Substantial similarity

Substantial similarity, in US copyright law, is the standard used to determine whether a defendant has infringed the reproduction right of a copyright. The standard arises out of the recognition that the exclusive right to make copies of a work would be meaningless if copyright infringement were limited to making only exact and complete reproductions of a work. Many courts also use "substantial similarity" in place of "probative" or "striking similarity" to describe the level of similarity necessary to prove that copying has occurred. A number of tests have been devised by courts to determine substantial similarity. They may rely on expert or lay observation or both and may subjectively judge the feel of a work or critically analyze its elements.

To win a claim of copyright infringement in civil or criminal court, a plaintiff must show he or she owns a valid copyright, the defendant actually copied the work, and the level of copying amounts to misappropriation. Under the doctrine of substantial similarity, a work can be found to infringe copyright even if the wording of text has been changed or visual or audible elements are altered.

Confusion arises because some courts use "substantial similarity" in two different contexts during a copyright infringement case. In the first context, it refers to that level of similarity sufficient to prove that copying has occurred, once access has been demonstrated. In the second context, it is used after it has been shown that a defendant had copied to determine if what had been copied is legally actionable or amounts to misappropriation. Some courts use "striking" or "probative" instead of "substantial" to describe the level of similarity needed in the first context to avoid confusion. The second meaning, which Justice Jon O. Newman referred to in 1997 as the more proper use, defines "the threshold for determining that the degree of similarity suffices to demonstrate actionable infringement" exists, "after the fact of copying has been established."

Direct evidence of actual copying by a defendant rarely exists, so plaintiffs must often resort to indirectly proving copying. Typically, this is done by first showing that the defendant had access to the plaintiff's work and that the degree of similarity between the two works is so striking or substantial that the similarity could only have been caused by copying, and not, for example, through "coincidence, independent creation, or a prior common source". Some courts also use "probative similarity" to describe this standard. This inquiry is a question of fact determined by a jury.

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