Law and Morality: The Decisive Oath as a Means of Resolving Legal Disputes

How does a society resolve legal disputes lacking evidence? Although such cases would typically be dismissed by modern courts, an interesting but little-known alternative element observed in some legal systems is the option to request a decisive oath in a lawsuit. Distinct from ordinary (testimonial) oaths that are routinely administered to witnesses at trial with the aim of eliciting only truthful testimony, a decisive oath can end a lawsuit in cases where the plaintiff has no evidence. We argue that the oath option may improve on burden-of-proof rules because standard theory overlooks the possibility that some defendants will truthfully admit their guilt after having professed an oath to be truthful. The reason is that people with strong religious or moral beliefs have internalised an ethic of honesty that overrides material consequences. We show that when the fraction of such people is large enough, the oath option achieves a more accurate resolution of disputes than burden-of-proof rules. Evidence from the Ottoman courts in Istanbul during the 16th and 17th centuries supports the argument regarding the positive relationship between religiosity and the use of the decisive oath for dispute resolution. The viability of the decisive oath declined in recent centuries, despite remaining ‘on the books’ in some countries. We suggest that its global demise can be attributed to rising intellectual opposition, increasing complexity of laws and/or legal disputes, falling cost of gathering evidence, and a general trend toward the separation of religion and state.