Sender demeanor is an individual difference in the believability of message senders that is conceptually independent of actual honesty. Recent research suggests that sender demeanor may be the most influential source of variation in deception detection judgments. Sender demeanor was varied in five experiments (N = 30, 113, 182, 30, and 35) to create demeanor-veracity matched and demeanor-veracity mismatched conditions. The sender demeanor induction explained as much as 98% of the variance in detection accuracy. Three additional studies (N = 30, 113, and 104) investigated the behavioral profiles of more and less believable senders. The results document the strong impact of sender effects in deception detection and provide an explanation of the low-accuracy ceiling in the previous findings.