What is the test for an aider and abettor to be found guilty as a principal in a crime?

"All persons concerned in the commission of a crime, . . . whether they directly commit the act constituting the offense, or aid and abet in its commission, or, not being present, have advised and encouraged its commission, . . . are principals in any crime so committed." ( 31.) The rationale for finding the defendant guilty as an accomplice has been described as follows: " '[W]hen an accomplice chooses to become a part of the criminal activity of another, she says in essence, "your acts are my acts," and forfeits her personal identity. We euphemistically may impute the actions of the perpetrator to the accomplice by "agency" doctrine; in reality, we demand that she who chooses to aid in a crime forfeits her right to be treated as an individual.' [Citation.]" (People v. Prettyman

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(1996) 14 Cal.4th 248, 259.) To be found guilty as a principal of a crime the defendant did not personally commit, the prosecution must show that the "aider and abettor act[ed] with knowledge of the criminal purpose of the perpetrator and with an intent or purpose either of committing, or of encouraging or facilitating commission of, the offense. [Citations.]" (People v. Beeman (1984) 35 Cal.3d 547, 560.) Factors the jury may consider when assessing aiding and abetting "include presence at the scene of the crime, companionship, and conduct before and after the crime, including flight." (People v. Haynes (1998) 61 Cal.App.4th 1282, 1294.)

A person may not be found guilty of a crime as an aider and abettor simply because he or she was present when the crime was committed or failed to take action to prevent it; those factors, however, are ones that a trier of fact may consider in assessing culpability. (People v. Nguyen (1993) 21 Cal.App.4th 518, 529-530.) Similarly, a person's knowledge of another's criminal purpose is insufficient of itself to support aider and abettor culpability; the person "must also share that purpose or intend to commit, encourage, or facilitate the commission of the crime. [Citation.]" (Id. at p. 530.)