Historically, many subjects could be tested for purity—even language itself.Chris Carlson / AP

At last night’s Democratic presidential debate, the most memorable words spoken were no doubt “wine cave.” In a clash over big-dollar donors, Elizabeth Warren laid into Pete Buttigieg for a recent Silicon Valley fundraising event that she described as being “held in a wine cave, full of crystals and served $900-a-bottle wine.” Warren added, “Billionaires in wine caves should not pick the next president of the United States.”

Buttigieg was ready with his response, relying on a different two-word phrase: “purity tests.” After pointing out that he was “the only person on this stage who is not a millionaire or a billionaire,” he shot back at Warren: “This is the problem with issuing purity tests you cannot yourself pass.” Noting that Warren, too, has previously engaged in the same kind of big-ticket fundraising, Buttigieg concluded that “these purity tests shrink the stakes of the most important election.”

In calling out the hypocrisy and divisiveness of “purity tests,” Buttigieg was echoing a sentiment recently raised by former President Barack Obama, when he assessed the Democratic-primary field at a question-and-answer session with party donors a day after the last debate. “We will not win just by increasing the turnout of the people who already agree with us completely on everything,” Obama said. “Which is why I am always suspicious of purity tests during elections. Because, you know what, the country is complicated.”

The notion of a purity test by which candidates are measured is similar to another political metaphor drawn from chemical analysis: the litmus test. But a purity test suggests an even more stringent scrutiny to weed out any ideological impurities.

Historically, many subjects could be tested for purity—even language itself. In the preface to his landmark 1755 dictionary, Samuel Johnson wrote that he found “our speech copious without order, and energetic without rules,” while “adulterations were to be detected without a settled test of purity.” As a fixed phrase, purity test started showing up in the scientific literature a century later. One correspondent to Scientific American in 1864 expressed a desire for a “purity test of air” that would quantify the oxygen levels in “our densely crowded and often sickening public rooms.”  

In the late 19th and early 20th centuries, chemical testing became more commonplace in agriculture and food preparation, with “purity tests” developed to ferret out contamination in everything from plant seeds and sugarcane to wine and beer. Purity tests also worked their way into advertising—most famously for Ivory soap, with its slogan, first rolled out by Procter & Gamble in 1882, pledging that each bar was “99 44⁄100 percent pure.”

One early political foray for the purity test occurred in 1912, when Kansas was deciding whether to adopt an amendment to the state constitution extending full suffrage to women. One Kansan woman wrote to The Hutchinson Daily Gazette to oppose the amendment, arguing that there were “better ways of purifying politics” than granting women the right to vote. If mothers trained their boys to be “pure, honest, upright and patriotic,” then when they become men, “their votes and the laws they will make will stand the purity test.”

Purity tests took a decidedly less puritanical turn a decade later, when the Jazz Age ushered in a loosening of conventional morals, personified by the fun-loving flapper. A 1922 “defense of the flapper” in The Daily Home News of New Brunswick, New Jersey, described the fad at women’s colleges known as the “purity test,” intended “to discover how many girls were given to flapper habits.” “Have you ever smoked, drank, petted?” constituted typical questions. “Girls who have ever taken one puff or one sip or who had held hands with the boy next door, were forced to say ‘Yes’ along with those who indulged habitually,” the writer explained.

The following year, the Des Moines Register reproduced a “purity test” that was circulating among Iowa college students, with 10 questions running from “Have you ever been drunk?” and “Do you gamble?” to “Do you make a practice of promiscuous petting?” In Des Moines, at least, the test was administered by college girls as a way of sizing up the boys, and “purity” was far from a desired quality. As one “little bobbed-haired blonde” slangily divulged, “An awfully pure youth is the same as a dreadful oil can or a poor fish or a terrible dumbbell. Being ‘pure’ is a social error.”

Lurid reports of collegiate “purity tests” continued through the ’30s. The Columbia University humor magazine The Jester caused a commotion when it published the results of a Barnard Purity Test in 1935, though the magazine wryly observed that “no actual correlation between the purity index and actual purity has been found.”

Likewise, the folklorists Alan Dundes and Carl R. Pagter wrote of the purity-test genre that “it is obviously doubtful whether anyone would answer the questions posed on the test in an honest and truthful fashion.” In their 1978 book Work Hard and You Shall Be Rewarded, Dundes and Pagter presented one such test that circulated at Indiana University in 1939, and a much more obscene “Official Purity Test” from Caltech that they obtained from the Kinsey Institute for Sex Research. The Caltech test included 100 questions covering all sorts of sex acts, “formulated by the Student Houses Intellectual, Theological, & Obscenely Naturalistic Youth Organization, Unlimited (S.H.I.T.-O.N.Y.O.U.).”

While college campuses were awash in the prurient kind of purity test, the phrase continued making inroads in politics. In 1934, The Washington Post detailed how the Republican National Committee was debating how best to disburse funds, with their party out of power during Franklin D. Roosevelt’s first term. “In advancing money for speakers and organization work,” the Post reported, “there is a strong sentiment within the party for using this on Republicans who can pass the purity test rather than those whose voting records in the last Congress are scarcely distinguishable from those of Democratic members.”

Fast-forward 75 years, and Republicans were still arguing over administering purity tests to reward the party faithful. In 2009, with Obama ascendant, conservative activists proposed a purity test for candidates that would withhold funding from anyone disagreeing with more than two of 10 core principles. After much dissension (“Put Me in the No Camp on the Purity Test,” announced Erick Erickson on the Red State blog), the RNC ended up approving a greatly diminished version of the resolution.

Leading Democrats, too, have bristled at an imposition of a purity test as a doctrinaire standard of partisan loyalty. In the 2016 presidential race, it was Hillary Clinton objecting to such requirements from Bernie Sanders, and as with the Warren-Buttigieg dustup, campaign fundraising was at issue. Clinton argued in a primary debate that according to Sanders’s criteria for what counts as “progressive,” even Obama wouldn’t measure up, “because he took donations from Wall Street.” By invoking the specter of purity tests in last night’s debate, Buttigieg surely had this recent party history in mind—and not the long-forgotten history of flappers and their promiscuous petting.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.