Photos: Getty Images / Photoillustration by Jaclyn LoRaso
By Anne VanderMey
August 22, 2014

His recent U.S. tour drew standing-room-only crowds. Producers scrambled to book him on primetime. Internet songstresses wrote ballads about him. And countless blogs mentioned his jauntily unbuttoned shirts.

The man in question isn’t a teen heartthrob. He is Thomas Piketty, a 43-year-old French economist, whose book Capital in the Twenty-First Century has whipped the U.S. into an inequality fervor since its publication in March.

There are few, if any, other academic economists who inspire so much passion in their fans, or—let’s be honest—have fans. But Piketty’s dense volume on the wealth gap has transcended the usual audience for academic texts to become a pop-culture phenomenon. It sold out on Amazon.com, eventually selling some 400 times more copies than the publisher originally thought it would need. It was the first book in Harvard University Press’ 101-year history to get to No. 1 on the New York Times nonfiction bestseller list—where it remained for three weeks. (Right now, it’s holding at No. 9.) Of the rapturous reception, the book’s publicist Lisa LaPoint says, “It’s really not something you even dream of happening.”

So how did a data-heavy economics tome give way to Piketty-Mania?

For a new Fortune series, my colleagues and I have set out to investigate the puzzling realm of contagion—chronicling the spread of things as disparate as the deadly MERS-coV virus (here and here), corporate takeover rumors, mass panic in the stock market, a narcissistic meme like the “selfie.”—and, yes, the bizarre pop-culture embrace of a European academic. In the case of Piketty, the “rockstar economist” managed to achieve a status typically reserved for actual rockstars: He became cool. And while it doesn’t seem that Piketty has much in common with such traditional T-shirted trendsetters, one might argue that he (and his ideas) actually had many of the elements conducive for a viral sensation from the start.

To begin with, there was a there there. Singer CeeLo Green once said, “It’s taken me 18 years to become an overnight sensation.” Piketty, too, didn’t exactly come out of nowhere. Capital had been reasonably successful in its original French, and the author was already known in the U.S. for his groundbreaking inequality research with Emmanuel Saez, a MacArthur Fellow and highly regarded economist at the University of California, Berkeley. Even before the national press and the fawning reviews, Harvard University Press anticipated that the book would be a hit. The publisher ordered a larger-than-usual printing of 10,000 copies.

The moment, after all, seemed right for an academic study that could roam effortlessly from Quentin Tarantino’s Django Unchained to Jane Austen’s Sense and Sensibility to the “Cobb-Douglas production function.” In a nutshell, the book argues that, as the rate of return on tradable assets like stocks and real estate (which Piketty interchangeably terms “capital” and “wealth”) continues to outpace the growth of the broad economy, the wealth gap will only get wider—and dangerously so. And it was released into a intellectual climate primed for a discourse on inequality. Occupy Wall Street’s cries of “We are the 99%” were still fresh in a national psyche battered by reports of both intractable unemployment, and violin-shaped swimming pools. In December, President Barack Obama called the gap between rich and poor the “defining issue of our time.”

Shortly after the President’s speech, and three months before the book was slated to hit store shelves, Harvard University Press got the first signs it might be sitting on a bonafide blockbuster. Only weeks after LaPoint sent the first galleys to journalists, then-Slate writer Matt Yglesias wrote that the book was “going to be a big deal.”

“We don’t tend to see that when someone gets a 700-page book of economics,” LaPoint says. Then, in late January, Thomas Edsall, a contributing op-ed writer at the New York Times, flagged the book in an online post and cited a Journal of Economic Literature article saying, “we are in the presence of one of the watershed books in economic thinking.”

By the time Edsall’s article was published, the first run of 10,000 copies were already spoken for on pre-order. The publisher ordered a second printing of 10,000. It wouldn’t be enough.

Piketty arrived in the U.S. for a media tour in April. Though most of the interviews had been booked beforehand, with the tour the press attention snowballed, including multiple mentions by New York Times columnist Paul Krugman, major network appearances, and a Colbert Report interview where the host called Piketty the “King Merde of Economics.” As the author’s public profile soared, sales followed.

Six months after it came out the book had sold 400,000 copies, making it the most successful first year for a Harvard University Press book by a factor of seven—and it was only June.

But good press, good timing, and the fact that Piketty had an impressive record of achievement already aren’t the only factors behind the viral spread of Capital. There are plenty of well-reviewed books that no one reads. Chalk up the difference (well, it’s a theory, anyway) to Piketty’s aforementioned cool factor—one that may be unprecedented in the modern history of academia. For evidence, consider the breakdown of the 400,000 copies of Capital sold: Of that prodigious amount, only 60,000 were e-books. People, it seems, didn’t just want to read the book, they wanted to be seen reading the book. (If you read a book on a Kindle and no one sees it, did it really happen?) Twitter, Instagram and Facebook were full of shots of the cover. When Capital sold out in stores and on Amazon, the added element of exclusivity only increased its cachet.

In some ways, the book’s popularity might say more about us than it does about inequality. Wharton marketing professor Jonah Berger, who has studied and written about why things catch on, thinks the primary reason we share something online is to make ourselves look good. Take something as banal as the enormously popular laughing baby YouTube video (nine million views and counting). As consumers, when we post that video to social media or share it over e-mail, it shows our friends we’re empathic, we’re part of the conversation, and we’re good curators of content. The statement of self-identity made by someone who shares a picture of themselves reading Capital is even more obvious—it reveals to to the world that follows that person that he or she is part of the conversation, intellectually ambitious, and concerned with the pressing issues of the day.

One more reason Piketty’s book might have become catnip for a digital age is a bit counterintuitive: it isn’t rocket science. According to Berger, messages spread faster when they’re easy to understand. And while Capital may be cloaked in dense economic theory, the central argument boils down to a simple equation: r > g, or return on capital exceeds economic growth. That is to say, returns for the rich, who invest their wealth, accumulate faster than the economy as a whole grows.

The extension of his theory contains what Berger says is another irresistibly click-able element—fear. If the wealth divergence continues unabated, Piketty writes, the consequences are “potentially terrifying.” Growing inequality is propelling us toward another Victorian era, where a wasteful, rent-seeking upper crust rules over a sprawling proletariat underclass.

But despite the resonance of its warning, the final—and, perhaps, fundamental—reason the book caught on is also the simplest of all: it’s very good. Regardless of your opinion of the criticism in the Financial Times, Piketty’s data gives substance and clarity to an urgent national debate. Matt Yglesias makes the point that thanks to the superstar effect, gains for the very best—whether it’s an author, an athlete, or a rock star—tend to accumulate much faster than the gains to the runners up. This is especially true in a globalized, e-commerce economy where consumers can buy anything from everywhere, and the best-in-category products are readily available to everyone.

Ironically, in the marketplace, the superstar effect itself is a driver of income inequality. In the book world, as Yglesias writes, it means that “being the book on economic inequality is much more lucrative than being the fourth-best book on economic inequality.”

There will be more very good academic books, but Piketty-like contagion will likely be difficult to replicate. With its intellectual scoops, its perfect timing, and its comprehensive data sets, Capital was, in many ways, primed for success. To replicate it, an author would have to conjure up another freakishly perfect mixture of skill and luck, cool and clarity, resonance and readability. That doesn’t happen often. In the end, while there are a lot of YouTube videos of babies who are laughing, there’s only one laughing baby.

 

For more inside the world of contagion, see

• CONTAGION—How things spread. Introducing a new Fortune series

• Part 1: How a bat virus became a human killer

• Part 2: How the MERS virus made it to Munster, Indiana

• Part 3: How M&A rumors spread

• Part 4: How market selloffs happen

• Part 5: How Americans fell in love with a 685-page economics treatise

• Part 6: How the “selfie” became a social epidemic

• Part 7: How studying Twitter became an academic craze

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