Proving Copying

Proof that a defendant actually copied from a copyrighted work is a critical part of a claim for copyright infringement. Indeed, absent such copying, there is no infringement. The most common method of proving copying involves the use of circumstantial evidence, consisting of proof that a defendant had “access” to the protected work, and a showing of “similarities” between the copy and the protected work. In inferring copying from the combination of such evidence, courts have for many decades developed a framework known as the “inverse ratio rule,” which allows them to modulate the level of proof needed on access based on the level of similarity, and vice-versa.

While analytically sound, the inverse ratio rule has proven to be a persistent source of confusion among some courts, most prominently the Ninth Circuit, causing some to misapprehend the very nature of the inquiry into copying as well as the manner in which circumstantial evidence operates. This Article explains how the inverse ratio rule emanates from crucial insights about the manner in which circumstantial evidence operates, wherein multiple inferences are combined to enhance the probative value of evidence that is inconclusive in isolation. This flexible formula builds on the notion of a combined narrative that is a hallmark of all circumstantial evidence. The Article unpacks the theoretical and normative logic behind the inverse ratio rule to show how it is essential to circumstantial proof of copying, reveals how purported rejections of the doctrine by courts have been driven by a basic misunderstanding of the manner in which circumstantial inferences interact, and concludes by drawing broader insights about the interplay of copyright and evidence law.