Custom, Noncustomary Practice, and Negligence

The rule that evidence of compliance with or departure from custom is admissible to prove negligence reflects the notion that recurring patterns of conduct have a bearing on what constitutes reasonable care. In contrast, evidence of the incidence of practices that are not sufficiently widespread to qualify as customs is not admissible to prove negligence. This latter rule treats each negligence case as more nearly unique. In other words, the traditional custom rule reflects an “experiential” conception of negligence, whereas the practice rule reflects an “individuated” conception. This Essay examines precisely what makes custom evidence at least potentially probative of reasonable care, and analyzes the arguments for and against permitting the introduction of evidence of the incidence of noncustomary practices. In addition, because negligence law permits the parties to deploy, and juries to consider, evidence and arguments based on the different perspectives that the custom and practice rules reflect, we have a conception of negligence that is often ambiguous. In a concluding discussion, this Essay considers the varying sources of authority underlying negligence law, and teases out some of the broader implications these differences reveal for our understanding of negligence.