Criminal sanctions are usually public, stable and predictable. In contrast, the practices governing the determination of the probability of detection and conviction reinforce uncertainty. We invoke psychological insights to illustrate that criminals prefer a scheme in which the size of the sentence is uncertain while the probability of detection and conviction is certain. Consequently, the choice to increase certainty with respect to the size of the sentence and to decrease certainty with respect to the probability of detection and conviction can be justified on the grounds that such a scheme is disfavored by criminals and consequently has better deterrent effects.