The test of substantial similarity (or "probative similarity") is whether, considering the two works as a whole, including both the copyrightable elements and the uncopyrightable ones (such as basic ideas or public domain expressions that are not eligible for copyright), a reasonable person would conclude that the defendant had actually copied the work from the original.
This standard focuses on the works' similarities rather than their differences. Thus, "[i]t is enough that substantial parts [of a copyrighted work] were lifted; no plagiarist can excuse the wrong by showing how much of his work he did not pirate."
Note that this test is designed to determine whether copying occurred, not necessarily whether that copying constituted infringement. If the court determines that actual copying has occurred, only then does it assess whether the copying was substantial enough to constitute infringement. Unfortunately, many courts also refer to this test as one of "substantial similarity," which can lead to confusion. To avoid this confusion, many courts prefer to use the term "probative" similarities to show "actual copying," and "substantial similarity" to show "actionable copying."
In practice, a plaintiff demonstrates "substantial" or "probative" similarity, as well as infringement, by comparing the suspect copy side-by-side against an authentic original. Although it is generally better to compare against the original maintained on file at the Register of Copyrights, it is not absolutely necessary — an authenticated duplicate of the original work will suffice.
- See Positive Black Talk Inc. v. Cash Money Records, Inc., 394 F.3d 357, 370 n.9 (5th Cir. 2004); McCulloch v. Albert E. Price, Inc., 823 F.2d 316, 318-19 (9th Cir. 1987), disagreed with on other grounds, Fogerty v. Fantasy, Inc., 510 U.S. 517 (1994); Atari, Inc. v. North American Philips Consumer Elec. Corp., 672 F.2d 607, 614 (7th Cir. 1982).
- United States v. O'Reilly, 794 F.2d 613, 615 (11th Cir. 1986) (affirming conviction for infringement of copyright in videogames where approximately 70% of defendant's code was identical to the copyrighted original) (quoting Sheldon v. Metro-Goldwyn Pictures Corp., 81 F.2d 49, 56 (2d Cir. 1936) (L. Hand, J.)).
- See, e.g., Sid & Marty Krofft Television Prods., Inc. v. McDonald's Corp., 562 F.2d 1157, 1164-65 (9th Cir. 1977) (referring to the test of whether copying occurred as an "extrinsic" test of substantial similarity, while calling the test of whether infringement occurred, i.e., whether copyrightable elements were copied, an "intrinsic" test of substantial similarity).
- See Positive Black Talk Inc. v. Cash Money Records, Inc., 394 F.3d 357, 370 (5th Cir. 2004); Dam Things from Denmark, 290 F.3d at 562 & n. 19.
- See United States v. O'Reilly, 794 F.2d 613, 615 (11th Cir. 1986); United States v. Shabazz, 724 F.2d 1536, 1539 (11th Cir. 1984).