When the Times informed Montana Senator John Walsh last week that one of his graduate-school papers contained unattributed passages by other writers, Walsh tried out three responses. First, he told the Times that he did not do “anything intentional.” The next day, Walsh, a Democrat who spent thirty-three years in the military, suggested that his plagiarism was connected to post-traumatic stress disorder from service in Iraq. The public was unmoved by that explanation, and, on Friday, Walsh said that P.T.S.D. did not have “any impact” on the case. Instead, he urged voters to look ahead. “I made a mistake here and I’m going to move on,” he told the local CBS station.

In his aversion to attribution, Walsh has earned a footnote in the history of those whom the Roman poet Martial, in the first century, called the “plagiarii”—those who “kidnap slaves,” or, in Martial’s experience, his words. “If you allow them to be called mine, I will send you my verses gratis; if you wish them to be called yours, pray buy them, that they may be mine no longer,” he wrote. Walsh is in good company: Wordsworth, Swift, Coleridge, Wilde. In describing Cleopatra on her barge, Shakespeare digested a description from Sir Thomas North, though after comparing the two, Richard Posner, the judge and author of “A Little Book on Plagiarism,” concluded, “If this is plagiarism, we need more plagiarism.”

As he confronts the consequences, Senator Walsh might feel fortunate that he is a senator, not an academic or a college freshman or a blogger. Last year, an assistant professor of English at Brown University lost her tenure-track jobafter she was found to have included unattributed passages in her book; undergraduates could flunk a course, or worse. Last Friday, as Walsh declared his intention to move on, BuzzFeed was firing one of its writers, Benny Johnson, after the site found forty-one “instances of sentences or phrases copied word for word from other sites.”

If one is going to plagiarize, it pays to be in politics, where the expectation for remorse, and the likelihood of punishment, are minimal. In 1950, Joseph McCarthy thundered about Communists using the unattributed words of Richard Nixon—but when he was asked, in a hearing, “Have you no sense of decency?” it wasn’t because of that. In 2008, Barack Obama took the words of Massachusetts Governor Deval Patrick, but fended off criticism by explaining later that Patrick “gave me the line and suggested I use it.” Vladimir Putin was awarded an advanced degree by the St. Petersburg Mining Institute with the help of a dissertation that, as two Brookings researchers discovered, included sixteen stolen pages—and, remarkably, not a single set of quotation marks. Putin has simply avoided answering questions about it...