The previous chapter explored the individual characteristics taken into account in the reasonable person standard and the ways in which these characteristics are assessed under the standard. This chapter explores external factors, such as customs and statutes.

The reasonable person standard is largely an objective test, though sometimes it takes individual incapacities into account. Judge Learned Hand's Carroll Towing opinion (previous chapter) suggests that the reasonable person standard operates as a cost-benefit test at its core, though it often incorporates factors that cannot be quantified as part of such a test.

The reasonable person standard also takes into account rules that people follow as customs, norms, or laws. This takes the test even further away from the cost-benefit formulation suggested by Judge Hand.

This chapter looks at the extent to which customs and statutes affect the application of the reasonable person test. Compliance with custom would appear to be a natural way of determining whether an actor's conduct was reasonable. Similarly, the violation of a statute would appear to be a natural way of determining whether an actor's conduct was unreasonable. Of course, any student of history knows that there have been customs and statutes in the past that have been harmful to society overall but were imposed because the winners had greater political power than the losers. Because of this, not every custom or statute should be presumed to be reasonable. Still, the customs and statutes that have the broadest application, that burden and benefit almost everyone in the same way, such as rules of the road, are likely to be the sort that have a strong claim to being regarded as reasonable.

One question lying slightly beneath the surface throughout this chapter is the extent to which an external standard, such as a custom or a statute, supplants or displaces the reasonable person test. No court has argued that external standards entirely displace the reasonable person test. However, one court famously held that compliance with custom should be treated as negating any theory of negligence, and another court said that violation of a safety statute is “negligence in itself.” The case law has not uniformly supported such strong statements, especially the claim regarding custom, but has indicated factual conditions under which custom is a defense, and has provided legal tests for determining the effects of statutes on the negligence assessment.