For nearly four decades, economic analysis has dominated academic discussion of tort law. Courts also have paid increasing attention to the potential deterrent effects of their tort decisions. But at the center of each economic model and projection of cost and benefit lies a widely accepted but grossly undertested assumption that tort liability in fact deters tortious conduct. This article reports the results of a behavioral science study that tests this assumption as it applies to individual conduct. Surveying over 700 first-year law students, the study presented a series of vignettes, asking subjects to rate the likelihood that they would engage in a variety of potentially tortious behaviors under different legal conditions. Students were randomly assigned one of four surveys, which differed only in the legal rules applicable to the vignettes. In summary, the study found that although the threat of potential criminal sanctions had a large and statistically significant effect on subjects' stated willingness to engage in risky behavior, the threat of potential tort liability did not. These findings call into question widely accepted notions about the very foundations of tort law.