In Kansas v Hendricks, the Supreme Court held that it did not violate double jeopardy or substantive due process to commit a person indefinitely to a locked state-run facility after he had completed his maximum prison sentence. Although the state is barred from incarcerating such a person to condemn his past behavior or to deter future misbehavior, it may incapacitate him if he suffers from a mental illness that makes him likely to commit a new violent sex crime characteristics the Supreme Court found to be true of so-called sexually violent predators (SVPs). In this Article, we question a core empirical foundation for the Court s holding that SVPs are so extremely dangerous that they have a high likelihood of committing repeat acts of predatory sexual violence if they are not locked away. If SVPs are as dangerous as the Court asserts then we would expect to see an incapacitation effect a negative impact on the incidence of sex crimes after passage of SVP laws. In conducting our analysis, we use original data that we gathered directly from states with SVP laws. To examine whether the laws have had an impact on the incidence of forcible rape and sex-related homicide, we employ panel data on U.S. states for the last few decades. We also use data collected in the National Child Abuse and Neglect Data System (NCANDS) to examine the impact of SVP legislation on the incidence of non-fatal child sexual abuse. Finally, since underreporting poses problems in accurately measuring the incidence of sex crimes, we examine gonorrhea rates, a common proxy for the prevalence of sexual abuse. Our findings suggest that SVP laws have had no discernible impact on the incidence of sex crimes. These results imply that states could more effectively reduce sex crimes by allocating these resources elsewhere. More importantly, they challenge the only constitutionally permissible justification for SVP legislation.