Litmus is a substance, made of lichen-based dyes, that is absorbed in paper and used to test acidity. Blue litmus turns red when exposed to acidic materials, and red litmus turns blue when exposed to nonacidic materials. This is the origin of litmus test in its figurative sense—i.e., a test that draws broad conclusions based on a single factor. By extension, it also refers to a single factor that is useful for drawing a broad conclusion.
For instance, if experience shows that you get along with people who like cats and that you clash with those who don’t, then you might perform a litmus test on people you meet by asking whether they like cats. Those who say no are not qualified to be your friends—no analysis or additional questions necessary.
The phrase is often used in political contexts. For example, if you care about the funding of animal shelters more than any other issue, you might use each candidate’s position on animal shelters as a litmus test for whether or not to vote for him or her, even when you might disagree with other parts of the candidate’s platform.
The earliest instances of litmus test used this way come from the middle of the 20th century, right around 1950, and most of the early examples from the ’50s and ’60s are in American texts. A few of these are below, followed by a few more recent examples.
Asked his opinion of the North Atlantic Treaty and the Marshall Plan—which delegates here regard as the litmus test for distinguishing between Communists and supporters of the United States—Dr. Houdek refused to comment. [New York Times (1950)]
What counts is whether the jargon contributes to illumination and clarification, and unfortunately we have no litmus test for that. ["English Philosophy at Midcentury," Morton White (1951)]
WCPO-TV is staging a litmus test in film programing by scheduling the same half-hour film series across the board Mondays thru Fridays. [Billboard (1955)]
The fate of the tax legislation is being closely watched … as a litmus test on Tokyo’s ability to start controlling its runaway fiscal deficits. [Wall Street Journal]
The third lesson of pessimism emerges in what we might call the ultimate litmus test of authenticity: Did behavior actually change? [The Moral Imagination, John Paul Lederach]
The way in which we organise our healthcare is surely a litmus test of the moral limits of markets. [Guardian]