What is the test for establishing causation in a personal injury action?

(1985) 163 Cal.App.3d 396, 403.) "The law is well settled that in personal injury action causation must be proven within a reasonable medical probability based upon competent expert testimony. Mere possibility alone is insufficient to establish a prima facie case. [Citations.]" (Id. at pp. 402-403; accord Cottle v. Superior Court (1992) 3 Cal.App.4th 1367, 1385 [in personal injury case, "causation must be founded upon expert testimony and cannot be inferred from the jury's consideration of the totality of the circumstances unless those circumstances include the requisite expert testimony on causation"].)

Expert testimony is required to establish causation to a reasonable medical probability because knowledge of injuries that are wholly subjective is generally beyond the common experience of the average fact finder. (See Evid. Code, 801, subd. (a).) Accordingly, in establishing causation in a personal injury case, a victim's own self-interested testimony may not serve as a substitute for the testimony of a medical expert. (See, e.g., Oliveira v. Warren (1938) 24 Cal.App.2d 712, 716-717.)