concurring.
I concur in affirming the district court’s grant of habeas corpus relief to Robert Cruz McLain on the basis of Hamilton error. I write separately because two additional bases also support the grant of habeas relief. The exclusion of testimony regarding McLain’s ability to adapt to life in prison violates Skipper v. South Carolina, 476 U.S. 1, 106 S.Ct. 1669, 90 L.Ed.2d 1 (1986). The California Supreme Court and the district court were correct in finding Skipper error. People v. McLain, 46 Cal.3d 97, 249 Cal.Rptr. 630, 757 P.2d 569, 575 (1988); McLain v. Calderon, 1995 WL 769176, *71 (C.D.Cal.1995). In addition, McLain received ineffective assistance of counsel, which forms a third basis for habeas relief.
I. Exclusion of Defense Testimony
A. Eighth Amendment Violation
McLain sought to introduce the expert testimony of Dr. Dean Clair, a prison staff psychologist. Dr. Clair had treated McLain, reviewed McLain’s prison records, and interviewed McLain for litigation purposes. Dr. Clair was to testify that given a life sentence instead of death, McLain would likely adjust well to prison, and would not pose a threat to prison staff or fellow inmates. McLain v. Calderon, 1995 WL 769176, *55. However, the trial court excluded Dr. Clair’s testimony, thus violating the mandates of the Eighth and Fourteenth Amendments as interpreted by the Supreme Court in Skipper.
The Eighth Amendment requires that the sentencer in a capital case must be allowed to consider “as a mitigating factor, ‘any aspect of a 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.’” Skipper, 476 U.S. at 4, 106 S.Ct. at 1670-71 (quoting Eddings v. Oklahoma, 455 U.S. 104, 110, 102 S.Ct. 869, *1387874, 71 L.Ed.2d 1 (1982); Lockett v. Ohio, 438 U.S. 586, 604, 98 S.Ct. 2954, 2964, 57 L.Ed.2d 973 (1978)(plurality)). In Skipper, the Court applied this general rule to “a defendant’s disposition to make a well-behaved and peaceful adjustment to life in prison,” Skipper at 7, 106 S.Ct. at 1672. Stated differently, “evidence that the defendant would not pose a danger if spared (but incarcerated)” must be allowed. Id. at 5, 106 S.Ct. at 1671. Here, McLain’s right to introduce Dr. Clair’s mitigating evidence was clearly violated.
B. Due Process Violation
In Gardner v. Florida, 430 U.S. 349, 97 S.Ct. 1197, 51 L.Ed.2d 393, the Supreme Court held that it was a violation of due process to apply a sentence of death “on the basis of information which [the defendant] had no opportunity to deny or explain.” Id. at 362, 97 S.Ct. at 1207. In Skipper, the Court unanimously stated: “Where the prosecution specifically relies on a prediction of future dangerousness in asking for the death penalty, it is not only the rule of Lockett and Eddings that requires that the defendant be afforded an opportunity to introduce evidence on this point; it is an elemental due process requirement ...” Skipper, 476 U.S. at 5, n. 1, 106 S.Ct. at 1671, n. 1. The prosecution in this case relied, albeit impliedly, on the argument that McLain would pose a persistent threat to prison safety if incarcerated for life. The prosecution submitted evidence that McLain twice attempted to escape from prison, once restrained a prison guard using a homemade knife, and repeatedly threatened that given the opportunity, he would act violently. In closing argument, the prosecutor underscored this evidence, contending that it showed McLain to be “far from a model prisoner.” The prosecution thus created the inference that McLain would pose a danger if sentenced to life imprisonment. The Due Process Clause, as interpreted in Gardner and Skipper, entitles McLain to rebut this inference.
C. Prejudice
On collateral appeal, relief is granted for a constitutional error where it “ ‘had [a] substantial and injurious effect or influence in determining the jury’s verdict.’ ” Brecht v. Abrahamson, 507 U.S. 619, 637, 113 S.Ct. 1710, 1722, 123 L.Ed.2d 353 (1993)(quoting Kotteakos v. United States, 328 U.S. 750, 776, 66 S.Ct. 1239, 1253 (1946)). If the error does not rise to that level, it is considered harmless. Bonin v. Calderon, 59 F.3d 815, 824 (9th Cir.1995), cert. denied, - U.S. -, 116 S.Ct. 718, 133 L.Ed.2d 671 (1996). “When a federal judge in a habeas proceeding is in grave doubt as to whether a trial error of federal law had a ‘substantial and injurious effect or influence in determining the jury’s verdict,’ that error is not harmless. And, the petitioner must win.” O’Neal v. McAninch, 513 U.S. 432, 436, 115 S.Ct. 992, 994, 130 L.Ed.2d 947 (1995).
Dr. Clair would have had considerable credibility with the jury because he was employed by the state department of corrections, and not in any way biased in favor of McLain. His expertise in the assessment and treatment of prisoners further augmented his credibility. According to expert testimony presented to the district court, Dr. Clair’s testimony would have been crucial to the jury, as future behavior in prison is a major consideration in the penalty phase of capital cases.
Contrary to the state’s argument, other evidence and testimony in this case do not duplicate Dr. Clair’s testimony. The state contends that the submission of McLain’s prison records supplanted the need for an expert opinion based in part on those same records. Without expert interpretation, however, these records are considerably less persuasive. Moreover, Dr. Clair relied on his first hand knowledge of McLain, in addition to records, to prepare his testimony.
Similarly, the state argues that other witness testimony effectively substituted for Dr. Clair’s opinion. Each of these witnesses, however, lacked either Dr. Clair’s credibility, expertise, or foundation of knowledge. Furthermore, only one of these witnesses mentioned that McLain would adapt well to life in prison, and this was not the thrust of his testimony, but only a passing remark.
The trial court did instruct the jury to disregard character evidence of future dan*1388gerousness in imposing the death penalty. However, this instruction did not protect McLain’s right to offer evidence of future adaptability to prison. Thus, the prosecution’s evidence went unrebutted, preventing the jury from considering McLain’s future behavior as a basis for leniency.
While the facts of McLain’s brutal crimes are significant aggravating factors, the district court found the mitigating evidence to be significant. McLain v. Calderon, 1995 WL 769176, *67-68.1 In this case, the aggravating evidence did not so overwhelm the mitigating evidence to render harmless the exclusion of Dr. Clair’s testimony.
In view of the credibility, persuasiveness, and distinct nature of Dr. Clair’s testimony, as well of the significance of the mitigating evidence, we should affirm the district court’s conclusion that the exclusion of Dr. Clair’s testimony had a substantial and injurious effect on the jury.
II. Ineffective Assistance of Counsel
A. Failure to Investigate
McLain has shown that his attorney’s performance was deficient and that it caused him prejudice. This constitutes ineffective assistance of counsel under Strickland v. Washington, 466 U.S. 668, 687-90, 104 S.Ct. 2052, 2064-66, 80 L.Ed.2d 674 (1984); Hendricks v. Vasquez, 974 F.2d 1099, 1109 (9th Cir.1992). To establish prejudice, “[t]he defendant must show that there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different. A reasonable probability is a probability sufficient to undermine confidence in the outcome.” Strickland, 466 U.S. at 694, 104 S.Ct. at 2068.
Specifically, “a particular decision not to investigate must be directly assessed for reasonableness in all the circumstances, applying a heavy measure of deference to counsel’s judgments.” Strickland, 466 U.S. at 691, 104 S.Ct. at 2066. Here, McLain’s attorney, Steve Pell, failed to: 1) discover all relevant mitigating evidence; 2) develop a social history based on all available records; or 3) provide McLain’s mental health expert with all available social history records. Evidence of McLain’s childhood sexual abuse, diagnosed brain damage, multiple head injuries, physical abuse, abject poverty, attempted suicides, and lack of treatment was all omitted. Pell did call McLain and two other members of McLain’s family to give testimony regarding McLain’s background. Dr. Jerome Evans also testified, regarding McLain’s mental condition. However, Pell never obtained releases to review McLain’s confidential records. Therefore, Pell’s decision to present this testimony was not based on adequate investigation. As the district court stated: “Pell’s failure to investigate this mitigating evidence cannot be construed as reasonable because he did not consciously explore the possibility of presenting it.” McLain v. Calderon, 1995 WL 769176, *100 (C.D.Cal.). Pell’s failure to investigate and present important mitigating evidence, taken alone, constitutes ineffective assistance of counsel.
CONCLUSION
I agree with the majority that Hamilton error merits a grant of habeas corpus relief in this case. McLain’s defense was further harmed by the exclusion of persuasive expert testimony and inadequate investigation by counsel. I write separately to note that these errors provide additional bases upon which to grant habeas corpus relief and CONCUR in the opinion.
. Evidence of McLain's deprived and violent childhood and his resulting mental disorders was found to be mitigating and are discussed in Part II, infra.