The Cultural Evolution of Oaths, Ordeals, and Lie Detectors

Abstract

In a great variety of cultures oaths, ordeals, or lie detectors are used to adjudicate in trials, even though they do not reliably discern liars from truth tellers. I suggest that these practices owe their cultural success to the triggering of cognitive mechanisms that make them more culturally attractive. Informal oaths would trigger mechanisms related to commitment in communication. Oaths used in judicial contexts, by invoking supernatural punishments, would trigger intuitions of immanent justice, linking misfortunes following an oath with perjury. These intuitions would justify the infliction of costs on oath takers in a way that appears morally justified. Ordeals reflect the same logic. Intuitions about immanent justice link a worse outcome following the ordeal with a guilty verdict. This link justifies the application of the ordeal, and the fixed costs involved (burning, poisoning). Lie detectors also rely on the creation of a link between a specified outcome and a guilty verdict. However, they do not rely on intuitions about immanent justice, but on a variety of intuitions ranging from the plausibly universal to the culturally idiosyncratic. As a result, lie detectors involve lower fixed costs than ordeals, and are less cross-culturally successful than oaths or ordeals.