The term, “burden of proof,” represents a party’s obligation to prove a fact to the court with the requisite degree of certainty. California Evidence Code section 115 lists those degrees of certainty as “rais[ing] a reasonable doubt, . . . establish[ing] the existence or nonexistence of a fact by a preponderance of the evidence, by clear and convincing proof, or by proof beyond a reasonable doubt.”
In Beck Development Co. v. Southern Pacific Transportation Co. (1996) 44 Cal.App.4th 1160, 1205, the Third District Court of Appeal discussed the burden in civil cases:
In a civil case the party with the burden of proof must convince the trier of fact that its version of a fact is more likely than not the true version. Stated another way, it requires the burdened party to convince the trier of fact that the existence of a particular fact is more probable than its nonexistence—a degree of proof usually described as proof by a preponderance of the evidence.
Accordingly, the burdened party will prevail if he or she establishes a fact as more likely than not—even by the slimmest of margins. But what happens when that party can only establish a fact to an exact 50% likelihood? A favorite law school hypothetical presents this situation in this manner: One of a pair of identical twins commits a tortious act against a victim. In the ensuing civil case, the victim has the burden of proving by a preponderance of the evidence that a certain individual harmed him or her. While it can be proven that one of the twins committed the act, the victim cannot determine which of the two the actual perpetrator was. As far as the victim can tell, each twin is exactly as likely to be the tortfeasor as not—in other words, each would be exactly 50% likely to be the cause of the injury. What would a court do in such a situation? A British court was faced with this precise question when the victim of an assault could not determine which of two twins had battered him. The court came to the same conclusion that the law student would in the hypothetical—the victim was unable to meet the burden of proof so neither twin was liable for the injuries.
Aside from indistinguishable twin scenario, there is another context in which twins are frequently the subject of law school hypotheticals: Situations where only one of two conjoined twins commits a crime. Would the inseparable innocent twin also be punished? Apparently law professors and students aren’t the only ones to puzzle over this question. In 2009, Slate readers selected it as the question of the year. The article explores historical examples of conjoined twins tried for crimes and possible methods of punishing the perpetrating twin without harming the innocent one. In the end, however, how a court would handle such a situation remains to be seen.
For those interested in learning how to meet one’s burden of proof, the Alameda County Law Library has numerous popular titles on evidence, including Witkin’s and Wigmore’s treatises and Jefferson’s benchbook. In addition, the Library has AmJur Proof of Facts, the standard resource for information on how to establish elements necessary to prove a case. To the great disappointment of law professors and students, none of these resources discuss issues dealing with indistinguishable identical twins or conjoined twins.