Researchers have found that asking probing questions of message sources does not enhance deception detection accuracy. Probing does, however, increase recipient and observer perceptions of source honesty, a finding we label the probing effect. This project examined 3 potential explanations for the probing effect: behavioral adaptation, confidence bias, and a probing heuristic. In Study 1, respondents (N = 337) viewed videotaped interviews in which probes were present or not present, and in which message source behaviors were controlled. Inconsistent with the behavioral adaptation explanation, respondents perceived probed sources as more honest than nonprobed sources, despite the fact that source behaviors were constant across conditions. The data also were inconsistent with the confidence bias explanation. Studies 2 and 3 investigated the probing heuristic explanation. The data from Study 2 (N = 236) were ambiguous, but the results of third study (N = 143) were consistent with the heuristic processing explanation of the probing effect.