ABSTRACT Previous findings from perceptual studies of deterrence lead to the conclusion that perceived certainty of punishment (but not perceived severity of punishment) is inversely related to involvement in illegal behavior. The conclusion concerning severity is inconsistent with the rational image of man in utilitarian thought, the philosophical framework in which modern deterrence theory is grounded. We question the validity of previous measures of perceived severity and argue that the appropriate hypothesis never has been tested with a valid measure. In our own analysis we use a refined measure which we believe is more consistent with the utilitarian perspective. The results support the hypothesis that perceived severity, at relatively high levels of perceived certainty, has a significant deterrent effect.