This essay extends the recent work of Levine, Park, and McCornack (1999) on the veracity effect in deception detection. The probabilistic nature of a receiver's accuracy in detecting deception is explained, and a receiver's detection of deception is analyzed in terms of set theory and conditional probability. Detection accuracy is defined as intersections of sets, and formulas are presented for truth accuracy, lie accuracy, and total accuracy in deception detection experiments. In each case, accuracy is shown to be a function of the relevant conditional probability and the truth-lie base rate. These formulas are applied to the Levine et al. results, and the implications for deception research are discussed.