Truth-default theory (TDT) is a communication theory which predicts and explains the use of veracity and deception detection in humans. It was developed upon the discovery of the veracity effect - whereby the proportion of truths versus lies presented in a judgement study on deception will drive accuracy rates. This theory gets its name from its central idea which is the truth-default state. This idea suggests that people presume others to be honest because they either don't think of deception as a possibility during communicating or because there is insufficient evidence that they are being deceived. Emotions, arousal, strategic self-presentation, and cognitive effort are nonverbal behaviors that one might find in deception detection. Ultimately this theory predicts that speakers and listeners will default to use the truth to achieve their communicative goals. However, if the truth presents a problem, then deception will surface as a viable option for goal attainment.

As an alternative view of deception and detection, truth-default theory was introduced by Timothy R. Levine. Levine is a Professor and Chair of Communication Studies at University of Oklahoma. While experimenting with deception detection, Levine found that, even in high suspicion situations, truth-bias still occurred. At first, truth-bias was thought of as flawed cognitive processing but later found to be functional and adaptive. After enough focus on truth-bias, truth-default theory began to take shape.

Knowing that you are intentionally misleading a person when communicating with them is considered deception. Deception in most cases is looked at as a negative thing that often leads to feeling of betrayal and distrust. There are several different types of deception such as lies, equivocations, concealments, exaggerations, and understatements. There are many reasons why people choose to use deception. Based on interpersonal deception theory, people often use deception to avoid punishment, maintain relationships, and preserve self image.

Deception motives refer to the theory that the majority of individuals only lie when they deem it unavoidable. People communicate honestly or choose to deceive with the same intention to achieve one goal, and when the truth allows that goal to be reached, people will not lie. It is only when the truth serves as an obstacle to their goal that people choose to deceive instead of using honest communication. Also, when deceivers try to save their self image and want to avoid hurting the other person, they use falsification tactics. Males tend to view deception more acceptable than females, therefore they tend to deceive more. Another study shows that women are more likely to deceive to protect their partner's self image, while men are more likely to deceive to protect themselves. For example, if a women's partner were to get a new haircut that she did not like she is more prone to lie and say she likes it to protect their self-image. A man might tell others that they make more money than they actually do in order to raise their self-image. Being able to successfully detect deception does not come easily to most, and that is why so many people are just automatically truth-biased. Studies have shown that people who are successful at detecting deception either receive a confession by the deceiver or has some preexisting knowledge of the situation.

Truth bias is people's inclination towards believing, to some degree, the communication of another person, regardless of whether or not that person is actually lying or being untruthful. It is human nature to believe communication is honest, which in turn makes humans highly vulnerable to deception. Consequently, a person's ability to detect deception is weakened, particularly when the source of deception is unfamiliar. As long as a person already has the perception that everything they are told is true, they are still considered to be truth-biased.

The term "truth bias" was first coined in 1984 by deception researcher Steven McCornack and his mentor Malcolm "Mac" Parks, while conducting an experiment that led to them to posit the McCornack-Parks Model of Deception Detection. On page 24 of Zuckerman, DePaulo, and Rosenthal's meta-analysis, the authors described having observed a "truthfulness bias" in which detectors under certain conditions were more likely to perceive truthfulness in sources. Parks and McCornack had observed the same pattern amongst dating partners, and so they shortened the name to "truth bias," and added it to their causal model. In subsequent works, McCornack and fellow deception scholar Timothy Levine broadened its inclusiveness to enfold a general tendency toward judging the communication of others as truthful. An example of truth bias is if a person were given a series of truths and lies, generally, the accuracy with which they detect truths would likely be above 50%, and the accuracy with which they detect lies would likely be below 50%. The results of deception research conducted by Timothy Levine illustrates that this is due in part to the "truth-lie base-rate," which is a part of the "Park-Levine Probability Model."

The theory states that there are two reasons an individual will assume the communication is honest:

This is the central premise to the truth-default theory. Unless an individual finds active evidence to believe they are being deceived, the individual will take the communication as honest. This concept is also referred to as The Projective Motive Model, or the idea that individuals are less vulnerable to deception when they are already suspicious of the communication.