospitalized during this period with a blood sugar level of well over 1,100. He committed the crime a few months later.
A toxicologist testified at the evidentia-ry hearing that, in addition to suffering the acute toxicity experienced immediately after ingesting PCP, chronic abusers suffer long-term impairments. For instance, chronic PCP abuse aggravates preexisting psychological conditions, such as cognitive impairment, depression, and erratic mood swings. Its other effects can include “occasional bizarre episodes of acting out, including violent behavior,” impaired judgment, decreased impulse control, decreased stress tolerance, and impaired reality testing.17 Unlike acute PCP toxicity, chronic abuse generally does not result in memory loss.
The psychiatrist testified at the eviden-tiary hearing that Mayfield’s denial to the police and the State’s psychologist of a substance abuse problem was normal; substance abusers often deny they have a problem, particularly to figures of authority-
[932]*932Several witnesses testified that Mayfield was helpful and generous with his time. Mayfield’s younger brother, Cicero, and sister, Teresa, testified that Mayfield was a protective older brother. Cicero testified that Mayfield supported him by coming to his basketball practices after school. Mayfield babysat for Harper and assisted her in preparing projects for the local chapter of Head Start, where she was a teacher’s aide. He cared for his wheelchair-bound uncle, Calvin Hawkins, by helping him with rehabilitation and taking him to medical appointments and to collect his disability payments.
Several witnesses indicated that May-field loved and interacted well with children. Wanda Griffin testified that she had been pregnant with Mayfield’s son, who was born just before trial. She visited Mayfield in jail with their son when he was seven days old, and Mayfield seemed “happy and proud and blushing” and, although he tried to hide it, he got “watery eyes.” Teresa Mayfield testified that Mayfield loved her children and that he had helped her son overcome a speech problem.
Hazel Hawkins testified that Mayfield’s uncle, Willie Willingham, would have testified on his behalf. Willingham had overcome substance abuse problems and become a preacher and a drug and alcohol counselor. Several witnesses also testified that Mayfield was generally not a violent person. Mayfield’s friends and family testified that they would have told the jury that they loved Mayfield and would have asked the jury to spare his life.
The jury deliberated for approximately four hours before sending a written question to the judge: “Must all 12 jurors agree for the sentence of life without parole?” The judge responded: “All jurors must agree if either verdict is reached.” The jury deliberated an additional full day before it reached a verdict of death.
In light of the quantity and quality of the mitigating evidence Ames failed to present at trial, the duration of the jury’s deliberations, and the jury’s communication to the trial judge, we are not confident that, with the additional evidence presented at the evidentiary hearing, a unanimous jury would still have returned a sentence of death.18 If the jury had considered the testimony of experts in endocrinology and toxicology, or of friends and family members relating additional humanizing stories, there is a “reasonable probability that the omitted evidence would have changed the conclusion that the aggravating circumstances outweighed the mitigating circumstances and, hence, the sentence imposed.” 466 U.S. at 700, 104 S.Ct. 2052. Accordingly, we conclude that Mayfield was prejudiced by Ames’ deficient performance at the penalty phase. We hold that Mayfield received ineffective assistance of counsel at the penalty phase in violation of his Sixth and Fourteenth Amendment rights and reverse the district court’s denial of Mayfield’s petition for a writ of habe-as corpus on this ground.
Ill
For the foregoing reasons, we grant Certificates of Appealability only with regard to Mayfield’s claims of ineffective assistance of counsel at the guilt and penalty phases. We hold that, whether or not Mayfield’s counsel performed deficiently at the guilt phase, Mayfield’s claim for inef[933]*933fective assistance at the guilt phase fails for lack of prejudice. We also hold, however, that a reasonable probability exists that the jury might not have sentenced him to death if the jury had considered the additional evidence presented at the state evidentiary hearing. We reverse the district court’s denial of Mayfield’s petition for habeas corpus. We remand the case to the district court with instructions to issue the writ on the ground that Mayfield received ineffective assistance of counsel at the penalty phase and we further direct the district court to return the case to the San Bernardino County Superior Court to conduct a new sentencing proceeding.
AFFIRMED in part; REVERSED in part; REMANDED.