Does the oath enhance truth-telling in eyewitness testimony? experimental evidence

Objective. Eyewitness testimony strongly influences conviction and sentencing; yet false testimony is a leading cause of wrongful convictions. We investigate whether the truth-telling oath — common in legal systems — reduces eyewitness deception. Hypothesis. While prior work emphasizes eyewitness reliability, little is known about whether courtroom procedures like the oath increase truthfulness. Drawing on psychological and economics literature, we hypothesize that oath-taking acts as a commitment device that reduces deception. Method. In a 22 between-subjects experiment combining legal psychology and experimental economics, participants watched a video and answered questions under varying incentive and oath conditions. Performance-based incentives conflicted with honesty, allowing us to isolate the effect of the oath. Results. The oath reduced deception by 39.2% (95% CI [20.1, 59.5]) even under incentives to lie, without impairing truth-tellers' performance. Conclusions. The courtroom oath is not merely symbolic — it plays a measurable and substantial role in enhancing the integrity of eyewitness testimony. Eyewitness testimony is the most powerful form of evidence in a court of law. Eyewitnesses affect both the odds of conviction and the severity of sentences of the guilty. But eyewitnesses also lie, and false testimony is the primarily cause of wrongful convictions. Most of the extant literature focuses on eyewitness reliability and credibility assessment, but very little is known about the efficiency of the main mechanism used in-field to foster eyewitness honesty: a solemn truth-telling oath—the most ancient and worldwide institution used in the solemn legal ceremony underpinning criminal cases. Herein we examine how the truth-telling oath actually affects the level of eyewitness deception. Using a controlled experimental test designed to address this question, we show that an eyewitness who is exogenously incentivized to lie and takes a solemn oath is significantly less likely to use deception. In contrast with the related literature focusing on the detection of lies, we show that an oath actually works to improve truth-telling. The oath is not just ceremonial, it plays a key role in improving efficiency within the court.