*423Arizona state prisoner Theodore Washington appeals the district court's denial of his petition for a writ of habeas corpus under 28 U.S.C. § 2254. In 1987, a jury found Washington guilty of six crimes involving the robbery and murder of Sterleen Hill in her Arizona home. The court sentenced Washington to death.
In his habeas corpus petition, Washington challenges his conviction and sentence on the first-degree murder charge. He asserts that he is entitled to habeas relief on several grounds, the majority of which are addressed in a separate memorandum disposition filed concurrently with this opinion. This opinion solely addresses Washington's certified claim for ineffective assistance of trial counsel. Washington contends that his counsel did not investigate and present mitigating evidence at the penalty phase, including evidence of diffuse brain damage, childhood abuse, and substance abuse. The Arizona court previously considered and rejected this claim on post-conviction review.
Because review under the Antiterrorism and Effective Death Penalty Act of 1996, Pub. L. No. 104-122, 100 Stat. 1214 ("AEDPA"), does not apply in this case, we are not bound by the highly deferential "double deference" in considering Washington's claim of ineffective assistance of counsel and its proper analysis. See Hardy v. Chappell , 849 F.3d 803, 824-26 (9th Cir. 2016) (explaining the interaction of 28 U.S.C. § 2254(d) and the standard for deficiency under Strickland v. Washington , 466 U.S. 668, 104 S.Ct. 2052, 80 L.Ed.2d 674 (1984) ). Instead, we apply the familiar standard articulated in Strickland , and assess the state court record to determine whether Washington's counsel was constitutionally deficient and whether the deficient performance resulted in prejudice. See Bobby v. Van Hook , 558 U.S. 4, 7, 130 S.Ct. 13, 175 L.Ed.2d 255 (2009) (applying the Strickland analysis in a pre-AEDPA case). Because Washington's counsel did not properly investigate Washington's background, the trial court at the penalty phase was not presented with substantial mitigation evidence regarding Washington's education and incarceration, his diffuse brain damage, and his history of substance abuse. This raises a reasonable probability that, had the court been presented with the mitigation evidence in the first instance, the outcome would have been different. The sentencing judge might have decided that Washington should be spared death and be imprisoned for life.1 We reverse the district court's denial of habeas relief and remand with instructions to grant habeas relief against the death penalty, unless within a reasonable time the state retries the penalty phase or decides to modify the sentence to life in prison.
I
At around 11:45 p.m. on the night of June 8, 1987, two men forced their way into Ralph and Sterleen Hill's Yuma, Arizona *424home in what turned out to be a disastrously violent home invasion. The men forced the Hills to lie face down on the floor of the master bedroom and bound their hands behind their backs. One of the men intermittently "screwed" a pistol in Ralph's ear while both men yelled at the couple demanding that the Hills give them drugs or money. Ralph glimpsed one of the assailants as he ransacked the drawers and closets in the room. The Hills were discovered lying face down in their bedroom. Both had been shot in the back of the head. Ralph survived the horrendous shot to his head, but was seriously injured. Sterleen did not survive the shooting.
Police arrested Fred Robinson shortly after the incident. Robinson was the common law husband of Susan Hill, Ralph Hill's daughter from a prior marriage. Police also arrested Jimmy Mathers and Theodore Washington in connection with the crimes. The state charged the three men with (1) first degree murder for the death of Sterleen Hill, (2) attempted first degree murder, (3) aggravated assault causing serious physical injury, (4) aggravated assault using a deadly weapon, (5) burglary in the first degree, and (6) armed robbery. The three men were tried together, and the jury convicted on all counts.
A
The penalty phase of the trial commenced on January 8, 1988. In this appeal, we are concerned with the penalty phase of Washington's trial and the death penalty sentence he received.
Washington's trial counsel, Robert Clarke, called three witnesses to testify on Washington's behalf: Washington's friend, Steve Thomas; Washington's mother, Willa Mae Skinner; and Washington's half-brother, John Mondy.
Steve Thomas testified that he knew Washington for two years. He testified that Washington was easily influenced but not violent. He also testified that Washington was a dedicated father. When asked if Washington had a drug problem, Thomas testified that he had not noticed one. Willa Mae Skinner testified that Washington was a good child and that he dropped out of school when he was in high school. She also testified that Washington was a good father, and that he was gentle and "liked to party." Finally, John Mondy reiterated that Washington was affable but easily led. He also confirmed that Washington had trouble in school as a child.
During closing argument, Clarke focused primarily on attacking the sufficiency of the court's findings under Enmund v. Florida , 458 U.S. 782, 102 S.Ct. 3368, 73 L.Ed.2d 1140 (1982), and Tison v. Arizona , 481 U.S. 137, 107 S.Ct. 1676, 95 L.Ed.2d 127 (1987). Regarding mitigation, Clarke did not entirely ignore all mitigation, but rather urged the court to consider Washington's age, his relatively minor criminal record, his good relationship with his son, and his general demeanor as a caring individual. This appeal is concerned primarily with the mitigation evidence and argument that Clarke did not present.
The trial court found that the state had established two aggravating factors beyond a reasonable doubt: (1) that the murder was committed in an especially cruel, heinous, or depraved manner, and (2) that the murder was committed for, or motivated by, pecuniary gain. With respect to mitigation, the court found that Washington's age was not a mitigating factor and that the remaining mitigating factors did not outweigh the aggravating factors. The court sentenced all three defendants to death on the first-degree murder charges.
B
Washington, Robinson, and Mathers each appealed their convictions and sentences *425to the Arizona Supreme Court. The state high court affirmed Washington and Robinson's convictions and sentences, State v. Robinson , 165 Ariz. 51, 796 P.2d 853, 865 (Ariz. 1990), but found insufficient evidence to convict James Mathers and vacated his conviction, State v. Mathers , 165 Ariz. 64, 796 P.2d 866 (Ariz. 1990).
Following the direct appeal process, Washington and Robinson challenged their convictions and sentences on post-conviction review ("PCR"). The court held a joint PCR hearing on September 8, 1993. The Honorable Stewart Bradshaw, the same judge who presided over the trial, also presided over the post-conviction review proceeding. Washington, through his appellate counsel, argued that Clarke was ineffective at the penalty phase due to his failure to present mitigating evidence. Specifically, Washington argued that Clarke erred by failing to conduct a more thorough review of his school, medical, and incarceration records. He also argued that Clarke should have obtained a psychological evaluation and presented the results to the court.
The bulk of the new evidence presented at the PCR hearing was elicited through the testimony of Dr. Roy, the defense counsel's retained psychologist. Dr. Roy evaluated Washington in 1992. He conducted clinical interviews and several psychological tests. Dr. Roy's interviews with Washington revealed that he suffered abuse as a child in the form of daily whippings with straps and belts and that adults in the home used alcohol as a means to sedate him as a child. His review of Washington's school and Department of Corrections ("DOC") records revealed that he was placed in classes for the "educable mentally retarded" when he was five years old and that he had been marked as low-IQ while incarcerated. However, Dr. Roy testified that these records conflicted with his own clinical findings because Washington tested at a low-average IQ of 96.
Dr. Roy's interviews with Washington also revealed that Washington had substance abuse problems relating to alcohol and cocaine use. Washington told Dr. Roy that he began drinking recreationally at age eight and was a functional alcoholic by age fourteen. He also told Dr. Roy that he was heavily intoxicated on the night of the murder. Washington also said that he was a heavy cocaine user and that he consumed about $ 175 in cocaine per day at the time of the crime.
Finally, Dr. Roy testified that he believed that Washington suffered from diffuse brain damage resulting from early and prolonged drug and alcohol use and numerous traumatic head injuries. Dr. Roy testified that diffuse brain damage can result in disinhibition and poor social judgment as well as poor impulse control and an inability to appreciate the long-term consequences of one's actions. Dr. Roy testified that, in his opinion, Washington's cocaine addiction and his impaired impulse control likely contributed to his ability to be manipulated by others into making poor decisions.
The state called Dr. Eva McCullars, a psychiatrist who also evaluated Washington. Dr. McCullars reviewed Dr. Roy's report and conducted clinical interviews with Washington in June 1993. Dr. McCullars testified that she did not review Washington's DOC records, school records, or adult incarceration records. Dr. McCullars agreed that Washington suffered from diffuse brain damage, but concluded that Washington also suffered from antisocial personality disorder. On direct examination, the state asked Dr. McCullars whether diffuse brain damage could cause hyperkenesis (hyperactive behavior or attention deficit disorder ). Dr. McCullars explained that "[hyperkenesis] is one example *426of diffuse brain damage." She went on to explain that several prominent individuals including Walt Disney and Thomas Edison exhibited hyperkinetic behavior as children. When questioned on cross examination, Dr. McCullars acknowledged that Washington came from a "significantly dysfunctional family." She also admitted that several of the markers for antisocial personality disorder, such as early truancy and an inability to maintain employment, were more frequently associated with lower socio-economic status black adolescents when compared to the general population.
Robert Clarke, Washington's trial counsel, also testified at the PCR hearing. Clarke testified that he did not request Washington's education or corrections records because he believed his interviews with Washington, Skinner, Mondy and Bryant were sufficient. Clarke testified that he had "very extensive discussions" with Washington about what his life was like and any possible substance abuse issues. He also testified that he had "relatively extensive" discussions with Washington's mother, half-brother, and girlfriend. Clarke testified that, based on these interviews, "there wasn't anything that clued me in that there was a special problem that would suggest I should obtain those types of records." With respect to Washington's drug use, Clarke testified that Washington never told him that he was addicted to cocaine or that he was using cocaine on the night of the murder. When questioned on the matter, Clarke acknowledged that Bryant had told Clarke that Washington had a "cocaine problem," but that he did not investigate further.
In a written order, Judge Bradshaw held that Washington was not entitled to relief for ineffective assistance of counsel at the penalty phase. Judge Bradshaw credited Dr. McCullars's findings that Washington had antisocial personality disorder and was poorly adjusted to living in society. However, Judge Bradshaw concluded that "there is nothing ... which lessened his ability to differentiate right from wrong or conform his actions with the law." Judge Bradshaw also explained that he had been aware at the time of sentencing that Washington had been doing well while incarcerated. Judge Bradshaw further reasoned that any drug and alcohol dependency "taken separately or with any other mitigating circumstance or circumstances would [not] have mitigated against the sentence [Washington] has received."
C
On April 25, 1995, the Arizona Supreme Court summarily denied Washington's petition for review of the PCR court's decision. Washington then commenced his habeas action in the federal district court, culminating in this appeal.
In his amended federal habeas corpus petition, Washington raised 17 claims. The district court determined that claims 1, 2, 3 (in part), 6, 7, 8 (in part), 9, 11 (in part), 12, 14 (in part), 16, and 17 were procedurally barred. On April 22, 2005, the district court rejected the remaining claims on their merits and dismissed the petition. Washington filed a motion to alter the judgment on May 5, 2005, which the district court denied on June 8, 2005.
On July 11, 2005, Washington filed an untimely notice of appeal from the district court's denial of habeas relief. A three-judge panel of this court held that it lacked jurisdiction and affirmed the district court's denial of Rule 60(b) relief. Washington v. Ryan , 789 F.3d 1041 (9th Cir. 2015). The court then granted Washington's motion for en banc rehearing. Washington v. Ryan , 811 F.3d 299 (9th Cir. 2015). In a 6-5 decision, the en banc panel *427held that Washington was entitled to relief under Rule 60(b)(1) and (6) from his untimely notice of appeal and ordered the district court to "vacate and reenter its judgment denying Washington's petition for writ of habeas corpus, nunc pro tunc , June 9, 2005," to render the notice of appeal timely. Washington v. Ryan , 833 F.3d 1087, 1102 (9th Cir. 2016). The United States Supreme Court denied the state's petition for writ of certiorari. Ryan v. Washington , --- U.S. ----, 137 S.Ct. 1581, 197 L.Ed.2d 736 (2017) (mem.).
In his opening brief, Washington raised three certified issues and four uncertified issues. In this opinion, we address Washington's certified claim for ineffective assistance of counsel at the penalty phase. The remaining issues are addressed in a memorandum disposition filed concurrently with this opinion.
II
We review de novo a district court's decision to grant or deny a habeas petition under 28 U.S.C. § 2254. See Bean v. Calderon , 163 F.3d 1073, 1077 (9th Cir. 1998). Because Washington filed his habeas petition before the enactment of AEDPA, the provisions of AEDPA do not apply to this case. Id. (citing Jeffries v. Wood , 114 F.3d 1484, 1495-96 (9th Cir. 1997) (en banc)). Instead, we review the claim under the familiar standard set out in Strickland and its progeny without the added deference required under AEDPA.
III
To prevail on his claim for ineffective assistance of counsel, Washington must establish that Clarke's performance was deficient and that he suffered prejudice as a result. See Strickland , 466 U.S. at 687-89, 104 S.Ct. 2052. To establish deficient performance, Washington must show that "counsel's representation fell below an objective standard of reasonableness." Id. at 688, 104 S.Ct. 2052. To establish prejudice, Washington must show that there is "a reasonable probability that, but for counsel's unprofessional errors, the result of the proceeding would have been different." Id. at 694, 104 S.Ct. 2052. These are formidable barriers to habeas corpus petition relief in federal court to state prisoners even absent application of AEDPA.
A
To prevail under Strickland's performance prong, Washington must show that his "counsel's representation fell below an objective standard of reasonableness." See Strickland , 466 U.S. at 688, 104 S.Ct. 2052. Even without the added layer of deference to the state court decision under AEDPA, "[s]urmounting Strickland's high bar is never an easy task." Padilla v. Kentucky , 559 U.S. 356, 371, 130 S.Ct. 1473, 176 L.Ed.2d 284 (2010). In articulating the standard against which counsel's performance should be judged, the Strickland Court emphasized the deference due to a lawyer's decisions both as to scope of investigation and decisions made after investigation: "[S]trategic choices made after thorough investigation of law and facts relevant to plausible options are virtually unchallengeable. ..." Strickland , 466 U.S. at 690, 104 S.Ct. 2052. We have likewise recognized the wide latitude to be given to counsel's tactical choices. See, e.g. , United States v. Ferreira-Alameda , 815 F.2d 1251, 1253 (9th Cir. 1986) ("Review of counsel's performance is highly deferential and there is a strong presumption that counsel's conduct fell within the wide range of reasonable representation.").
But our deference to counsel's performance is not unlimited. As the Court explained in Strickland , counsel's strategic *428choices made after less than complete investigation are reasonable only to the extent that "reasonable professional judgments support the limitations on investigation." 466 U.S. at 690-91, 104 S.Ct. 2052.
The mitigation obligation applies even when a person is clearly guilty. See Penry v. Lynaugh , 492 U.S. 302, 318-20, 109 S.Ct. 2934, 106 L.Ed.2d 256 (1989), abrogated on other grounds by Atkins v. Virginia , 536 U.S. 304, 122 S.Ct. 2242, 153 L.Ed.2d 335 (2002) ("[R]ather than creating a risk of an unguided emotional response, full consideration of evidence that mitigates against the death penalty is essential if the jury is to give a 'reasoned moral response to the defendant's background, character, and crime.' " (quoting Franklin v. Lynaugh , 487 U.S. 164, 184, 108 S.Ct. 2320, 101 L.Ed.2d 155 (1988), abrogated on other grounds by Atkins , 536 U.S. 304, 122 S.Ct. 2242, 153 L.Ed.2d 335 )). Indeed, that is a key point of the penalty phase of a capital case, which proceeds only after a determination of guilt in the earlier phase. See Gregg , 428 U.S. at 191-92, 96 S.Ct. 2909 (endorsing the use of a bifurcated trial to determine guilt and penalty). In the penalty phase, the focus shifts from guilt to culpability, and evidence on both aggravating and mitigating factors is properly considered. See Ariz. Rev. Stat. Ann. § 13-751 (2018) (identifying statutory aggravating and mitigating circumstances in capital-eligible cases); Buchanan v. Angelone , 522 U.S. 269, 275-76, 118 S.Ct. 757, 139 L.Ed.2d 702 (1998) (observing that the penalty phase of capital sentencing involves a determination of eligibility, where "the jury narrows the class of defendants eligible for the death penalty, often through consideration of aggravating circumstances," and selection, where "the jury determines whether to impose a death sentence on an eligible defendant" and must consider "any constitutionally relevant mitigating evidence").
At the penalty phase, "[a] decision not to ... offer particular mitigating evidence is unreasonable unless counsel has explored the issue sufficiently to discover the facts that might be relevant to his making an informed decision." Lambright v. Schriro , 490 F.3d 1103, 1116 (9th Cir. 2007) ; see also Ainsworth v. Woodford , 268 F.3d 868, 874 (9th Cir. 2001) (holding counsel's performance was ineffective where counsel "failed to adequately investigate, develop, and present mitigating evidence to the jury even though the issue before the jury was whether [the defendant] would live or die").
Washington asserts that Clarke erred by not reviewing his education records and incarceration records. "In preparing for the penalty phase of a capital trial, defense counsel has a duty to 'conduct a thorough investigation of the defendant's background' in order to discover all relevant mitigating evidence." Robinson v. Schriro , 595 F.3d 1086, 1108 (9th Cir. 2010) (quoting Correll v. Ryan , 539 F.3d 938, 942 (9th Cir. 2008) ). We have recognized that, "[a]t the very least, counsel should obtain readily available documentary evidence such as school, employment, and medical records." Id. at 1109. Clarke did not request or review any such records and provided no tactical reason for his failure to do so. Instead, Clarke relied entirely on his interviews with Washington, Skinner, Mondy, and Bryant and testified that he did not believe other sources of information would have been fruitful. Clarke's failure to review these basic and readily-available sources of mitigating information fell below prevailing professional norms at the time. See id.
Washington also alleges that Clarke erred by not retaining an expert to *429conduct a psychological evaluation. A psychological evaluation at the penalty phase is not always required. Gentry v. Sinclair , 705 F.3d 884, 900 (9th Cir. 2013). Instead, "[t]rial counsel has a duty to investigate a defendant's mental state if there is evidence to suggest that the defendant is impaired." Douglas v. Woodford , 316 F.3d 1079, 1085 (9th Cir. 2003).
In assessing counsel's performance, we must keep in mind that "[i]t is imperative that all relevant mitigating information be unearthed for consideration at the capital sentencing phase." Caro v. Calderon , 165 F.3d 1223, 1227 (9th Cir. 1999). Whereas evidence of mental impairment is relevant at the guilt phase of a capital trial if it tends to negate the mens rea and criminal liability, evidence of mental impairment is relevant at the penalty phase for broader purpose; namely, where it "might provide a basis for refusing to impose the death penalty on a particular individual, notwithstanding the severity of his crime or his potential to commit similar offenses in the future." Abdul-Kabir v. Quarterman , 550 U.S. 233, 246, 127 S.Ct. 1654, 167 L.Ed.2d 585 (2007) ; see also McKoy v. North Carolina , 494 U.S. 433, 440, 110 S.Ct. 1227, 108 L.Ed.2d 369 (U.S. 1990) ("Relevant mitigating evidence is evidence which tends logically to prove or disprove some fact or circumstance which a fact-finder could reasonably deem to have mitigating value." (quotation omitted)). In light of the broad scope of evidence relevant to mitigation at the penalty phase of a capital case, we have recognized that counsel's duty to investigate possible psychological mitigation evidence is higher at the penalty phase than it might be during the guilt phase of trial: "At the penalty phase, counsel's duty to follow up on indicia of mental impairment is quite different from-and much broader and less contingent than-the more confined guilt-phase responsibility." Bemore v. Chappell , 788 F.3d 1151, 1171 (9th Cir. 2015).
We may conclude, arguendo , that based solely on the limited facts known to Clarke during his investigation-which included Clarke's discussions with Washington and his friends and family, but not Washington's education and incarceration records-Clarke's decision not to seek a psychological evaluation was not objectively unreasonable. But our analysis does not end there. In a case such as this, the question under Strickland is "whether the investigation supporting [counsel's] decision not to introduce mitigating evidence ... was itself reasonable." Id. (alterations in original) (quoting Wiggins v. Smith , 539 U.S. 510, 523, 123 S.Ct. 2527, 156 L.Ed.2d 471 (2003) ).
As discussed above, Clarke performed unreasonably by not reviewing Washington's education and incarceration records. We must therefore determine whether, if Clarke had performed reasonably in reviewing those records, he would have uncovered information that would have prompted him to obtain a psychological evaluation for Washington. See Rompilla v. Beard , 545 U.S. 374, 390-93, 125 S.Ct. 2456, 162 L.Ed.2d 360 (2005). We conclude that an objectively reasonable lawyer would have done so. If Clarke had reviewed Washington's education and incarceration records, he would have seen that Washington's elementary school had placed Washington in classes for the educable mentally retarded and that the DOC had indicated that Washington had a low IQ. Although the Supreme Court had not yet recognized that the Eighth Amendment bars the execution of mentally retarded individuals at the time of Washington's penalty trial, Atkins , 536 U.S. at 321, 122 S.Ct. 2242, the Court had made clear that evidence of mental retardation was *430important mitigation evidence that should be presented at the penalty phase, Penry , 492 U.S. at 322-24, 109 S.Ct. 2934 (noting that evidence of a defendant's mental retardation may render him less culpable for his crime). A reasonable attorney would have investigated the potential that Washington may have been mentally retarded after reviewing Washington's education and incarceration records.
In sum, although Clarke did not perform unreasonably by not requesting a psychological evaluation based on the evidence known to him at the time, his unreasonable failure to review Washington's education and incarceration records prevented Clarke from gaining information that would have led him to request a psychological evaluation. See Rompilla , 545 U.S. at 390-93, 125 S.Ct. 2456 (concluding that counsel erred by failing to review defendant's prison file, which would have prompted counsel to further investigate potential psychological mitigation evidence). As discussed below, these errors prevented Washington from presenting substantial mitigation evidence at the penalty phase.
Washington also argues that Clarke erred by not investigating and presenting evidence of his childhood abuse. Through his conversations with Dr. Roy, Washington revealed that he suffered physical abuse as a child in the form of daily whippings and beatings. Roy was also told that Washington was given alcohol as a child to control his behavior. None of this information later given to Dr. Roy had come to Clarke's attention during the trial. By contrast, both psychological experts who testified at the PCR hearing agreed that Washington's childhood was significantly dysfunctional. Unlike other categories of information that are easily verified through documentary evidence, Clarke had to rely entirely on the word of Washington and his family members in determining whether Washington suffered childhood abuse. Because neither Washington nor his family members had then indicated that Washington suffered abuse as a child, Clarke did not err by not further investigating Washington's childhood abuse, to the extent that he could have, or by not presenting the information he did not have regarding abuse at the sentencing hearing. See Strickland , 466 U.S. at 691, 104 S.Ct. 2052 ("[W]hen a defendant has given counsel reason to believe that pursuing certain investigations would be fruitless or even harmful, counsel's failure to pursue those investigations may not later be challenged as unreasonable.").
Finally, Washington alleges that Clarke erred by not investigating and presenting evidence of his substance abuse. Again, Clarke did not err by failing to investigate Washington's substance abuse, because Clarke reasonably relied on his conversations with Washington and his friends and family, wherein substance abuse was not indicated. Washington had told Clarke that he was heavily intoxicated on the night of the crimes, but he denied that he had any ongoing problems with drugs or with alcohol. Similarly, Washington's mother described him as someone who "liked to party," but also did not say that Washington had problems with addiction. Perhaps the single clue Clarke had that might have raised his suspicions about substance abuse was the statement of Washington's girlfriend that Washington had a "cocaine problem." However, when set against Washington's own statements and those of his family members, Clarke's decision not to further investigate Washington's drug abuse was not objectively unreasonable.
In summary, Clarke performed ineffectively by not reviewing Washington's education and incarceration records. If Clarke *431had performed effectively, he would have known about and acted on information regarding Washington's potentially impaired cognitive function. While we recognize that, as a general rule, deficient lawyer performance should be found only in exceptional cases presenting extraordinarily poor performance, we nonetheless conclude that the record here amply demonstrates that Clarke's representation of Washington at the penalty phase was objectively unreasonable and deficient for the reasons articulated above.
B
Relief for ineffective assistance of counsel under Strickland requires both deficient performance in representation and prejudice. Even in light of Clarke's performance, Washington can succeed on his ineffective assistance of counsel claim only if Clarke's performance resulted in prejudice. To establish prejudice, Washington 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. "A reasonable probability is a probability sufficient to undermine confidence in the outcome." Id. To determine whether a reasonable probability exists that the sentencing judge would not have imposed the death sentence in light of the mitigation evidence, we "reweigh the evidence in aggravation against the totality of the available mitigating evidence." Wiggins , 539 U.S. at 534, 123 S.Ct. 2527.
Here, as a result of Clarke's performance, the sentencing court did not hear any evidence concerning Washington's potentially impaired cognitive functions or his possible mental retardation. However, to establish that those omissions prejudiced him, Washington must show that there is a reasonable probability that he would have received a different sentence if those materials and evidence had been introduced at sentencing. Strickland , 466 U.S. at 694, 104 S.Ct. 2052. Washington faces a significant obstacle in doing so, because we need not guess at whether the outcome would have been different if that evidence had been available to the court at sentencing. The sentencing judge, Judge Bradshaw, considered that evidence during the PCR proceeding and concluded in a written order that those materials would not have made a difference. Judge Bradshaw's unequivocal ruling might ordinarily persuade us that Washington cannot show prejudice. The materials that Clarke missed, though they undoubtedly had mitigating value, were not so overwhelming that they influenced Judge Bradshaw's no-prejudice finding.
However, the case of Washington's co-defendant, Fred Robinson, casts a long shadow on our prejudice analysis here.2 In Robinson v. Schriro , we considered Fred Robinson's habeas corpus petition. 595 F.3d at 1098-99. That case is important here because Washington and Robinson were tried and sentenced together, and their convictions and sentences were affirmed in state court following *432joint PCR proceedings, in nearly identical written orders. Like Washington, Robinson alleged that he received ineffective assistance of counsel based on his trial counsel's failure to present mitigation evidence at the penalty phase. Id. at 1108-10. Judge Bradshaw (like here) concluded that the mitigation evidence Robinson produced in the state PCR proceeding would not have made a difference. Yet, we determined that Judge Bradshaw, who presided at both Robinson's and Washington's trial and PCR proceedings, applied an unconstitutional "nexus test" in considering Robinson's newly-presented mitigation evidence.3 Therefore, we concluded that Judge Bradshaw had not "properly evaluated the mitigating evidence offered in the evidentiary hearing," 595 F.3d at 1113, and that the possibility that Judge Bradshaw would have imposed a sentence other than death if he had applied the correct standard when considering that evidence was enough to establish prejudice. Id .
Turning now to this case, Judge Bradshaw committed precisely the same error by imposing the nexus test when evaluating Washington's mitigation evidence. Evidence of brain damage and mental disorders is important to the mitigation analysis. See, e.g. , Daniels v. Woodford , 428 F.3d 1181, 1209 (9th Cir. 2005) ; Caro v. Woodford , 280 F.3d 1247, 1258 (9th Cir. 2002) ("More than any other singular factor, mental defects have been respected as a reason for leniency in our criminal justice system."); Mitchell v. United States , 790 F.3d 881, 903-04 (9th Cir. 2015) ("Evidence that [Petitioner] was a chronic user of alcohol and drugs from a young age is the kind of 'classic mitigating evidence' that counsel must pursue at the penalty phase ...."). However, when evaluating Washington's newly-presented evidence of diffuse brain damage, Judge Bradshaw discounted the value of the evidence because "[there was nothing] at the time of the offenses, which lessened his ability to differentiate right from wrong or to conform his actions with the law." As we concluded in Robinson , Judge Bradshaw's analysis erroneously demanded that the newly-presented evidence relate to Washington's guilt for the charged offense, a standard that has been squarely rejected by the United States Supreme Court. See, e.g. , Smith , 543 U.S. at 45, 125 S.Ct. 400 ; Tennard , 542 U.S. at 283, 124 S.Ct. 2562.
Thus, we must conclude here that there is a reasonable probability that the outcome of sentencing would have been different if the trial court had been presented with evidence of Washington's cognitive defects and had properly evaluated that evidence consistently with the Supreme Court decision in Smith . As in Robinson , our confidence in the Arizona court's imposition of the death sentence has been undermined, and we remand.
IV
Washington received ineffective assistance of counsel at the penalty phase and *433was prejudiced thereby. He is thus entitled to relief in the form of a new penalty phase trial. We reverse the district court's denial of a writ of habeas corpus as to the penalty phase and remand with instructions to grant the writ of habeas corpus unless the state court undertakes resentencing proceedings within a reasonable time to be determined by the district court.4
REVERSED and REMANDED for issuance of a writ of habeas corpus.5