“Capital in the 21st Century,” by the French economist Thomas Piketty, has become this year’s most unlikely best seller, a crossover from the world of scholarship into general public discussion of a kind that seems rarer than it used to be. The book’s thesis — that economic inequality in the developed world is increasing, with potentially dire consequences for social justice and democratic governance — has struck a nerve in the American body politic. But its implications extend beyond the realm of political economy. Though it is expectedly dry, careful and data-driven, the book invites the re-examination of deeply held assumptions about the world.

According to a widely accepted story, the expansion of the middle class — the collapse of older social hierarchies, the decline of inherited privilege and the rise of a new meritocratic order — unfolded according to something like a natural law. The nature of capitalism, we have been taught to believe, tends toward greater equality, wider opportunity and the leveling of archaic, invidious distinctions based on pedigree. Mr. Piketty throws cold water on this conventional wisdom, which has been part of the intellectual birthright of nearly everyone born in the West since the end of World War II.

The three decades between roughly then and the 1970s, known in France as “les trente glorieuses” (the “glorious thirty”), are remembered in the United States in various ways, not all of them glorious. The Cold War. The Space Age. The infancy, adolescence and reluctant adulthood of the baby boomers. The civil rights and sexual revolutions. The Golden Age of Middlebrow, whose end we may be mourning whether we realize it or not. That may sound odd, since “middlebrow” is the kind of word rarely said without a sneer. How can pretension and mediocrity enjoy a golden age? Like the later, sociologically related terms “yuppie” and “hipster,” middlebrow is a name you would never call yourself, but rather a semantic shoe that belongs on someone else’s foot. It is also, however, a workable synonym, in the sphere of art and culture, for democracy.