In a region filled with the palatial homes of the rich and famous, one mansion stands out. Measuring an astonishing 38,000 square feet (plus 17,000 more on the exterior), it was crafted with the finest and most expensive materials. The interior boasts 12 bedrooms, 21 bathrooms and three kitchens, plus six bars and a 40-seat theater. The whole thing comes pre-loaded with extensive, curated collections of fine art, vintage wine and classic cars.

Was this “The Breakers” built by the Vanderbilts in the 1890s in Newport, Rhode Island? Or maybe “The Biltmore” in Asheville, North Carolina? Actually, it’s a new property in Bel Air, California—fittingly called “Billionaire”—that just went on sale this summer for a cool $250 million. But beware to any billionaire buyer thinking this would put them at the top of the real-estate heap: Another Bel Air mega-mansion is slated to go on sale later this summer—for twice the price.

Welcome to the Second Gilded Age, where the opulence is unapologetic and the ranks of the hoi polloi can be seen swelling outside the gates. Scores of books and articles have been published in recent years on the topic of a new Gilded Age. Many activists and politicians invoke the phrase because they see startling parallels with the first Gilded Age, the period from roughly 1870 to 1900 marked by increased poverty, rising inequality and growing concern about corporate influence in politics.

What are the parallels, really? A look at the original Gilded Age reveals it as an era marked, not unlike ours, by a powerful duality. It was both the best of times and the worst of times. It was an age of both enthusiasm and anxiety.

On the enthusiasm side of the ledger, nothing loomed larger than the booming industrial economy. Between 1860 and 1900, U.S. factory output soared from $1.9 billion to $13 billion, an increase of nearly 600 percent. By 1900 the U.S. boasted the most powerful industrial economy in the world. In recent decades, America has experienced a similar economic boom, albeit one interrupted by periodic recessions. (The same was true in the Gilded Age.) ...