Judges and juries often make factual decisions even if the facts are disputed and there is no clear-cut evidence available. Despite this common state of uncertainty, verdicts are thought of as having clear winners and losers––either the plaintiff wins and receives a full remedy, or the defendant wins and the plaintiff gets nothing. In private disputes, factfinders base their binary factual determinations on the preponderance of the evidence. There are, however, several doctrines that allow for partial remedy, discounted by the probability that the facts support the plaintiff’s case, given the available evidence (proportional liability). This Article offers a general theory for proportional liability in private law. It identifies three types of factual uncertainty—mutual uncertainty, unilateral uncertainty, and institutional uncertainty—and shows that legal economists should support proportional liability when the state of uncertainty is shared by the parties and the court (mutual uncertainty), and they should adopt an all-or-nothing rule whenever the information is observable but unverifiable (institutional uncertainty). In cases where one party holds private information (unilateral uncertainty), proportional liability is sometimes, but not always, superior to an all-or-nothing rule.

According to common conception, laws should make actors internalize all the costs and benefits of their actions to make them behave efficiently. This article shows that even when only partial internalization is possible, private law can create efficient incentives by ensuring that each actor internalizes an identical proportion of the costs and benefits. This proportional internalization principle has profound implications. In tort law, it offers a new mechanism for dividing liability between multiple parties. In contract law, it suggests a new default rule for joint ventures. And, in restitution law, it presents an alternative doctrinal formulation for restitution for unrequested benefit.

Behaviours are primarily regulated to reduce the risks of a negative outcome to others. This article discusses the use of outcomes as evidence of violations of a legal standard (outcome evidence). The current debate over outcome evidence centres around limited rationality. Opponents argue that factfinders’ estimations are distorted by hindsight bias, while supporters argue that factfinders properly update the probability of fault, given information about the outcome. The article adopts the rationality assumptions and argues that factfinders should nevertheless disregard outcome evidence in most cases unless the outcome can provide evidence that works for or against the defendant or when the law creates inefficient incentives to comply with the legal standard, then using adverse outcomes as evidence may help solve the problem of undercompliance. The article further shows that when evidence cannot be excluded, changes to the law governing primary behaviour are warranted to account for the distortionary effect of outcome evidence.

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