The Conjunction Problem and the Logic of Jury Findings

Abstract

For several decades, evidence theorists have puzzled over the following paradox, known as the "conjunction problem." Probability theory appears to tell us that the probability of a conjunctive claim is the product resulting from multiplying the probabilities of its separate conjuncts. In a three element negligence case (breach of duty, causation, damages), a plaintiff who proves each element to a 0.6 probability, will have proven her overall claim to a very low probability of 0.216. Either the plaintiff wins the verdict based on this low probability (if the jury focuses on elements) or the plaintiff loses despite having met the condition of proving each element to the stated threshold. To solve this "conjunction problem," evidence theorists have advanced such proposals as changing the rules of probability, barring probability theory entirely from analysis of adjudicative factfinding, abandoning the procedural principle that the defendant need not present a narrative of innocence or non-liability, or dispensing with the requirement that the overall claim must meet an established burden of proof. This article argues that the conjunction paradox in fact presents a theoretical problem of little if any consequence. Dropping the condition that proving each element is a sufficient, as opposed to merely a necessary condition for proof of a claim, makes the conjunction problem disappear. Nothing in logic or probability theory requires this "each element/sufficiency" condition, and the legal decision rules reflected in most jury instructions do not mandate it. Once this "each element/sufficiency" condition is removed, all that is left of the conjunction problem is a "probability gap," an intuitive but ill-founded impression that the mathematical underpinnings of the conjunction problem are "unfair" to claimants. This probability gap is considerably narrowed by recognizing the probabilistic dependence of most facts internal to a given claim, and by applying the correct multiplication rule for probabilistically dependent events. Finally, the article argues that solving the conjunction problem is an insufficient ground either to abandon probability theory as a useful analytical tool in the context of adjudicative factfinding, or reform decision rules for trial factfinders.