Partial Concurrence and Partial Dissent by Judge Nguyen
OPINION
REINHARDT, Circuit Judge:
INTRODUCTION
As in many capital cases, the facts involved in this case are deeply disturbing and the crimes brutal. In April 1983, a jury convicted Francis Hernandez of two counts of first degree murder, two counts of rape, and two counts of forcible sodomy. After finding that each murder occurred during the commission of rape and sodomy—special circumstances permitting capital punishment—the jury returned a verdict of death. The gruesome nature of the facts makes applying an objectively simple legal standard inherently difficult for any jurist, for as some astute observer will undoubtedly note someday, “bad facts make bad law.”
The ultimate question in this case is whether there is a reasonable probability—that is, even less than a fifty-fifty chance—that at least one juror would have declined to convict Hernandez of first degree murder if his counsel had presented a diminished capacity defense based on mental impairment. Counsel failed to present this defense because he was ignorant of the law. As a result of his incompetence, the jury did not hear that Hernandez had suffered from a variety of mental impairments since childhood which stemmed from a genetic inheritance that all but guaranteed that he would suffer, severe mental illness, coupled with numerous in útero traumas, physical and sexual abuse at the hands of a psychotic adoptive mother, and head injuries from nearly a dozen motorcycle accidents. Most important, the jury was not told that such evidence, if believed, provided a legal defense to ¡first degree murder. With respect to sentencing, the district court concluded that had the jury heard similar evidence during the penalty phase, there was a reasonable probability that at least one juror would have voted against the death penalty, and as a result it vacáted Hernandez’s death sentence. Hernandez v. Martel, 824 F.Supp.2d 1025, 1120 (C.D. Cal. 2011).1 A similar analysis leads us to conclude that had the jury been told' of the evidence of Hernandez’s mental impairments and that such evidence could as a matter of law provide a defense to 'first degree murder, at least one juror would have had reasonable doubt as to whether Hernandez could have formed the requisite mental state for that offense.2 Put differently, our confidence in the outcome of Hernandez’s trial is undermined: we believe it likely that at least one juror would have concluded that Hernandez suffered from the mental impairment required for a successful defense of diminished mental capacity.3'
BACKGROUND
I. Factual Background
A. Francis Hernandez
Francis Hernandez was born to a fourteen-year-old mother with a history of mental illness who abused drugs and was herself physically abused throughout her pregnancy. He inherited an “extremely strong predisposition” to “severe mental illness” with “psychiatric illness of psychotic proportions” going back three generations, including both his biological parents. These illnesses include schizophrenia, bipolar disorder, seizure disorder, and depression, in addition to an “extraordinary degree of chemical dependency.”
Hernandez was adopted as a baby by Frank and Naomi Hernandez, who were no better situated to care for a child with •special needs than were his biological parents. Naomi, diagnosed with schizophrenia, was episodically psychotic throughout Hernandez’s childhood and was hospitalized at least ten times. After each hospitalization, she was too heavily medicated to care for her adopted son. She left the family when Hernandez was in middle school.
The experts who testified at his habeas hearing described Hernandez’s childhood as “a daily hell,” and rightly so. For discipline, Naomi would sit on Hernandez until he stopped struggling, slap him, tie him to chairs, chase him around the house with a baseball bat, and forcibly administer enemas to Hernandez twice a week. Frank suspected that Naomi sexually abused Hernandez. Frank also abused his adopted son, beating him with belts and leaving scars on his buttocks consistent with cigarette burns. Naomi was not spared from Frank’s violence.
After Naomi left, Frank was largely absent. Drug dealers set up shop in the home, terrorizing Hernandez and supplying him with drugs and alcohol, which he began using in sixth grade. By fourteen or fifteen, he' was declared a ward of the state. He was sentenced to the California Youth Authority in 1979, and upon his release, he discovered that Frank had sold the family home and left his adopted son an old van in which to live. One of those experts who testified at the evidentiary hearing reported that in the months leading up to the crimes Hernandez “was an eighteen-year-old, unemployed parolee who was homeless, isolated from his family, drug addicted, and living in a van.”
B. The Crimes
The nude bodies of Edna Bristol and Kathy Ryan were found five days apart near schools in Long Beach, California in' the winter of 1981. Their injuries and causes of death were similar: both died of asphyxiation due to strangulation or suffocation and suffered unusual pre-mortem bruising and tearing in the anal and vaginal areas, suggesting a large object—consistent with a baseball bat—had been inserted. They both had bite marks on their breasts, and their pubic hair was singed. Ryan’s abdomen had been cut in a tic-tac-toe pattern post-mortem.
Hernandez was arrested on February 4, 1981. After five hours and seventeen minutes of unrecorded interviews, he confessed to the crimes in a taped statement. Hernandez later claimed that he was willing to tell the police anything during the interview because they promised him psychiatric help.
In the taped confession, Hernandez said that on the night of Bristol’s murder, he was drunk and “didn’t have any control of myself. ... I was in a crazy mood.” He picked up Bristol hitchhiking but when he got lost, he got mad and stopped the van. When Bristol would not leave, he hit her and dragged her out of the vehicle. According to Hernandez, Bristol 'said “she’d do anything” so the two had consensual sex in the back of the van. He explained, “I didn’t really have her—you know—forcibly. I guess maybe she thought I did, but I don’t know.” At some point, Bristol began kicking and screaming. In response, Hernandez “went bezerk [sic].” He taped her wrists, legs, and mouth, and “proceeded to •fuck her in the ass,” While doing so, he pushed Bristol against the hot engine cowling to burn her chest. Hernandez put his hand over her face to quiet her and “just might have left it there too long” until she stopped moving. He thought Bristol was still alive when he left her at the school, which he, chose “so someone hopefully [would] find her.” Before he left, he lit a cigarette and flicked matches onto, Bristol’s pubic hairs to hurt her for kicking him and damaging his van.
On the night of Ryan’s murder, a group of young people, including Ryan and Hernandez, met in a local park before going to a pizza parlor to play pool. Four witnesses testified that Hernandez was drinking but did not appear very drunk. Hernandez told one of the witnesses that he wanted to make a “sandwich” out of Ryan and “fuck her in the butt until she screams.” He told the witness, “You watch. I’ll get some tonight or tomorrow night.”
In his taped confession, Hernandez not only talked about Bristol’s death but about Ryan’s. He said that after the group gathering in the park and at the pizza parlor, Ryan told him to stop by her house to “go kick back for a little while” after the group disbanded. When he arrived at her house, Ryan came outside and got in the van. Hernandez suggested they make out, but Ryan was hesitant because Hernandez had a girlfriend. Eventually, the two started kissing. Hernandez thought the encounter was consensual: although “she was sort of hesitant at first,” “she said oh, okay, cuz I pushed her arms back” and then “she took off her clothes.” The two had sex, but stopped when Ryan was “starting to bleed or starting her period or something.” When Ryan turned over, Hernandez thought she wanted to have anal sex, which they did until Hernandez stopped because Ryan said it hurt. Like Bristol, Ryan started screaming and kicking, and Hernandez put a hand over her mouth to “keep her quiet.” He thought he “must have used too much pressure” because she “stopped struggling,” but he did not realize she was dead until he took her body out of the van. He then singed Ryan’s pubic hair and cut her stomach with glass. At the interviewer’s prompting, Hernandez acknowledged that he “might” have bitten Ryan’s nipple. He added that, when he left Ryan’s body, “it started dawning on [him] what had happened before with the other girl.” He said, “there was thoughts going through my head like, how the hell can I do these things, and—you know—I was thinking maybe I was doing it on purpose, I didn’t know, you know, cause I hadn’t been planning anything.”
In addition to the taped confession, a variety of physical evidence linked Hernandez to the crimes.
II. Trial and Subsequent History
Hernandez’s trial counsel tried to establish that Hernandez lacked the specific intent necessary for a conviction of first degree murder. He based a diminished capacity defense solely on voluntary intoxication. Counsel presented some evidence that Hernandez had been drinking on the nights of both murders as well as expert testimony that an alcoholic would not be able to form the specific intent to rape or kill. Counsel also argued that in his intoxicated state, Hernandez had believed the encounters were consensual and had intended only to quiet the victims.
The jury found Hernandez guilty on two counts of first degree murder, two counts of rape, and two counts of forcible sodomy. People v. Hernandez, 47 Cal. 3d 315, 351, 253 Cal.Rptr. 199, 763 P.2d 1289 (1988). It found true six special circumstances. The jury then imposed a sentence of death as to each murder. For the rape and sodomy convictions, Hernandez received sentences of eight years to be served consecutively for a total of thirty-two years. On direct appeal, the California Supreme Court in a reasoned opinion vacated one multiple-murder special circumstance charge but otherwise affirmed. In the direct appeal, appellate counsel raised some arguments regarding trial counsel’s ineffective assistance, but failed to raise a claim that counsel did not investigate or present a diminished capacity defense based on mental impairment. Id. at 369, 253 Cal.Rptr. 199, 763 P.2d 1289.
III. Habeas Proceedings
In 1989, Hernandez filed a state habeas petition in the California Supreme Court, in which he raised the ineffective assistance of counsel claims at issue here. The California Supreme Court summarily denied that petition. Hernandez filed a timely federal habeas petition, and then returned to state court to exhaust his claims. The California Supreme Court summarily denied the second state habeas petition as untimely and on the merits, and Hernandez filed a timely amended federal petition. After granting in part the state’s motion for summary judgment, the district court ordered a bifurcated evidentiary hearing on three juror misconduct claims and two ineffective assistance of counsel claims. This evidentiary hearing, all on written materials, lasted six years.
In 2011, the district court granted in part Hernandez’s petition for writ of habe-as corpus. It vacated the death penalty for several reasons, including counsel’s failure to present mitigating mental health and social history evidence at the penalty phase. It denied, however, guilt phase relief. On appeal, the state has not challenged the grant of penalty phase relief. Hernandez, however, appeals his convictions on the first degree murder counts.
JURISDICTION
The district court granted a certificate of appealability (“COA”) on Hernandez’s claim that his counsel was ineffective at the guilt phase for failing to call a key witness. It did not certify Hernandez’s remaining ineffective assistance claims, including his claim that counsel was ineffective for failing to investigate or present a defense of diminished capacity based on mental impairment. We treat Hernandez’s appeal from the district court’s ruling on the uncertified issues as an application for a COA, Fed. R. App. P. 22(b)(2), and hereby grant the application pursuant to 28 U.S.C. § 2253(c)(2).
STANDARD OF REVIEW
Hernandez filed his federal habe-as petition before the enactment of the Antiterrorism and Effective Death Penalty Act of 1996 (“AEDPA”). Accordingly, pre-AEDPA standards of review apply. Carrera v. Ayers, 699 F.3d 1104, 1106 (9th Cir. 2012) cert. denied, 569 U.S. 965, 133 S.Ct. 2039, 185 L.Ed.2d 899 (2013). “Ineffective assistance of counsel claims present mixed questions of law and fact.” Id. We review the district court’s partial denial of Hernandez’s habeas petition, and its resolution of mixed questions of law and fact de novo; we review its findings of fact for clear error. Id.
DISCUSSION
Hernandez contends that his counsel provided constitutionally ineffective assistance of counsel during the guilt phase of his trial by failing to investigate and present evidence in support of a diminished capacity defense based on mental impairment.4 His petition specified counsel’s errors in great detail. The district court concluded that Hernandez’s counsel’s performance was ineffective because he failed to know or to find out that a diminished capacity defense based on mental impairment was available under California law. As a result, he failed to develop “various materials gathered since trial but that were reasonably available to counsel before trial,” including “records and background information regarding petitioner’s birth family as well as social history information from petitioner’s adopted family, preschool teacher and others.” The district court concluded, however, that despite counsel’s ineffective performance, Hernandez failed to show prejudice.
On this appeal, Hernandez must show both that his counsel was ineffective and that the district court erred in finding that counsel’s ineffective performance was not prejudicial. Strickland v. Washington, 466 U.S. 668, 687-88, 104 S.Ct. 2052, 80 L.Ed.2d 674 (1984).
I. Deficient Performance
We- agree with the district court that counsel was ineffective in not knowing or finding out that a diminished capacity defense based on mental impairment was available to Hernandez under California law and, based on that lack of knowledge, in failing to investigate and present such a defense.5
Deficient performance requires a showing that counsel’s guilt phase performance “fell below an objective standard of reasonableness” at the time of the trial. Strickland, 466 U.S. at 688,104 S.Ct. 2052. Defense counsel is “strongly presumed to have rendered adequate assistance and made all significant decisions in the exercise of professional judgment.” - Id. at 690, 104 S.Ct. 2052, However, deference to counsel.is owed only to strategic decisions made after “thorough investigation of law and facts .relevant to plausible options.” Id,.
A. We Consider Only Counsel’s Stated Reason for His Challenged Conduct
As an initial matter, the parties dispute whether we consider counsel’s stated reasons for the challenged course of conduct or any hypothetical strategic reasons that could have supported the challenged course of conduct. Unlike many lawyers called to testify before a habeas court, Hernandez’s counsel did not attempt to justify his failure to perform effectively by invoking a strategic decision' on his' part; rather, he admitted that he would have investigated and advanced the diminished capacity defense based on mental impairment had he realized that he could have done so. Citing Harrington v. Richter, 562 U.S. 86, 109, 131 S.Ct. 770, 178 L.Ed.2d 624 (2011), the state counters that counsel’s “subjective state of mind is irrelevant” to our analysis, and asserts that “a reasonable defense attorney could have decided to present a guilt-phase defense based on intoxication alone” because the experts who examined Hernandez before trial were either inconclusive or found that Hernandez could have formed the requisite intent, and there was “potentially damaging psychiatric, evidence that Hernandez, was a sociopath.” Hernandez argues in response that the state’s proffered explanations, for counsel’s conduct are “mere post-hoc rationalization with no place in the analysis.”
Hernandez is correct. Where counsel has provided the reason for his conduct, and we have no reason to doubt the validity of that explanation, the relevant inquiry is whether the stated reason was objectively unreasonable. The Supreme Court has repeatedly declared that courts are not to “indulge ‘post hoc rationalization’ for counsel’s decisionmaking that contradicts the available evidence of counsel’s actions.”Richter, 562 U.S. at 109, 131 S.Ct. 770 (quoting Wiggins v. Smith, 539 U.S. 510, 526-27, 123 S.Ct. 2527, 156 L.Ed.2d 471 (2003)). As a result, “we credit the statements of defense counsel as to whether their decisions at trial were—or were not—based on strategic judgments.” Doe v. Ayers, 782 F.3d 425, 445 (9th Cir. 2015). In Doe, trial counsel “unequivocally said” that he did not have a strategy in failing to investigate extensive mitigating evidence. Id. at 444. We concluded that to apply the presumption that counsel acted reasonably in such a case would “contradict [trial counsel’s] testimony rather than fíll[] a gap in memory, contravening the Supreme Court’s admonition” against post hoc rationalizations in Harrington and Wiggins. Id. at 445 (quoting Heishman v. Ayers, 621 F.3d 1030, 1040 (9th Cir. 2010)) (alterations in original); see also Duncan v. Ornoski, 528 F.3d 1222, 1237 n.7 (9th Cir. 2008) (explaining that “[i]n light of the Supreme Court’s admonitions that reviewing courts may not substitute their own strategic reasoning for that of trial counsel in order to find that counsel’s performance was justified, we [would] not consider [the district court and state’s] additional speculative justifications to be [counsel’s] actual reasons for declining to test [petitioner’s] blood” (citing Wiggins, 539 U.S. at 526-27, 123 S.Ct. 2527)).
Only where counsel’s conduct is not explained in the record, or the explanation contradicts the record, do we “entertain the range of possible reasons [ ] counsel may have had for proceeding as he did.” Leavitt v. Arave, 646 F.3d 605, 609 (9th Cir. 2011); Richter, 562 U.S. at 109, 131 S.Ct. 770 (finding, under AEDPA, that court of appeals erred in dismissing range of strategic considerations where counsel provided no reason for course of conduct); Cullen v. Pinholster, 563 U.S. 170, 196, 131 S.Ct. 1388, 179 L.Ed.2d 557 (2011) (directing court of appeals “to affirmatively entertain the range of possible ‘reasons’’’ where counsel could not recall and the record was ambiguous as to the extent of penalty-phase investigation).
Were the record ambiguous or silent as to why Hernandez’s counsel did not present the diminished capacity defense, we might consider the state’s hypothetical strategic reasons. But it is not, and we don’t. In thé habeas proceedings before the district court, counsel said exactly why he failed to pursue the defense: he did not know that he could and did no research to uncover his mistake of law. The state, moreover, suggested no credible basis for doubting the truth of Hernandez’s counsel’s statements. Finally,'the district'judge found the statements credible and that finding was not clear error.
B. Mistakes of Law Constitute Defi- ; cient Performance
“An attorney’s- ignorance of ■ a point of law that is fundamental to his case combined with his failure to perform basic research on that point is a quintessential example of unreasonable performance under Strickland.” Hinton v. Alabama, — U.S. —, 134 S.Ct. 1081, 1089, 188 L.Ed.2d 1(2014); see also Morris v. California, 966 F.2d 448, 454-55 (9th Cir. 1992) (finding that counsel provided “clearly deficient performance” because he was not “familiar with the law” -and had not “done his homework” to become familiar with the relevant law). In United States v. Span, 75 F.3d 1383 (9th Cir, 1996), we held that a mistake about the availability of a defense constitutes a mistake of law that gives rise to deficient performance. In that case, trial counsel failed to request a jury instruction that would cover an excessive force defense because he .mistakenly believed the unlawful. arrest jury instruction would include excessive force. Id. at 1390. This was deficient performance because his “errors with the jury instructions were not a strategic decision to forego one defense in favor of another,” but rather “the result of a misunderstanding of the law.” Id. No “strategy, save an ineffective one, would lead a lawyer to deliberately omit” his client’s best defense. Id.
So, too, here. Hernandez’s counsel’s failure to investigate and present a diminished capacity defense based on mental impairment—what the district court recognized as Hernandez’s “best possible defense”—was “quintessential” deficient performance. His counsel was simply wrong that evidence of mental impairment would not support a diminished capacity defense. Doing even minimal homework, he would have learned that at the time of the crimes, California recognized a diminished capacity defense where “someone [ ] is unable, because of intoxication or mental illness, to comprehend his duty to govern his actions in accord with the duty imposed by law.” People v. Saille, 54 Cal. 3d 1103, 1110-11, 2 Cal.Rptr.2d 364, 820 P.2d 588 (1991) (emphasis added) (citing People v. Conley, 64 Cal. 2d 310, 322, 49 Cal.Rptr. 815, 411 P.2d 911 (1966)); see also People v. McDowell, 69 Cal. 2d 737, 746-47, 73 Cal.Rptr. 1, 447 P.2d 97 (1968) (describing the diminished capacity defense based on “mental abnormality” as “settled” and “commonplace”). In McDowell, for instance, the California Supreme Court held that trial counsel was constitutionally deficient for failing to present a diminished capacity defense based on “mental abnormality” because he erroneously believed the defense was limited to a defendant’s “sexual propensities.” 69 Cal. 2d at 747, 73 Cal.Rptr. 1, 447 P.2d 97. Because of Hernandez’s counsel’s failure to determine the applicable law, counsel did not present the “substantial credible evidence” of the defendant’s mental health that could have negated his intent at the time of the crimes. Id. at 749, 73 Cal.Rptr. 1, 447 P.2d 97.
II. Prejudice
Although the district court found that Hernandez’s counsel performed incompetently, it concluded that his deficient performance did not prejudice Hernandez. That conclusion is in error.
To establish prejudice, Hernandez must show a “reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different.” Strickland, 466 U.S. at 694, 104 S.Ct. 2052. The Strickland Court specifically rejected a preponderance of the evidence standard; Id. at 693, 104 S.Ct. 2052. Instead, a reasonable probability is “a probability sufficient to undermine confidence in the outcome.” Strickland, 466 U.S. at 694, 104 S.Ct. 2052.
“When a defendant challenges a conviction, the question is whether there is a reasonable probability that, absent the errors, the factfinder would have had a reasonable doubt respecting guilt.” Id. at 694-95,104 S.Ct. 2052. “[B]ecause the jury was required to reach a unanimous verdict on each count, the outcome could have differed if even ‘one juror 'would have struck a different balance.’” Weeden v. Johnson, 854 F.3d 1063, 1071 (9th Cir. 2017) (quoting Wiggins, 539 U.S. at 537, 123 S.Ct. 2527). For guilt-phase claims, we contrast “the evidence that actually was presented to the jury with that which could have been presented had counsel acted appropriately.” Daniels v. Woodford, 428 F.3d 1181, 1201 (9th Cir. 2005) (citing Bonin v. Calderon, 59 F.3d 815, 834 (9th Cir. 1995)). We then ask whether that omitted evidence might have created reasonable doubt in the mind of at least one reasonable juror. Rios v. Rocha, 299 F.3d 796, 813 (9th Cir. 2002).
Hernandez’s jury was presented with two paths to first degree murder: willful, deliberate, and premeditated murder, or felony murder with rape as the predicate felony.6 Under the first theory, the jury was required to find that the killing was intentional, willful, deliberate, and premeditated with malice aforethought. Conley, 64 Cal. 2d at 318-19, 49 Cal.Rptr. 815, 411 P.2d 911. Under the second, the jury was required to find that Hernandez had the specific intent to commit the predicate felony of rape. Hernandez, 47 Cal. 3d at 346, 253 Cal.Rptr. 199, 763 P.2d 1289. At the time of Hernandez’s trial, evidence of diminished capacity could negate the existence of a specific mental state essential to an offense, including malice aforethought and specific intent. People v. Poddar, 10 Cal. 3d 750, 757, 111 Cal.Rptr. 910, 518 P.2d 342 (1974), superseded by statute-, see also Saille, 54 Cal. 3d at 1110, 2 Cal.Rptr.2d 364, 820 P.2d 588. A successful diminished capacity defense would therefore have reduced the first degree murder convictions to a lesser offense under either theory by establishing that Hernandez lacked the capacity to form the requisite mental state of either malice aforethought or specific intent to rape.7 Poddar, 10 Cal. 3d at 758 n.11, 111 Cal.Rptr. 910, 518 P.2d 342.8
Accordingly, we must determine whether there is a reasonable probability that at least one juror, upon hearing the evidence of diminished capacity based on mental impairment, would have concluded that the prosecution had failed to carry its burden of proof regarding Hernandez’s, mental state and, as a result, declined to vote to convict Hernandez of first degree murder.
A. The Evidence Presented at Trial
Counsel presented three reasons why Hernandez lacked the requisite mental state necessary for first degree murder: (1) he had intended to quiet, not kill the victims; (2) he believed in his intoxicated state that the sex was consensual; and (3) he was unable to form specific intent based on a diminished capacity defense due to intoxication. These were undeniably all weak defenses. There was little evidence to support Hernandez’s claim that he attempted to quiet the victims beyond his own confession, and the physical evidence of forcible rape and sodomy was compelling.
The California Supreme Court emphasized that the diminished capacity defense based on intoxication was equally weak. Hernandez, 47 Cal. 3d at 350-51, 253 Cal.Rptr. 199, 763 P.2d 1289. Hernandez’s counsel presented only uncorroborated testimony that Hernandez was intoxicated the night of Bristol’s killing, and four witnesses testified that he did not appear very drunk on the night of Ryan’s killing. After testifying that Hernandez was an alcoholic who would not be capable of forming the requisite intent for first degree murder, Hernandez’s expert, Dr. Amer Rayyes, conceded on cross-examination that it would be possible for an alcoholic to form the specific intent to MU, rape, or sodomize while drinMng.
We have emphasized that we are particularly likely to find prejudice from a failure to present a mental defense “where the defense that was presented at trial was weak or meritless.” Daniels, 428 F.3d at 1207; see also Jennings, 290 F.3d at 1019 (same). Here, it unquestionably was both.
B. The Evidence that Could Have Been Presented at Trial
The picture the jury received of Hernandez was of a man who, as the prosecution said in closing, “is out to hurt people,” and who was maybe a little drunk the night of the crimes. That picture was woefully incomplete. Counsel failed to pursue and present what the district court recognized as “the best possible defense at guilt: that due to mental deficiency, neurological deficits and inadequate parenting, petitioner lacked the capacity to form the specific intent to rape and Mil his victims.” ■
At the time of Hernandez’s trial, a diminished capacity defense based on mental health included, among other factors, consideration of a defendant’s biological background, familial history of mental illness, social history, living situation prior to the crime, and potential neurological impairments. See McDowell, 69 Cal. 2d at 741-43, 73 Cal.Rptr. 1, 447 P.2d 97. Evidence supporting all of thesé considerations was readily available to Hernandez’s counsel had he only looked.
During the district court’s evidentiary hearing, several witnesses testified to what counsel could have found and presented had he understood that a mental defense based on diminished capacity was available. Psychologist Dr. Clausen, psychiatrist Dr. Lewis, and criminologist Sheila Balkan provided detailed social histories of Hernandez, relying on interviews with Hernandez, family, friends, and teachers as well as on various records including adoption and medical records, and psychological assessments from 1967, 1979, and 1982. Neuropsychologist Dr. Gur, who reviewed similar materials and administered a series of neurological tests, also testified on behalf of Hernandez. The state presented only the testimony of clinical psychologist Dr. Martell, who also interviewed Hernandez.
Dr. Lewis explained, “[i]t is impossible to understand Francis Hernandez’s psychiatric condition ... without a clear understanding among his genetic vulnerabilities to severe mental illness which he inherited from his biological mother and father, the effects of in útero exposure to alcohol and drugs, repeated head injuries beginning in early childhood, arid an upbringing in a psychotic, physically and sexually abusive, and severely neglectful adoptive family.” Hernandez’s social history reveals a biological “prescription for disaster.” The in útero injuries'—stemming from his mother’s drinking and abuse throughout the pregnancy—combined with the use of forceps during Hernandez’s delivery contributed to his “neurological and psychological vulnerabilities.”
The experts identified significant parallels between the crimes and the extreme abuse Hernandez suffered. In addition to suspected sexual abuse, his adoptive mother would sit on Hernandez, tie him to chairs, chase him around the house with a baseball bat, and forcibly administer enemas to Hernandez twice a week—a practice Dr. Lewis described as “a form of sodomy.” Dr. Lewis explained, “Children who have had objects shoved into their rectums repeatedly against their will are at high risk of perpetrating similar acts on others.” In addition, it is significant with respect to the victims’ pubic hair that Hernandez burned that he himself has scars on his .buttocks consistent with cigarette burns.
Experts Clausen, Lewis, Balkan, and Gur diagnosed Hernandez with dissociative disorder, bipolar disorder, organic brain damage, and impaired reality testing.
The experts concluded that Hernandez had begun dissociating in childhood to cope with trauma. Dr. Gur explained that dissociation can lead to “a state in which a person can engage in a complex set of behaviors without intent or premeditation.” Evidence of Hernandez’s dissociation include his, inability to remember traumatic “watershed events” from childhood, and friends’ descriptions of “spells” in which Hernandez would become “non-responsive” and unaware of his surroundings. Hernandez, himself described elementary school fights in which he “would find himself thirty feet from where he last remembered being.” The experts concluded that dissociation accounted for why Hernandez appeared to know what happened during the crimes, but did not remember portions of his actions.
Dr. Lewis also diagnosed Hernandez with bipolar disorder. She believed that at the time of the crimes, Hernandez was in a “manic or hypomanic state” while simultaneously experiencing dissociative ■symptoms.
In addition, Dr. Gur concluded that Hernandez had organic brain damage, probably reflecting a neurodevelopment disorder exacerbated by Hernandez’s perinatal stressors as well as postnatal head' injuries. In addition to the in útero abuse and use of forceps during his birth, Hernandez suffered “sequential' head injuries” that “exacerbate existing psychiatric illness” and make- one “especially susceptible to the effects of alcohol.” His adoptive father purchased Hernandez an adult motorbike when he was five and Hernandez was involved in a dozen motorcycle accidents that sent him to the hospital, including one at age 17 in which he lost consciousness and went into convulsions. After administering psychological tests, Dr. Gur testified that Hernandez’s results were “highly abnormal” -and that he hadn’t “seen profiles like that in a long time.” When he had, “they’ve always been associated with severe brain damage.”
Drs. Clausen, Lewis, and Gur all emphasized that from an early age, Hernandez exhibited impaired reality testing, or difficulty interpreting and responding to others’ emotions. His preschool teacher noted that Hernandez was incapable of interpreting social cues and that he misinterpreted any action as a threat. The psychologist who evaluated Hernandez in relation to his adoptive family’s unsuccessful attempt to adopt a second child in 1967 found that Hernandez “phantasized [sic] so profusely that he is unable to readily accept reality.” Dr. Clausen found the origins of Hernandez’s misperception of emotion in his-genetic predispositions as well as his exposure to his adoptive parents’ psychological problems. Dr. Gur concluded that Hernandez’s brain damage most likely caused Hernandez to misperceive the victims as consenting to sex. .
All experts but Dr. Martell, whom the district judge thought not credible, concluded that due to these mental impairments, Hernandez lacked the capacity to form the specific intent necessary to-support a first degree murder conviction. Further, Dr. Gur testified that "a comprehensive neuropsychological evaluation at the time of the trial would have disclosed the mental impairments identified by himself and the others.
C. Ninth Circuit Precedent Demonstrates that the Ineffective Performance in this Case Was Prejudicial
We have repeatedly held that defense counsel in a first-degree murder trial was prejudicially ineffective where there was some evidence of the defendant’s mental impairments in the record, but counsel failed to investigate and present a mental impairment defense to the charge. See, e.g., Daniels, 428 F.3d at 1208 (holding that counsel’s failure to investigate and present evidence of petitioner’s “severe mental illness and possible brain damage” at the guilt phase was prejudicial because “[ujnder the diminished capacity standard, a jury could well have found that he did not have the capacity to truly premeditate and understand the gravity” of the offense); Jennings, 290 F.3d at 1010, 1014-16 (holding that where “trial counsel failed adequately to investigate and present considerable evidence regarding petitioner’s psychological and family history that might have ... defeated the jury’s finding of the requisite intent for first degree murder in the guilt phase,” defendant was denied effective assistance of counsel); Seidel v. Merkle, 146 F.3d 760, 755-56 (9th Cir. 1998) (holding that petitioner was prejudiced where “trial counsel failed to conduct any investigation at all into his client’s psychiatric history and therefore neglected to pursue a potentially successful defense” at the guilt phase).
None of the evidence described in section 11(B), supra, was put before the jury and considered during its deliberations. The testimony from these qualified experts would have added an entirely new dimension to the jury’s assessment of the critical issue of Hernandez’s mental state. The jury did not have the benefit of testimony regarding Hernandez’s numerous head injuries or his genetic predisposition to mental illness, his traumatic childhood raised in a violent and psychotic adoptive family, his organic brain damage, and his history of dissociation and impaired reality testing. Instead, the jury was simply asked to find, based on weak or uncorroborated evidence, that because Hernandez might have been intoxicated, he could not form the specific intent to rape or kill the two victims. It is “especially egregious” to forgo investigation when “the entire defense strategy rest[s] on contesting the intent element of the crime, a defense which could have benefited enormously from readily available psychiatric evidence.” Turner v. Duncan, 158 F.3d 449, 456-57 (9th Cir. 1998).
We conclude that there is a reasonable probability that, upon hearing Hernandez’s “best defense” of mental impairment, at least one juror, and probably more, would have harbored substantial doubt about Hernandez’s capacity to form the specific intent to rape or kill. Thus, our confidence in the verdict is, without question, undermined.
D. The Arguments to the Contrary Do Not Change Our Conclusion that Hernandez Was Prejudiced
None of the arguments that the district court or the state offer to the contrary causes us to alter our conclusion.
The district court provided three primary reasons for its conclusion that Hernandez was not prejudiced: (1) the level of detail in Hernandez’s confession could have caused the jury to reasonably reject a defense that he lacked the capacity to form the requisite intent due to mental defect; (2) the victims suffered very similar injuries just days apart, suggesting some amount of premeditation or deliberation; and (3) the fact that California voters elected to abolish the diminished capacity defense eighteen months before Hernandez’s trial could make a jury less likely to accept the defense.9
Without a doubt, Hernandez’s taped confession is detailed and disturbing. However, the experts at the habeas hearing agreed that Hernandez’s “ostensible recollection of details on the tape” does not rule out a diminished capacity defense. Instead, as they testified, his taped confession confirms their view that Hernandez experienced mental impairments during the crimes because his statements demonstrate “that his thought processes were psychotic during the crimes.” At one point, Hernandez explained, “it was some way I wasn’t even feeling that I did it,” which suggested that he did not understand what he was doing or thinking during the crimes. When the police asked what thinking about these events made Hernandez feel, he responded, “I think I need psychiatric help. Definitely. Cause I don’t know what would make me do this.” Within the greater context of Hernandez’s biological background and horrific upbringing, and expert testimony pointing out the evidence of psychosis buried in the confession, we do not find the district court’s first point persuasive.
Second, the district court pointed to the fact that the victims suffered very similar injuries days apart to suggest that any diminished capacity defense would have been undermined by this evidence of premeditation and deliberation. Dr. Gur, however, explained that mental impairments such as Hernandez’s could cause a person to “engage in a complex set of behaviors without intent or premeditation.” The result may be “highly organized if somewhat ritualistic behavior.” In effect, the sheer bizarreness of the nearly identical crimes just days apart would have supported, rather than undermined, a diminished capacity defense based on mental impairment. So, too, would the seemingly irrational post-mortem injuries to the victims’ bodies. Most important, with Hernandez’s harrowing childhood placed in context, a juror could have reasonably concluded, like the experts who testified at the habeas hearing, that the nearly identical injuries of both victims reflected not premeditation, but rather Hernandez’s own, similar history of abuse at the hands of his adoptive parents, from the forcible sodomy to the genital burns, such that the similarity of his actions were a product of his abuse and mental impairments.
This Court has recognized the power of a diminished capacity defense to overcome even “[sjubstantial evidence supporting a finding of premeditation and deliberation.” Daniels, 428 F.3d at 1207-08 (alterations in original) (quoting People v. Cruz, 26 Cal.3d 233, 162 Cal.Rptr. 1, 605 P.2d 830, 835 (1980)); see also Bloom v. Calderon, 132 F.3d 1267, 1269, 1273, 1276 (9th Cir. 1997) (finding prejudice where counsel failed to present evidence of brain damage, schizotypal personality disorder, and transient psychotic episodes, notwithstanding evidence that petitioner planned the murder of his family in advance). Thus, however similar the crimes or detailed the taped statement, there is a reasonable probability that, hearing all of the expert evidence in support of a diminished mental capacity defense, a juror would have harbored reasonable doubt on the element of specific intent and, thus, on the counts of first degree murder.
The state offers two additional arguments that also do not affect our conclusion: one, that any psychiatric testimony would have been rebutted by a prosecution expert such as Dr. Martell and, two, that any additional evidence of impairment would have been cumulative.
First, while this Court takes into account, the possibility of any rebuttal evidence that could have been admitted when evaluating prejudice, Richter, 562 U.S. at 109, 131 S.Ct. 770, the district court found that Dr. Martell—-the state’s rebuttal expert—was neither credible nor qualified. The state cites Jones v. Ryan, 583 F.3d 626 (9th Cir. 2009), vacated on other grounds and remanded, 563 U.S. 932, 131 S.Ct. 2091, 179 L.Ed.2d 886 (2011), for the proposition that it was “improper [for thé district court] to weigh the testimony of the experts against each other in order to determine who was the most credible.”- That quotation, however, is taken out of context and has no relevance here, because Jones reaffirmed the well-established principle that in order to determine whether the failure to offer evidence was prejudicial, the judge must determine'the probable effect of-such evidence upon the jury, including how he believes the jury would assess the -credibility of the witnesses. See 583 F.3d at 641. That is what the district judge did here by finding Dr. Martell not credible’ as part of his prejudice analysis. While we do not agree with the district judge’s ultimate conclusion regarding prejudice, we do agree that, for the reasons he stated, a jury probably would have found Dr. Mar-tell’s testimony unpersuasive.
Second, contrary to the state’s assertion, and for the reasons discussed throughout this opinion, the evidence of mental impairment clearly would not have been cumulative of the weak evidence of intoxication that Hernandez’s counsel presented.
CONCLUSION
The jury in Hernandez’s trial heard the gruesome facts of the crimes and was asked to find that Hernandez acted in a diminished capacity because of weak evidence that he might have been intoxicated. What it did not hear, solely because his counsel was ignorant of the law, was that Hernandez had suffered from neurological impairments since childhood, dissociating to cope with the trauma of being raised and abused by a psychotic mother and struggling to comprehend others’ emotions;. that he suffered persistent and pervasive abuse that bore a striking resemblance- to that which he inflicted upon the victims; that he sustained head injuries from nearly a dozen motorcycle accidents, some of which occurred upon a motorcycle given to him when he was five years old; and that he inherited a genetic “prescription for disaster,” all but guaranteeing that he would have some neurological impairments as a result of a staggering legacy of mental illness and addiction in his biological family. We conclude that had counsel performed effectively and investigated and presented a diminished mental capacity defense based on mental impairment, there is a reasonable probability that at least one juror would have had a reasonable doubt as to whether Hernandez could have formed the requisite mental state for first degree murder.
We reverse the district court’s denial of a writ of habeas corpus as to Hernandez’s guilt phase claims relating to first degree murder, vacate Hernandez’s convictions on those counts, and remand with instructions to grant the petition for a writ of habeas corpus unless the state conducts a new trial on those charges within a reasonable period of time.