What is the test for an aider and abettor's liability for a principal's criminal act?

An aider and abettor's liability for a principal's criminal act was described as "derivative" in People v. Brigham, supra, 216 Cal.App.3d 1039, 1052, 265 Cal.Rptr. 486. "Technically, only the perpetrator can (and must) manifest the mens rea of the crime committed. Accomplice liability is premised on a different or, more appropriately, an equivalent mens rea. [Citation.] This equivalence is found in intentionally encouraging or assisting or influencing the nefarious act." (People v. Luparello (1986) 187 Cal.App.3d 410, 439, 231 Cal.Rptr. 832.) The aider and abettor's liability "is a question of legal causation independent of any intention that the result obtained. [Citations.]" (People v. Rogers (1985) 172 Cal.App.3d 502, 515, 217 Cal.Rptr. 809, emphasis added; People v. Brigham, supra, 216 Cal.App.3d 1039, 1053, 265 Cal.Rptr. 486.)