Custom, Non-Customary Practice, and Negligence

The rule that evidence of compliance with or departure from custom is admissible to prove negligence implements the idea 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. In de-emphasizing the importance of patterns of conduct that regularly recur but are not as widespread as customs, the practice rule treats each negligence case as more nearly unique, and leaves greater room for the risk-benefit and reasonable prudence conceptions of negligence to operate. In this Article, I attempt to enrich our understanding of the custom rule, including what the rule implies about the admissibility of practice evidence. I do this by examining precisely what makes custom evidence at least potentially probative of reasonable care, and by identifying two functions of custom evidence that have not been emphasized in previous work. Custom evidence is not only directly relevant to the negligence issue, but can also perform a knowledge-building, or educational function. And admitting custom evidence may help to prevent the jury from drawing unwarranted inferences from the absence of such evidence. Interestingly, there is virtually no case law addressing the admissibility or inadmissibility of the incidence of practices that are not customs. I identify the reasons for this case law vacuum, and consider the arguments for and against permitting the introduction of evidence of the incidence of such practices. Finally, precisely 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 brief concluding discussion, therefore, I return to the contrast between the different sources of authority in negligence law, and try to tease out some of the broader implications of this contrast for our understanding of negligence