Severe punishments have historically been the bedrock of criminal deterrence, but criminologists have long documented that such threats are often ineffective. Instead, it has been the certainty of sanctions that has been most emphasized and that has garnered empirical support. In a departure from prior research, the question motivating this study is whether increases in the threatened severity of sanction threats alter the perceived certainty of detection irrespective of any objective changes in detection certainty, and then how such perceptions relate to offending. To the authors’ knowledge, scant attention has been paid to examining the possibility of this “boundary-crossing,” or the extent to which two core dimensions of deterrence, objective and perceptual certainty, cross, intersect, or interact with one another. Using data from a sample of young adults, the authors find mixed support for “boundary-crossing”: Although combinations of objective certainty and severity did not necessarily result in substantive differences in perceptions of certainty and severity, an individual’s own perceived certainty and severity related to offending differently depending on the information provided to them about the objective certainty and severity of punishment.