Mark Twain accused himself of it. So did Rod Stewart, Robert Louis Stevenson, and George Harrison.

When confronted with evidence of a strong resemblance between their own work and the creations of others, they defended themselves in the same way, claiming to have appropriated unwittingly and “unconsciously.”

But is unconscious plagiarism a real phenomenon, or is it simply an excuse for carelessness, or even outright theft?

In conducting research for my new book, Strikingly Similar: Plagiarism and Appropriation from Chaucer to Chatbots, I examined over four thousand claims of appropriation, infringement, and plagiarism. I was struck by how often authors, songwriters, and other artists expressed astonishment when these similarities were pointed out to them.

In Twain’s case, the issue was the similarity between the dedication for his Innocents Abroad (1869) and the inscription in Oliver Wendell Holmes’s Songs in Many Keys (1862)—a book that Twain knew well. A friend pointed out the resemblance, and Twain would later publicly out himself at a dinner held in Holmes’s honor.

A decade after penning Treasure Island (1883), Robert Louis Stevenson noted—with a fair degree of dismay—a striking similarity between elements of his book and details mentioned in Washington Irving’s Tales of a Traveler (1824).

In Rod Stewart’s case, it was the strong resemblance between his “Da Ya Think I’m Sexy” (1978) and the Brazilian artist Jorge Ben Jor’s song “Taj Mahal” (1977). In his autobiography, Stewart speculated that he had heard “Taj Mahal” during the Carnaval do Rio de Janeiro, when Ben Jor’s song was........

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