I do not agree with the majority that the trial court here correctly instructed the jury on what is required, in a group beating context, for a finding under Penal Code section 1192.7, subdivision (c)(8), that a defendant personally inflicted great bodily injury. The instruction that the trial court read to the jury, CALJIC No. 17.20, presented two alternative theories under which the jury could find that defendant personally inflicted great bodily injury. As defendant persuasively argues, neither theory is correct. I agree with the Court of Appeal that the instruction was erroneous and that the error was prejudicial. Accordingly, I would affirm the Court of Appeal’s judgment setting aside the great bodily injury finding.
The majority correctly explains what is required for a finding that a defendant personally inflicted great bodily injury in a group beating context. The defendant must do more than merely aid or abet others who inflict great bodily injury; the defendant must physically participate' in the injury-producing conduct. On the other hand, the defendant need not have acted alone, and his conduct need not be the sole or even the predominate cause of the great bodily injury. The defendant’s physical participation need only be a *503substantial factor in producing the great bodily injury, or, in other words, the effect of the defendant’s conduct in producing the injury must be more than trivial or insignificant.
The trial court, however, did not accurately explain these requirements to the jury. The trial court gave this explanation: “When a person participates in a group beating and it is not possible to determine which assailant inflicted a particular injury, he or she may be found to have personally inflicted great bodily injury upon the victim, if one, the application of unlawful physical force upon the victim was of such a nature that, by itself, it could have caused the great bodily injury suffered by the victim; or two, that at the time that the defendant personally applied unlawful physical force to the victim, the defendant knew that other persons, as part of the same incident, had applied, were applying, or would apply unlawful physical force upon the victim and the defendant then knew or reasonably should have known that the cumulative effect of all the unlawful physical force would result in great bodily injury to the victim.” (Italics added.)
There was no need or justification for a jury instruction setting forth two theories. The majority succinctly summarizes the statute’s requirements in these terms: “[T]he statute calls for the defendant to administer a blow or other force to the victim, for the defendant to do so directly rather than through an intermediary, and for the victim to suffer great bodily injury as a result.” (Maj. opn., ante, at p. 493.) The majority adds the necessary qualification that the defendant’s conduct must “contribute substantially to the victim’s injured state.” (Id. at p. 494.) An instruction in these terms would have been accurate and sufficient.
The instruction’s first theory is that a defendant personally inflicts great bodily injury if “the application of unlawful physical force upon the victim was of such a nature that, by itself, it could have caused the great bodily injury suffered by the victim.” (Italics added.) The majority concludes that the jury would have understood that the words “ ‘application of unlawful physical force’ ” referred to defendant’s own acts. (Maj. opn., ante, at p. 500, fn. 10.) Although this is questionable, the more serious defect in the instruction is its use of the words “could have caused.”
When we say that someone “could have” done something, we mean only that the person had the ability or potential to do that thing. We do not mean that the person actually did the thing, and often we mean the opposite. For example, in the film On the Waterfront (Columbia Pictures 1954), a former boxer portrayed by the actor Marlon Brando famously said: “I could have been a contender. I could have been somebody, instead of a bum—which is what I am.” With these words, the former boxer was expressing regret that he had never been a contender.
*504Thus, to say that the defendant’s application of physical force “could have” caused the victim’s great bodily injury is only to say that it had the capacity or potential to cause such an injury, even though it may not actually have done so. This is not a correct description of what must be proved to support a finding of personal infliction of great bodily injury under Penal Code section 1192.7, subdivision (c)(8). The defendant’s force must actually have caused the great bodily injury in the sense that it contributed to the injury in a nontrivial way.
The instruction’s other theory is also wrong. The second theory is that a defendant personally inflicts great bodily injury if “at the time that the defendant personally applied unlawful physical force to the victim, the defendant knew that other persons, as part of the same incident, had applied, were applying, or would apply unlawful physical force upon the victim and the defendant then knew or reasonably should have known that the cumulative effect of all the unlawful physical force would result in great bodily injury to the victim.” In addition to the defendant’s application of force to the victim, this theory imposes two requirements: (1) the defendant must have known that others are also beating the victim; and (2) the defendant must have known that the cumulative effect of all the blows would result in great bodily injury. The second requirement includes within it a requirement that the cumulative effect of all the blows in fact results in great bodily injury.
As the majority cautiously observes, “the reference to what the defendant knew or should have known during the attack arguably imposes an additional evidentiary burden on the prosecution.” (Maj. opn., ante, at p. 501.) Although this aspect of the instruction’s second theory (the imposition of knowledge requirements) is almost certainly erroneous, it is not the aspect that misled the jury to defendant’s prejudice. Defendant was harmed by the instruction’s second theory insofar as it stated or implied that the causation requirement for a finding of personal infliction of great bodily injury may be satisfied by proof that defendant “personally applied unlawful physical force to the victim” during an incident in which others were also beating the victim and that “the cumulative effect of all the unlawful physical force . . . resulted] in great bodily injury to the victim.”
What the instruction should have explained, but did not, is that when an injury results from the cumulative effect of multiple blows, the defendant has personally inflicted that injury if, but only if, the blow or blows struck by the defendant were a substantial factor in causing the injury. (See People v. Catlin (2001) 26 Cal.4th 81, 155-156 [109 Cal.Rptr.2d 31, 26 P.3d 357]; In re M.S. (1995) 10 Cal.4th 698, 719-720 [42 Cal.Rptr.2d 355, 896 P.2d 1365].) The “substantial factor” test is explained in a standard Judicial Council jury instruction. As adapted to the situation here, the instruction reads: “There *505may be more than one cause of [great bodily injury]. An act causes [great bodily injury] only if it is a substantial factor in causing the [injury]. A substantial factor is more than a trivial or remote factor. However, it does not have to be the only factor that causes the [injury].” (See Judicial Council of Cal. Crim. Jury Instas. (2006) CALCRIM No. 240; see also CALJIC No. 3.41.) The instruction’s second theory is erroneous because it does not explain the “substantial factor” requirement, that the defendant’s own conduct be more than a trivial or remote factor in causing the great bodily injury. A finding that the cumulative effect of multiple blows from different individuals resulted in great bodily injury does not exclude the possibility that the blow or blows inflicted by one of those individuals were only a trivial factor and did not contribute substantially in producing the injury.
The majority’s defense of the instruction is unpersuasive. In response to defendant’s argument that the instruction’s first theory—that defendant’s application of force “could have caused” the injury—“invited speculation as to whether he personally inflicted harm” and permitted a finding “based solely on injuries caused by other assailants in the group,” the majority asserts that the instruction told the jury “that the defendant must have (1) participated in the group beating and (2) applied physical force directly to the victim, who (3) suffered great bodily injury as a result.” (Maj. opn., ante, at p. 500, italics added.) But the instruction does not state that the victim must have suffered bodily injury “as a result” of the physical force that defendant applied. The instruction’s first theory does not require that the victim suffer any injury as a result of the defendant’s use of force. The instruction’s only reference to the victim’s injury being a “result” of physical force is in its description of the second theory, and there the instruction referred to the injury being the result of the “cumulative effect” of the physical force applied by all participants, not by the defendant. Thus, in asserting that the instruction required the jury to find that the victim suffered great bodily injury “as a result” of the physical force that defendant applied, the majority reads into the instruction a concept that simply is not there.
In response to defendant’s argument that the instruction’s second theory is defective because it “substituted his knowledge of the force applied by others for the injury that he was personally required to inflict” (maj. opn., ante, at p. 501), the majority asserts that the instruction “requires] the defendant to apply physical force directly to the victim to such a significant degree that he adds to the ‘cumulative’ injurious effect” (id. at p. 501). Here also, the majority claims to find in the instruction a concept that it nowhere expresses. Nothing in the instruction required the jury to assess defendant’s contribution to the cumulative effect of the multiple blows for the purpose of determining whether it was more than a trivial or insubstantial factor in producing the injury.
*506Given the sharply conflicting evidence in this case, the instruction’s failure to explain the substantial factor causation requirement resulted in prejudice to defendant, requiring reversal of the finding under Penal Code section 1192.7, subdivision (c)(8), that defendant personally inflicted great bodily injury. Because the Court of Appeal correctly resolved this issue, I would affirm its judgment.
Appellant’s petition for a rehearing was denied September 20, 2006, and the opinion was modified to read as printed above. Kennard, J., was of the opinion that the petition should be granted.