Truth-Default Theory (TDT) is a new theory of deception and deception detection. This article offers an initial sketch of, and brief introduction to, TDT. The theory seeks to provide an elegant explanation of previous findings as well as point to new directions for future research. Unlike previous theories of deception detection, TDT emphasizes contextualized communication content in deception detection over nonverbal behaviors associated with emotions, arousal, strategic self-presentation, or cognitive effort. The central premises of TDT are that people tend to believe others and that this "truth-default" is adaptive. Key definitions are provided. TDT modules and propositions are briefly explicated. Finally, research consistent with TDT is summarized.