In Tison v. Arizona (1987) 481 U.S. 137, two brothers helped a pair of convicted murderers -- their father and his cell mate -- escape from prison, then stood by doing nothing while the two men kidnapped, robbed, and killed an innocent family. ( Tison, supra, at pp. 151-152.) The Supreme Court said that a special circumstance felony-murder penalty was supported in that case by the brothers' major participation in the kidnapping and their display of reckless indifference to human life. (Id., at p. 158.) The two brothers aided an escape by bringing guns into a prison and arming two murderers, one of whom they knew had killed in the course of a previous escape attempt. After the breakout, one of the brothers flagged down a passing car, and both fully participated in kidnapping and robbing the vehicle's occupants. Both then stood by and watched as these people were killed. The brothers made no attempt to assist the victims before, during, or after the shooting, but instead chose to assist the killers in their continuing criminal endeavors. (Tison, supra, 481 U.S. at pp. 151-152.) The Supreme Court held that the brothers could be sentenced to death despite the fact they had not actually committed the killings themselves, stating: "Reckless disregard for human life implicit in knowingly engaging in criminal activities known to carry a grave risk of death represents a highly culpable mental state, a mental state that may be taken into account in making a capital sentencing judgment when that conduct causes its natural, though also not inevitable, lethal result. The brothers' own personal involvement in the crimes was not minor, but rather, ... 'substantial.' Far from merely sitting in a car away from the actual scene of the murders acting as the getaway driver to a robbery, each ... was actively involved in every element of the kidnapping-robbery and was physically present during the entire sequence of criminal activity culminating in the murders ... and the subsequent flight. The Tisons' high level of participation in these crimes ... implicates them in the resulting deaths." (Id. at pp. 157-158.) The issue in Tison was whether imposition of the death penalty on the brothers as aiders and abettors who did not kill and did not have the intent to kill would violate the Eighth Amendment. Specifically, the question was whether the Eighth Amendment's requirement that the death penalty be proportional to the culpability of the defendant be satisfied if there was no intent to kill. Recognizing that a reckless indifference to the value of human life "represents a highly culpable mental state," the high court in Tison observed. "some nonintentional murderers may be among the most dangerous and inhumane of all -- . . . the robber who shoots someone in the course of the robbery, utterly indifferent to the fact that the desire to rob may have the unintended consequence of killing the victim as well as taking the victim's property. This reckless indifference to the value of human life may be every bit as shocking to the moral sense as an 'intent to kill.' " ( Tison, supra, 481 U.S. at p. 157.) Tison also teaches that the culpable mental state of "reckless indifference to life" is one in which the defendant "knowingly engages in criminal activities known to carry a grave risk of death." ( Tison, supra, 481 U.S. at p. 157.) California has adopted the Tison definition. (Pen. Code 190.2, subd. (d); Tapia v. Superior Court (1991) 53 Cal.3d 282, 298, fn. 16, 279 Cal. Rptr. 592, 807 P.2d 434; People v. Estrada, 11 Cal.4th at p. 577.) The United States Supreme Court found that two defendants who aided their father in his escape from prison, but who did not actually commit an act of murder, could nonetheless be sentenced to death for the brutal murders committed by their father during his escape. The majority notes that Arizona law allows the death penalty for first degree felony murder. The Supreme Court found "'major participation in the felony committed, combined with reckless indifference to human life'" sufficient to support a sentence of death. The United States Supreme Court considered whether the death penalty constitutionally may be imposed on a defendant convicted of felony murder who was not the actual killer and did not intend to kill. The Supreme Court concluded that it could be so applied if the defendant was a major participant in the underlying felony and displayed "reckless indifference to the value of human life." (At p. 152.) In discussing the facts of the case, the court noted the defendants were far from minor participants and it could be inferred they "subjectively appreciated that their acts were likely to result in the taking of innocent life." (Ibid.) The court described the "highly culpable mental state" of "reckless disregard for human life" as "implicit in knowingly engaging in criminal activities known to carry a grave risk of death." (Id. at pp. 157-158.) The Court held the culpability requirement of the Eighth Amendment is satisfied and the death penalty may be imposed upon a defendant who, without an intent to kill and without being the actual killer, is a major participant in a felony that results in the death of a victim, and acts with a mental state of reckless indifference to human life. (Tison, supra, at pp. 152, 157-158.) The court held that the Eighth Amendment to the U.S. Constitution is not offended when the death penalty is imposed for "major participation in the felony committed, combined with reckless indifference to human life." ( Tison, supra, 481 U.S. at p. 158.) The court explained that "the reckless disregard for human life implicit in knowingly engaging in criminal activities known to carry a grave risk of death represents a highly culpable mental state, a mental state that may be taken into account in making a capital sentencing judgment when that conduct causes its natural, though also not inevitable, lethal result." ( Id. at pp. 157-158.) The Supreme Court held the brothers could be sentenced to death despite the fact they had not actually committed the killings themselves or intended to kill, stating: "Reckless disregard for human life implicit in knowingly engaging in criminal activities known to carry a grave risk of death represents a highly culpable mental state, a mental state that may be taken into account in making a capital sentencing judgment when that conduct causes its natural, though also not inevitable, lethal result. The brothers' own personal involvement in the crimes was not minor, but rather, ... 'substantial.' Far from merely sitting in a car away from the actual scene of the murders acting as the getaway driver to a robbery, each ... was actively involved in every element of the kidnapping-robbery and was physically present during the entire sequence of criminal activity culminating in the murders ... and the subsequent flight. The Tisons' high level of participation in these crimes ... implicates them in the resulting deaths." (Id. at pp. 157-158.)