Prior to testifying in the United States legal system, virtually all witnesses are required to make an explicit commitment to be truthful. Federal Rule of Evidence 603 states, “Before testifying, a witness must give an oath or affirmation to testify truthfully. It must be in a form designed to impress that duty on the witness’s conscience”. Further, the US legal system views lying under oath—perjury—as a serious and punishable offense. Pursuant to United States Legal Code, unless otherwise noted in this paper, the terms “oath” and “sworn” include “affirmation” and “affirmed”, respectively. Ultimately, the intended effect of taking an oath is to appeal to the morality and conscience of the witness in an attempt to dissuade them from lying. 
While the roots of the oath reach back to early thirteenth century England and are grounded in the principle of judicium dei—divine judgment—the oath or affirmation observed in United States courtrooms nowadays is secularized. This project examines whether the secular version of swearing to tell the truth impresses upon the adult testifier’s conscience enough to influence truthful disclosure. In addition, this project investigates both the language and commitment contained within the oath and interaction between the oath and motivation to be perceived as innocent, while simultaneously exploring truth telling as a method of ethical dissonance reduction. 
In other words, this project presents participants with an opportunity to behave dishonestly. We then examine their tendency to tell the truth about the dishonest behavior. The key independent variables are the level of morality evoked by manipulating the language of the oath and whether participants are motivated to be perceived as innocent. As will be described in detail below, predictions about the effects of these variables are derived from the framework of ethical dissonance. This project expands research on ethical dissonance by investigating the effect of oath-taking on truth-telling in mock legal settings. Specifically, this project investigates whether people tell the truth after taking an oath as a method of ethical dissonance reduction. 
The foundation of the American trial system is derived from the English system, which was fundamentally entwined not only with the belief in the existence of, but also the active presence of God. As the trial system evolved, the oath remained an integral part of witness testimony. This section provides a brief overview of the oath’s history.
When trial methods were still in early development, lay people did not believe they were competent enough to judge a man’s guilt or innocence. The lack of confidence in being able to accurately determine whether or not a man was guilty was most likely founded in humility stemming from judicium dei, or divine judgment—the belief that only God could truly search men’s hearts for their past deeds, intentions, and feelings of remorse. Because of this belief in divine judgment, priests participated in trials until an edict in 1215 abolished their involvement, and the oath was introduced as a proxy for priest participation. The oath accompanied the colonists as they settled what became the United States, where it remains an integral part of trial proceedings 800 years after its inception.
