Fernando Caro appeals the District Court’s dismissal of his writ of habeas corpus pursuant to 28 U.S.C. § 2254.1 In this opinion, we consider whether Caro was entitled to an evidentiary hearing on his claim that counsel’s failure to investigate the combined effects of Caro’s extraordinary exposure to neurotoxicants, neurological impairments, and personal background constituted ineffective assistance of counsel at the penalty phase trial. We conclude that Caro is entitled to an evidentiary hearing and remand to the district court for further proceedings.
FACTUAL AND PROCEDURAL BACKGROUND
Caro was convicted on October 16,1981, of two counts of first degree murder, the kidnapping of one of the victims, and two counts of assault with intent to commit murder. On December 10, 1981, a sentencing jury returned a verdict of death. On October 6, *12251988, the California Supreme Court set aside one of the special circumstance findings as duplicative, but otherwise affirmed the judgment. People v. Caro, 46 Cal.3d 1035, 251 Cal.Rptr. 757, 761 P.2d 680 (1988). Caro subsequently filed two writs of habeas corpus with the California Supreme Court. The court denied both writs. Caro then filed a writ of habeas corpus in the United States District Court for the Northern District of California. The District Court denied Caro’s request for an evidentiary hearing, granted summary judgment for the State, and dismissed the petition.
Caro is of Mexican-American and Yaqui Indian heritage. He was raised in a racially divided rural community in conditions of abject poverty. The oldest son of farm laborers, Caro spent most of his childhood living in a shack surrounded by livestock yards and agricultural fields. The house had no running water or indoor plumbing and, due to its proximity to the livestock yards, was filled with flies and other insects that “made the sky black.” During the summer, Caro and his family joined the stream of migrant farm workers. The family slept in fields, open tents, or along the road, sometimes using empty crates as beds. Caro worked alongside the adults as soon as he was able to walk.
Caro’s role as the oldest son of a traditional family meant that his parents’ expectations of him were higher than those for his siblings. Declarations from his mother, sister, and other relatives all indicate that Caro was the most severely beaten of his brothers and sisters because he was the oldest. Both parents beat Caro severely throughout his childhood, hitting him with closed fists, sticks, belts, tools, and boards, and kicking him with work boots.
Caro suffered severe head injuries as a child. His mother reports that he was born with a three inch lump on his head due to the use of forceps during his difficult delivery. At the age of three, Caro was struck by a car and fell back and hit his head. A water cooler fell on his head that year as well.
Caro also suffered from acute and chronic exposure to neurotoxie chemicals throughout his life, some of which have been documented to cause otherwise inexplicable aggressive behavior. Caro’s family used water laden with pesticides both at home and in the fields for drinking, bathing, and cleaning. His family used insecticide on a daily basis to fight the insect infestation at home. At the age of one and a half, Caro drank a bottle of Clorox and went into convulsions, turning blue and foaming at the mouth, for four to six minutes. Throughout his childhood, Caro worked and played in fields covered with pesticides. One day, while playing on a tank of ammonia, Caro peered into the tank and lost consciousness from the fumes.
During high school, Caro worked as a “flagger” — someone who stands in the fields to indicate to the crop duster where to dump his load of pesticides. Caro was denied the protective clothing his supervisors wore as they watched him mix and load the chemicals prior to dumping. Because he had no access to a shower or time to change clothes before school, Caro would spend all day in pesticide-drenched clothes. The pungent smell of the pesticides earned him the nickname “Stinky” among his classmates.
As an adult, Caro worked for the two years prior to his arrest as a maintenance worker at a corporation that produced toxic pesticides. One of Caro’s primary tasks was to clean the air ventilation system and the mixing and heating equipment. Workers were not given protective respiratory devices. Rather, they were tethered with a rope to a waist belt so they could be pulled out of the system when they lost consciousness. Workers also cleaned contaminated equipment, holding pieces of equipment in vats of solvents with their hands. Plastic protective gloves regularly tore during this work. Caro volunteered to break down five or six transformers containing polychlorinat-ed byphenyls (“PCBs”), formerly banned from use. Caro also formed part of the plant’s Emergency Response Team, responding to at least three accidents during his employment. During one of these accidents, the cloud of Parathion that was released burned the paint off the walls.
STANDARD OF REVIEW
The District Court’s denial of an evidentiary hearing is reviewed for an abuse of *1226discretion. Swan v. Peterson, 6 F.3d 1373, 1384 (9th Cir.1993) (citing Greyson v. Kellam, 937 F.2d 1409, 1412 (9th Cir.1991)).
DISCUSSION
Caro is entitled2 to an evidentiary hearing if (1) he has alleged facts that, if proved, would entitle him to relief; and (2) the state court trier of fact has not, after a full and fair hearing, reliably found the relevant facts. See Turner v. Marshall, 63 F.3d 807, 815 (9th Cir.1995) (quoting Tinsley v. Borg, 895 F.2d 520, 530 (9th Cir.1990)); see also Townsend v. Sain, 372 U.S. 293, 83 S.Ct. 745, 9 L.Ed.2d 770 (1963), rev’d on other grounds by Keeney v. Tamayo-Reyes, 504 U.S. 1, 112 S.Ct. 1715, 118 L.Ed.2d 318 (1992); Siripongs v. Calderon, 35 F.3d 1308, 1314 (9th Cir.1994).
In order to demonstrate that his counsel was ineffective, Caro must show that his counsel’s performance was so deficient that it fell below an “objective standard of reasonableness.” Strickland v. Washington, 466 U.S. 668, 688, 104 S.Ct. 2052, 80 L.Ed.2d 674 (1984). Effectiveness must be judged as of the time the legal services were rendered so as to minimize the distortions of hindsight. Bonin, 59 F.3d at 835. In addition, Caro must show that the deficient performance rendered the results of his trial unreliable or fundamentally unfair’. See Strickland, 466 U.S. at 687, 104 S.Ct. 2052; see also Lockhart v. Fretwell, 506 U.S. 364, 369, 113 S.Ct. 838, 122 L.Ed.2d 180 (1993).
Failure to investigate a defendant’s organic brain damage or other mental impairments may constitute ineffective assistance of counsel. See, e.g., Hendricks v. Calderon, 70 F.3d 1032, 1043-44 (9th Cir.1995); Evans v. Lewis, 855 F.2d 631, 637-38 (9th Cir.1988). In addition, this Court has recognized that the failure to present evidence necessary to bridge a cultural gap may constitute ineffective assistance of counsel. Siripongs, 35 F.3d at 1316. Counsel have a duty to make a reasonable investigation such that they are able to make informed decisions about how best to represent their clients. Strickland, 466 U.S. at 691, 104 S.Ct. 2052; Sanders v. Ratelle, 21 F.3d 1446, 1456 (9th Cir.1994). “Thus, we have found counsel ineffective where he neither conducted a reasonable investigation nor made a showing of strategic reasons for failing to do so.” Sanders, 21 F.3d at 1456.
Caro has presented several declarations from experts stating that no evaluation of his mental capacities is complete without information concerning Caro’s exposure to neurotoxicants as well as his personal background. Caro was examined by four experts prior to trial, including one medical doctor, a psychologist, and a psychiatrist. None of the experts indicated that Caro suffered from a mental impairment severe enough to constitute legal insanity or diminished capacity. Nevertheless, the medical doctor who examined Caro prior to trial has since declared that, had he known of Caro’s extraordinary exposure to pesticides, the severe physical, emotional, and psychological abuse he suffered as a child, and his social and cultural history, he would have testified that Caro had a diminished mental capacity. In addition, the competence of the psychiatrist is subject to question, since he prompted a suicide attempt by telling Caro that he should have the right to take his own life. Finally, none of the experts were neurologists or toxicologists and none conducted the neurological testing needed to-evaluate the effects that the pesticides and chemicals had on Caro’s brain.
Counsel have an obligation to conduct an investigation which will allow a determination of what sort of experts to consult. Once that determination has been made, counsel must present those experts with information relevant to the conclusion of the expert. Counsel in this case was aware of Caro’s extraordinary acute and chronic exposure to neurotoxicants, and yet he failed to consult either a neurologist or a toxicologist, experts on the effects of the chemical poisoning. In *1227addition, he failed to provide those experts who did examine Caro with the information necessary to make an accurate evaluation of Caro’s neurological system.
The conclusion of the dissent is based upon a misunderstanding of the difference in the mental and medical issues discussed in Williams v. Calderon, 52 F.3d 1465 (9th Cir.1995), and those presented here. In Williams, referring to the guilt phase of the trial, we stated that “no psychiatrist would have made a difference.” Id. at 1470. We have set forth in our memorandum disposition which pertains only to the guilt phase of Caro’s trial that the lack of medical evidence discussed in this opinion was not ground for granting the writ in the guilt phase.
However, this case is now about the sentencing phase of Caro’s trial and the effect in that phase of Caro’s chemically-induced neurological damage. It has been demonstrated that such poisoning causes inexplicable and aggressive behavior. No amount of disdain for the Yale Medical School can change the fact that the jury here was never presented with the most important evidence of mitigation — the chemical poisoning of Caro’s brain. Furthermore, the dissent fails to understand the difference between chemical poisoning causing brain damage which results in aggressive behavior and the death of a brain cell caused by the lack of oxygen as the result of a stroke which does not cause aggressive behavior. As the dissent does not understand that difference, then it is clear that a jury must have such a difference explained by experts. It may be the difference between life and death.
It is imperative that all relevant mitigating information be unearthed for consideration at the capital sentencing phase. “The Constitution prohibits imposition of the death penalty without adequate consideration of factors which might evoke mercy.” Hendricks, 70 F.3d at 1044 (quoting Deutscher v. Whitley, 884 F.2d 1152, 1161 (9th Cir.1989), vacated and remanded on other grounds, 500 U.S. 901, 111 S.Ct. 1678, 114 L.Ed.2d 73 (1991)); see also Penry v. Lynaugh, 492 U.S. 302, 319, 109 S.Ct. 2934, 106 L.Ed.2d 256 (1989).
The government argues that Caro suffered no prejudice during the sentencing phase because the jury was presented with extensive mitigating evidence, there was overwhelming evidence of Caro’s guilt, and aggravating factors weighed against him. The sentencing jury was aware that Caro was beaten and suffered head injuries as a child. The jury also knew that Caro worked as a flagger in high school and at an agricultural chemical company as an adult. The jury did not, however, have the benefit of expert testimony to explain the ramifications of these experiences on Caro’s behavior. Expert evidence is necessary on such issues when lay people are unable to make a reasoned judgment alone.
In addition, this Court has held that overwhelming evidence of guilt does not ameliorate the failure to present mitigating evidence at the penalty phase. Hendricks, 70 F.3d at 1044. “The determination of whether to impose a death sentence is not an ordinary legal determination which turns on the establishment of hard facts. The statutory factors [in California] give the jury broad latitude to consider amorphous human factors, to weigh the worth of one’s life against his culpability.” Id. The type of mitigating evidence omitted here is precisely the type most likely to affect a jury’s evaluation of the punishment Caro should receive.
Babbitt v. Calderon, 151 F.3d 1170 (9th Cir.1998), is distinguished from this case. Babbitt is a case in which the defendant suffered from post traumatic stress disorder because of his experiences in Vietnam. Id. at 1173. At trial, Babbitt presented testimony from two experts concerning his condition. Id. Also, at the habeas proceeding in district court, trial counsel stated that he had hired an experienced death penalty investigator who conducted a thorough investigation into Babbitt’s history and found every piece of relevant evidence regarding Babbitt’s background that could bear in mitigation. Id. at 1176. The court held that, with respect to both guilt and penalty phases, counsel’s failure to pursue further evidence that was largely cumulative of the testimony given at the trial was not constitutionally deficient. Id. at 1174, 1176.
*1228All that is entirely different from Caro’s case. Caro’s is not a case of stress caused by a stressful history; rather, it is a case of chemical poisoning of the brain causing aggressive behavior. Unlike Babbitt’s case, not a single piece of evidence of Caro’s condition was introduced at trial. Counsel, although aware of the acute and chronic exposure to neurotoxie chemicals, did nothing with regard to acquiring experts on the effects of chemical poisoning. In addition, counsel failed to provide those who did examine Caro with the information that he had. All counsel had to do was ask the question “What did all that extraordinary exposure to chemicals do to his brain?” And then, in order to find the answer, he merely had to address the question to either a neurologist or a toxicologist. Similarly, he could have asked the question to the experts he did retain who would have then told him that he needed to consult others.
This cases differs from Babbitt where “[t]here was no need for counsel to seek out [additional experts] independently.” Id. at 1174. It is clear that, contrary to what the dissent believes, Babbitt and this case are not based “on materially similar facts,” see ante at newly added dissent footnote 1, because every piece of relevant evidence in mitigation was not presented here at trial.
We also held in Babbitt that even if there was deficient performance of counsel that the defendant could not show prejudice. Id. at 1175-76. In Caro’s case, on the issue of prejudice, the dissent does not state that the case for prejudice does not exist, only that it is “weak.” Ante, at 1177 n. 2. It is submitted that “weak” prejudice is prejudice nonetheless, particularly when the jury was not presented with the single most important evidence of mitigation — chemical brain poisoning causing aggressive behavior.
Finally, with reference to footnote one in the revised dissent, robust dissent always has been universally recognized as an essential part of the judicial process as it requires judges to focus with particularity upon those matters in the case that divide them. The majority welcomes the intensity of the dissent in this case. However, it is difficult to believe that the dissent would base its test for determining constitutional Sixth Amendment competency of attorneys on its personal unsubstantiated belief in what “good lawyers” often do, rather than on what actually happened in this case.
It must be understood that Caro’s brain injuries and poisoning are different from the facts of any other case that has been called to the court’s attention. Caro suffered severe head injuries at birth and afterwards. As a child, he drank and bathed in water contaminated with poisons and then, in high school,- worked in fields where he was drenched in pesticides. As an adult, he worked in vats filled with toxic pesticides without the benefit of any protective breathing devices. A lawyer who knows of but does not inform his expert witnesses about these essential pieces of information going to the heart of the case for mitigation does not function as “counsel” under the Sixth Amendment, let alone as a “good lawyer.” Furthermore, a lawyer whose expert witness advises his client to commit suicide must engage in what the dissent pejoratively labels “witness shopping.”
Despite Caro’s efforts to obtain an eviden-tiary hearing on this issue, neither the district court nor the state courts have reliably found the facts relevant to this claim. Caro is therefore entitled to an evidentiary hearing to determine whether he suffered brain damage as a result of his exposure to neuro-toxicants and his personal background. If so, counsel’s failure to investigate will have rendered the results of Caro’s penalty phase trial unreliable.
The judgment of the district court denying the petition for writ of habeas corpus as it pertains to the sentencing phase of the petition is remanded to the district court for an evidentiary hearing as set forth in this opinion.
REMANDED.
. By a memorandum disposition filed this date we have concluded that petitioner is not entitled to habeas corpus relief on the guilt phase issues raised in his petition.
. The Anti-Terrorism and Effective Death Penalty Act's amendments to 28 U.S.C. § 2254 do not apply retroactively. Lindh v. Murphy, 521 U.S. 320, 117 S.Ct. 2059, 138 L.Ed.2d 481 (1997); see also Jeffries v. Wood, 103 F.3d 827, 827 (9th Cir.1996) (en banc). We therefore apply pre-1996 law.