join as to the analysis in Part II, Concurring:
I.
I concur in the majority’s opinion and judgment that Mayfield is entitled to a new penalty phase hearing, or else to life imprisonment rather than death, because his counsel gave ineffective assistance in the penalty phase to his dire prejudice. I write separately to stress my views about prejudice at the penalty phase.
II.
It is important to recognize that the jury in the end was considering the fate of a young man barely out of his teens who did not have an extensive record involving major crimes or violence. In my view, there is a reasonable probability that the result would have been different if Mayfield’s counsel had called at least some family members or friends to testify in the penalty phase.
Mayfield is eligible for the death penalty because he was convicted, in a single proceeding, of two counts of first degree murder. See CaLPenal Code § 190.2(a)(3). As pertinent to my analysis, Mayfield contends that his counsel’s failure to present testimony from family members or friends to humanize Mayfield during the penalty phase was prejudicial. The district court rejected this claim, concluding that “had Ames presented the testimony of petitioner’s friends and family at the penalty phase, the Court finds that no reasonable probability exists that the jury would have returned a sentence of life.” Mayfield v. Calderon, 1997 WL 778685, at *18 (C.D.Cal.1997). I am not so sure this is right, indeed, I conclude it is wrong. On this prejudice issue Mayfield, for whom life or death hangs in the balance, deserves the benefit of the doubt.4
*934A claim of constitutionally ineffective assistance of counsel is shown when: (1) counsel’s efforts in defense are “outside the wide range of professionally competent assistance;” and (2) the defense thereby suffers prejudice: “a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different.” Strickland v. Washington, 466 U.S. 668, 690, 104 S.Ct. 2052, 80 L.Ed.2d 674 (1984). Ames’ deplorable efforts were far off the mark and plainly deficient for reasons explained by the majority opinion in part II G 1.
Although I share agreement with the majority’s conclusion that counsel’s performance offends the rule of Williams v. Taylor, 529 U.S. 362, 393, 399, 120 S.Ct. 1495, 146 L.Ed.2d 389 (2000), requiring counsel to present available mitigating evidence, I will comment on Ames’ deficient defense work. The calling of only one witness in the penalty phase, with no attention to family members or friends who could personalize and humanize Mayfield, under the circumstances of this case, meets the first part of the test for constitutional deficiency. The law governing the sentencer’s decision in the penalty phase gave the jury broad discretion to show mercy and spare life based on any consideration in May-field’s background or character appealing to the jury. See, e.g., CaLPenal Code § 190.3 (allowing evidence of “defendant’s character, background [and] history” to be presented during penalty phase); Cal.Penal Code § 190.3(k) (jury shall take into account “[a]ny other circumstance which extenuates the gravity of the crime even though it is not a legal excuse for the crime”). Notwithstanding the brutal murders for which he was convicted, family members or friends might have testified about humanizing aspects of Mayfield’s personality.
Naturally, we do not second guess trial counsel on strategic decisions that prove to be unsuccessful. See, e.g., Darden v. Wainwright, 477 U.S. 168, 184-87, 106 S.Ct. 2464, 91 L.Ed.2d 144 (1986) (rejecting Strickland claim when counsel made strategic decision not to have family members testify). But the record does not disclose that counsel made a strategic decision to avoid testimony from family members.5 To the contrary, counsel had not interviewed the family members, his investigator only interviewed a handful of people, and, from what appears in the record, Ames had no clue what they would say.6 *935Further, even if counsel knew all that was presented by the family members in the evidentiary hearing, I see no substantial strategic reason for counsel to fail to call members of Mayfield’s family, other than perhaps Mayfield’s mother, to testify during the penalty phase.7 Only Mayfield’s mother possessed allegedly damaging information that Mayfield beat his mother and that, as a twelve year old, he may have sexually abused a younger sister. In the state court evidentiary hearing, Mayfield’s sister and cousin were both specifically asked whether they had any knowledge of these incidents. Both said that they did not and in any event, it would not alter their opinion of Mayfield. There is no evidence that other family members had any knowledge of the possible sexual abuse.8 Nor is it clear that, had any witness known of the incident, relating it to the jury would have been of great prejudice to Mayfield, because he was only twelve at the time of the incident.9 I agree with the district court’s assessment that “the cross-examination of the witnesses at trial, with the exception of petitioner’s mother, would not have uncovered any information which would have outweighed the value of their testimony.” Mayfield, 1997 WL 778685, at *16.
I conclude that Ames’ failure in this case to call any family members or friends who could have humanized Mayfield falls well below “an objective standard of reasonableness,” as required by the first prong of Strickland, 466 U.S. at 688, 104 S.Ct. 2052.10
Having so concluded, I am not comfortable with the speculative conclusion reached by the district court that it would not have mattered. I take a different view of the *936analysis of prejudice than that of the district court. The district court seems to have believed that, despite the wealth of potentially humanizing evidence that Ames neglected to discover and introduce, the sentence was basically decided, along with the verdict of guilt, by the defendant’s videotaped confession and reenactment. Williams, however, reminds us: “Mitigating evidence unrelated to dangerousness may alter the jury’s selection of penalty, even if it does not undermine or rebut the prosecution’s death-eligibility case.” Williams, 529 U.S. at 398, 120 S.Ct. 1495.
And the Supreme Court in Penry v. Lynaugh, 492 U.S. 302, 109 S.Ct. 2934, 106 L.Ed.2d 256 (1989), has also unequivocally declared:
To provide the individualized sentencing determination required by the Eighth Amendment ... the sentencer must be allowed to consider mitigating evidence. Indeed, as Woodson v. North Carolina, 428 U.S. 280, 96 S.Ct. 2978, 49 L.Ed.2d 944 (1976), made clear, “in capital cases the fundamental respect for humanity underlying the Eighth Amendment ... requires consideration of the character and record of the individual offender and the circumstances of the particular offense as a constitutionally indispensable part of the process of inflicting the penalty of death.”
Penry, 492 U.S. at 316, 109 S.Ct. 2934 (internal citations omitted).
In cases involving even greater aggravating circumstances than here, we have found that counsel’s failure to present mitigating evidence during the penalty phase of the trial was prejudicial. See Mak v. Blodgett, 970 F.2d 614, 620-22 (9th Cir.1992) (affirming habeas relief when counsel failed to present mitigating evidence during the penalty phase; defendant was convicted of murdering thirteen people in a single night); see also Bean v. Calderon, 163 F.3d 1073, 1080-81 (9th Cir.1998) (finding prejudice where family portrait was an “unfocused snapshot;” other mitigating evidence was presented “only in the vaguest of terms”). In another case, we ordered an evidentiary hearing into a defendant’s ineffective assistance claim, rejecting the district court’s view that testimony from the defendant, the defendant’s ex-girlfriend, and a medical expert adequately covered defendant’s childhood. Hendricks v. Vasquez, 974 F.2d 1099, 1109-10 (9th Cir.1992). Three years later, we affirmed habeas relief in Hendricks’ case, writing:
The determination of whether to impose a death sentence is not an ordinary legal determination which turns on the establishment of hard facts. The [California] statutory factors give the jury broad latitude to consider amorphous human factors, in effect, to weigh the worth of one’s life against his culpability. Presumably the imposition of a death sentence is entrusted to a jury because it is a uniquely moral decision in which bright line rules have a limited place. In light of the whole record, and despite the substantial evidence of aggravation, we conclude that the failure of [counsel] to present mitigating evidence rendered the sentencing hearing neither fair nor reliable.
Hendricks v. Calderon, 70 F.3d 1032, 1044 (9th Cir.1995).
I conclude that this is such a case. Of course, we cannot be absolutely certain how testimony that was not presented would have affected the possibility that the jury would have shown mercy. But our law does not require certainty in this context. Instead, prejudice is shown where there is a “reasonable probability” of a different result: a “probability sufficient to undermine confidence in the outcome.” Strickland, 466 U.S. at 694, 104 S.Ct. 2052. *937As the Fifth Circuit has noted in Neal v. Puckett, 239 F.3d 683 (5th Cir.2001), in a state requiring a unanimous sentence, there need only be a reasonable probability that “at least one juror could reasonably have determined that ... death was not an appropriate sentence.” 239 F.3d at 691-92 (footnote omitted).
Applying that rule here, we must consider whether testimony from family members and friends could have humanized Mayfield, and we must assess the probable impact of Ames’ failure to do so on the jury’s penalty phase deliberations. In the best case scenario for Mayfield, such testimony from family members might have supported the view that Mayfield’s coldblooded murders of Ora Mae Pope and Edward John Moreno, while heinous, were out of character for him and at odds with his past behavior. But even if the humanizing witnesses could not have shown that much, there is still the real probability that they could have presented a sincere statement that Mayfield was not all bad. In this vein, the record developed in the state court evidentiary hearing suggests the following: a sister would have said that May-field was a gentle person and a peacemaker in the family; a brother would have said that Mayfield steered him away from gangs and drugs; an uncle would have said that Mayfield helped him when the uncle was disabled; and a cousin would have said that her children looked up to Mayfield and that Mayfield helped her four-year old overcome a speech impediment. See Wade v. Calderon, 29 F.3d 1312, 1323-25 (9th Cir.1994) (granting ha-beas relief because petitioner was severely prejudiced by counsel’s deficient performance at a capital trial, including his failure to prepare and investigate favorable witnesses; counsel in Wade was the same Ames who here represented Mayfield).
We have held that such humanizing testimony may be presented to a jury in a capital case and the failure to produce it can show prejudice. See Siripongs v. Calderon, 35 F.3d 1308, 1315-16 (9th Cir.1994) (granting an evidentiary hearing when counsel made only a “cursory investigation” of defendant’s background and made “no attempt to humanize him before the jury”); Mayes v. Gibson, 210 F.3d 1284, 1290-91 (10th Cir.2000) (granting an evi-dentiary hearing where trial counsel did not call family or friends to testify regarding defendant’s character).
The potential effect of mitigating testimony from family members is more likely to be significant when, as here, the aggravating factors are not overwhelming.1 In defendant’s case, the aggravating factors are: (1) two convictions in a single proceeding for first degree murder, (2) a 1982 conviction for a battery on the person of Mayfield’s ex-girlfriend, and (3) a 1982 arrest for discharging a firearm in an inhabited residence. Like Bean v. Calderon, “this is not a case in which a death sentence was inevitable because of the enormity of the aggravating circumstances.” 163 F.3d at 1081 (affirming habeas relief on Strickland claim when aggravating factors were: (1) burglary conviction, and (2) altercation in which defendant fired a shotgun). We have noted that prejudice is “especially likely” in cases like Bean where the aggravating factors are not overwhelming. Lambright v. Stewart, 241 F.3d 1201, 1208 (9th Cir.2001).
If it can be discerned, an examination of the jury’s deliberations is relevant to the question of prejudice. In Bean, we said:
*938[W]e find it noteworthy that the jury was initially divided over the appropriateness of the death penalty, deadlocking as to both murders before ultimately returning a death verdict.
163 F.3d at 1081. See also Murtishaw v. Woodford, 255 F.3d 926, 974 (9th Cir.2001) (granting habeas relief based, in part, on “length of the jury’s deliberations”); Mayes, 210 F.3d at 1291 (granting eviden-tiary hearing on Strickland claim due to the “relative weakness of the State’s case, the jury’s obvious struggle in deliberations, and the fact that only one aggravator was found”).
The jury only took two hours to arrive at a guilty verdict, concluding that there were two intentional murders, but the penalty phase was more difficult. Ames testified that the jury foreman told him that the jury “agonized over the question of penalty;” that on the first ballot during the penalty phase six jurors were in favor of life imprisonment, five in favor of the death penalty, and one undecided; and that eight ballots were taken before the jury finally arrived at a death sentence. While we cannot be certain of these points, we do know that the jury deliberated for nearly two days before reaching a sentence of death. We also know that the jury sent out a note asking whether all jurors must reach a unanimous verdict for a life sentence. The jury struggled on the limited penalty-phase record presented with whether death was the right penalty. We cannot be sure that the jury would not have concluded otherwise if family members had drawn a picture of Mayfield’s positive characteristics, had shown affection for him, and had directly or indirectly made a pitch for mercy. We cannot be sure that the jury would not have decided, in the words of John Milton, to “temper justice with mercy.” John Milton, Paradise Lost, book x, lines 77-78, (1674 ed.).
It must be acknowledged that some of our sister circuits, in cases predating Williams v. Taylor and involving differing facts, have concluded that failure to call witnesses during the penalty phase to humanize the defendant was not prejudicial. See Card v. Dugger, 911 F.2d 1494, 1508-11 (11th Cir.1990) (rejecting argument that counsel prejudiced defendant in relying on a psychologist as the sole witness during penalty phase rather then seeking more detailed testimony from family members); Williams v. Cain, 125 F.3d 269, 278-280 (5th Cir.1997) (finding no prejudice in failure to call family and friends when such testimony could have opened the door to testimony regarding drug use, expulsion from school, and discharge from job; positive testimony “would have had little mitigating effect against the aggravating evidence”). To a degree, these precedents argue against a determination of prejudice here. However, Strickland does not establish “mechanical rules.” 466 U.S. at 696, 104 S.Ct. 2052. To the contrary, the Strickland inquiry is fact-specific, requiring examination of “the totality of the evidence.” Id. at 695, 104 S.Ct. 2052. The facts of Mayfield’s case are distinguishable because family members, except for May-field’s mother, did not have damaging testimony and the aggravating circumstances were not as severe as in both of the aforementioned cases.
As Shakespeare reminded us: “The quality of mercy is not strain’d, It drop-peth as the gentle rain from heaven Upon the place beneath.”2 So too, in our analysis of prejudice, we must remind ourselves that the possibility of mercy, like the possibility of gentle rain, is not predictable with certainty. It was the jury’s duty to consider the possibility of mercy based on *939statutory factors that gave the jury a broad sway for action. Ames’ failure to humanize Mayfield lost a good chance for the jury’s mercy and undermines confidence in the penalty phase.
The jury’s struggle in this case without humanizing testimony suggests a reasonable probability of a life sentence had such testimony been presented. The family members had something to say. Given Mayfield’s youth and limited prior record, Mayfield meets his burden to show prejudice because there is a good chance that counsel’s substandard performance prejudiced the defendant at sentencing; there is a “probability sufficient to undermine confidence in the outcome.” Strickland, 466 U.S. at 694, 104 S.Ct. 2052.
The Sixth and Eighth Amendments, as incorporated by the Fourteenth Amendment’s due process clause, require us to grant the defendant’s habeas petition, vacate his sentence, and remand for re-sentencing. See Penry, 492 U.S. at 328, 109 S.Ct. 2934 (“Our reasoning in Lockett [v. Ohio, 438 U.S. 586, 98 S.Ct. 2954, 57 L.Ed.2d 973 (1978) ] ... thus compels a remand for resentencing so that we do not ‘risk that the death penalty will be imposed in spite of factors which may call for a less severe penalty.’ Lockett, 438 U.S. at 605, 98 S.Ct. 2954. ‘When the choice is between life and death, that risk is unacceptable and incompatible with the commands of the Eighth and Fourteenth Amendments.’ Lockett, supra, at 605, 98 S.Ct. 2954.”) (internal citations omitted).
Ill,
I agree with the opinion and judgment that it is correct to deny the petition for writ of habeas corpus as to the conviction, but grant the petition with respect to the sentence, giving the state the option of either accepting the imposition of a life sentence without parole or holding a new penalty phase hearing.
. Chief Judge Schroeder also concurs in Judge Graber's dissent, and in Judge Hawkins' separate opinion, and does not concur in the judgment of the Court.
. Judge Hawkins also concurs in Judge Gra-ber's dissent and does not concur in the judgment of the Court, as is reflected in his separately filed concurrence and dissent.
. Judge Berzon also concurs in the judgment of the Court, but not in the majority opinion.
. An ancient Roman legal maxim provided: in dubiis benigniora sunt semper prferenda. Dig. 50.17.56 ("Dig.” refers to the Digesta of the Byzantine emperor Justinian I, circa 530 A.D. The Digesla was gathered by a commission of sixteen lawyers who examined the writings of all known jurists, extracting whatever they deemed valuable. See New Encyclopedia Britannica, v.6 665-66.) This phrase can be interpreted to mean: "In case of doubt it is best to lean to the side of mercy.” H.L. Menken, Dictionary of Quotations 780 (Knopf 1966). Also, we find this maxim interpreted: “In doubtful cases, the more favorable are to be preferred.” S.S. Peloubet, Legal Maxims 106 (Rothman 1985). *934And further, we find it interpreted, "In all case of doubt the most merciful construction of facts should be preferred.” Coffin v. United States, 156 U.S. 432, 454, 15 S.Ct. 394, 39 L.Ed. 481 (1895). However interpreted, this ancient principle is not a stranger to our law. We require proof beyond a reasonable doubt to sustain a criminal conviction. See Coffin, 156 U.S. at 453-56, 15 S.Ct. 394 (collecting Roman law supporting the presumption of innocence and the requirement of guilt beyond a reasonable doubt); In re Winship, 397 U.S. 358, 361-64, 90 S.Ct. 1068, 25 L.Ed.2d 368 (1970). Cf. Lockett v. Ohio, 438 U.S. 586, 604-5, 98 S.Ct. 2954, 57 L.Ed.2d 973 (sen-tencer must be allowed to consider "any aspect of defendant’s character or record and any of the circumstances of the offense that the defendant proffers as a basis for a sentence less than death”); Callins v. Collins, 510 U.S. 1141, 114 S.Ct. 1127, 1129, 127 L.Ed.2d 435 (1994) (Blackmun, J., dissenting) (sentencer in capital case must be afforded "the power and discretion to grant mercy”).
. Indeed, in another case where counsel’s deficient performance was caused by a "complete lack of effort,” we noted that "[d]escrib-ing [counsel’s] conduct as ‘strategic’ strips that term of all substance.” Bloom v. Calderon, 132 F.3d 1267, 1277 (9th Cir.1997) (citations omitted); see also Smith v. Stewart, 140 F.3d 1263, 1269-71 (9th Cir.1998) (granting habeas relief when, with little effort, counsel could have developed mitigating evidence).
. Mark Hall, the defense investigator, interviewed Mayfield, Cicero Mayfield, Jr. (brother), Ivan Johnson (half-brother), Hazel Haw*935kins (mother), and Tommy Wydrmyr (friend). Although Hall gave transcripts of these interviews to Dr. Rath so that the information could be included in his testimony at the penalty phase, only Ms. Hawkins and Cicero Mayfield had humanizing testimony about Mayfield. Johnson and Wydrmyr’s testimony was only relevant to the question of diminished capacity.
. Counsel also stated that he made a strategic choice not to call family members because he did not want to put on a "parade of family members.” Mayfield, 1997 WL 778685, at *16. The district court correctly rejected this purported "strategy” because "trial counsel did not interview any of the potential witnesses, therefore, he had no informed basis upon which to make a tactical or strategic decision.” Id.
. In fact, Mayfield's mother testified during the evidentiary hearing that she told no one about the incident, other than one close friend, court officials, and Mayfield's defense investigator. When interviewed by the defense investigator, Mayfield’s mother told her other children to leave the room before discussing the possible sexual abuse.
. Even if this had come out through some family member’s testimony that was otherwise positive, it seems nonetheless speculative to assume that a jury would have voted to terminate Mayfield’s life in any substantial part because of an offensive and even perverted act as a twelve year old. It seems much more probable that a jury would have focused on the violence of the murders, on the one hand, and any exculpatory evidence of May-field's conduct that was more proximate to the murders, on the other. Doubtless the jury would care about twenty-two year old May-field’s conduct as a teenager, but the jurors would be much less likely to give controlling weight to an evil deed done at the age of twelve.
.I do not suggest that it is ineffective to fail to call all possible family members or friends or business associates or others who could humanize a defendant before a jury tasked with deciding life or death under broad statutory guidelines. Had counsel called any family member to testify, failure to call others would likely be seen as a strategic decision which could not properly be second-guessed in this proceeding. But to call none who would talk about Máyfield's human characteristics and even plead for mercy is to do too little.
. In fact, the trial court judge commented that he had never understood why the state sought death in Mayfield’s case.
. William Shakespeare, The Merchant of Venice, act IV, sc. 1.