Every intellectual property ("IP") right has its own definition of infringement. In this Article, we suggest that this diversity of legal rules is largely traceable to differences in the audience in IP cases. Patent, trademark, copyright, and design patent each focus on a different person as the fulcrum for evaluating IP infringement. That patent law, for example, focuses on an expert audience while trademark looks to a consumer audience explains many of the differences in how patent and trademark cases are decided. Expert audiences are likely to evaluate infringement based on the technical similarity between the plaintiff's and defendant's works. Consumers, by contrast, are likely to pay more attention to market substitution and less attention to how things work under the hood. Understanding the different audiences in IP infringement is critical to understanding how the IP regimes variously define infringement.
The focus on audience has normative as well as descriptive implications. Neither patent law, with its focus on experts and technical similarity, nor trademark law, with its market-based consumer focus, has it entirely correct. Rather, we suggest that as a general matter, infringement of an IP right should require both technical similarity and market substitution. An ideal IP regime should care about a defendant's conduct only if that conduct actually causes injury to the plaintiff's market and its work is sufficiently like the plaintiff's that it is reasonable to give the plaintiff control over that work. Assessing infringement through the expert's eyes ensures that the law prevents closely related works in the field while permitting sufficiently different contributions. The consumer vantage point ensures that we protect IP owners only when they have been harmed in the marketplace.
IP owners who want to show infringement should have to demonstrate both that the defendant's work is technically similar to their own from the expert's vantage point and that the defendant's use causes the plaintiff harm in the marketplace. Copyright law, which looks to both experts and consumers at various points in the infringement analysis, is on the right track.
At the heart of any IP case is the problem of deciding whether the de- fendant has infringed on the plaintiff's right. A principal question in IP in- fringement disputes is whether the defendant's product (or work, or brand, or idea) is too similar in some respect to the plaintiff's.
But who decides whether the defendant's product is too similar to the plaintiff's? Put another way, who is the audience in IP infringement? Is it the customer of the parties' products that must find the products too similar? Or perhaps an expert on the products' subject matter? Or instead an ordi- nary reasonable person? This choice can easily affect the judgment on simi- larity. For example, a chemist might think that two drugs for providing pain relief are not sufficiently similar because of their different chemical composi- tion or because they trigger a different pathway in the human body. A cus- tomer, by contrast, might find the two drugs highly similar because they provide comparable relief without side effects. An ordinary reasonable per- son might find insufficient similarity between two audiovisual works that are both aimed at children and depict fantasylands filled with fanciful crea- tures because a principal character in one work wears a "cummerbund," while a major character in the other wears a "diplomat's sash." By contrast, the typical child encountering the works would regard them as substantially similar.1 A musicologist might ignore differences in musical style, instead focusing on the underlying composition, in determining whether a pop song is similar to a calypso song. A consumer would do just the opposite.2 Con- sumers may focus on descriptive similarities in brand names or functional aspects of packaging that producers do not intend to indicate the product's source. …