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WSJ article "Black Swan': A Rare Disaster, Not as Rare as Once Believed"

Grappling with the coronavirus pandemic and its disastrous impact on financial markets, commentators have reached for a familiar metaphor: the "black swan."

"Coronavirus is the black swan of 2020," read a memo that the Silicon Valley venture-capital firm Sequoia Capital issued on March 5. Since then, the Covid-19 outbreak has borne out the warning, with businesses facing dire economic prospects as the global health crisis continues spiraling out of control.

A "black swan," for market prognosticators, is a rare, unpredictable event with serious and unavoidable effects. The theory of "black swan events" was developed by Nassim Nicholas Taleb in his 2001 book, "Fooled by Randomness: The Hidden Role of Chance in Life and in the Markets," and then fleshed out in his 2007 follow-up, "The Black Swan: The Impact of the Highly Improbable."

Mr. Taleb drew on an expression with classical roots, going all the way back to the Roman poet Juvenal and his "Satires," penned in Latin in the early 2nd century. Juvenal despairingly observed how difficult it is to find a wife with just the right set of virtues, describing such a woman as "rara avis in terris nigroque simillima cygno," or "a bird as rare upon the earth as a black swan."

From this famous line, "rara avis" came to mean a person or thing rarely encountered, and "black swan" became a common figure of speech for something with vanishingly little chance of being discovered. In Juvenal's era, swans were only known to have white feathers.

In 1697, a Dutch navigator made a startling discovery while exploring southwestern Australia: Large numbers of black swans.

In early modern English, the "black swan" was a frequent point of comparison for remarkable, hard-to-find figures. In a 1570 sermon, the English clergyman Thomas Drant recalled Cornelius, the Roman centurion said to have been the first Gentile to convert to Christianity. Drant was hard-pressed to find such a virtuous soul in his day: "Captaine Cornelius is a blacke Swan in this generation." And "The Ladies' Dictionary," published in 1694, wryly mentions "husbands without faults (if such black Swans there be)."

Soon thereafter, in 1697, the Dutch navigator Willem de Vlamingh made a startling discovery while exploring southwestern Australia. There were large numbers of black swans living there, though Europeans might have disbelieved early accounts of creatures long held to be as fanciful as unicorns.

The philosopher John Stuart Mill seized on the image of the black swan in "A System of Logic" in 1843 to demonstrate the perils of inductive reasoning: Since Europeans had never seen a black swan before the exploration of Australia, the statement "All swans are white" appeared to be true. Following Mill's lead, Karl Popper and Bertrand Russell invoked the black swan in their philosophical works as an example of the risks of declaring something impossible based only on one's limited experience.

Mr. Taleb sought to move beyond this logical fallacy by providing a more precise framework for "black swan events" in terms of their statistical improbability and unexpected impacts. But some have argued that the Covid-19 health crisis should not be considered a black swan because it was not so improbable. Nate Silver of FiveThirtyEight told Fox News that it would be better described as a "fat tail kind of black swan," with "fat tail" referring to a statistical scenario in which extreme outcomes are more likely to occur.

Policy analyst Michele Wucker has a different name for the pandemic and the attendant economic chaos. She calls it a "gray rhino," her term for events that are "obvious, visible, coming right at you, with large potential impact and highly probable consequences." Writing this week in the Washington Post, Ms. Wucker observed that "the black swan, only visible in hindsight, is a convenient narrative" for political and financial authorities who want to wash their hands of "unforeseeable" calamities when there were plenty of warning signs. "Paying attention to the gray rhino," she argues, "would be a far better use of our time than retroactively spotting black swans."

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