Robert H. Frank writes: Republicans have never wanted to talk about inequality, and many Democrats now seem afraid to. As a congressional Democratic adviser quoted by the New York Times reporter Jackie Calmes recently put it, the party is having difficulty articulating its position “in a way that doesn’t get us pegged as tax-and-spenders.”

The remarkable achievement of the Occupy Wall Street movement has been to make continuing silence about inequality politically unacceptable. Some have criticized the movement for not pressing specific demands. Yet most protesters wouldn’t pretend to have a sophisticated understanding of the forces that have been causing growing income disparities, or the policy experience to prescribe what might be done about them. But now that the movement has forced inequality onto the agenda, the time is ripe to focus on these issues.

Because many continue to deny that income inequality has been growing, it’s useful to start with a brief review of how income growth patterns have changed since World War II. The three decades after the war saw incomes grow at an almost uniform 3 percent annual rate for families up and down the income ladder. Since the early 1970s, however, virtually all income gains have accrued to those whose incomes were highest to begin with.

It’s a striking fractal pattern. Most of the gains have gone to the top 20 percent of earners, but the lion’s share of the gains within that group have gone to the top 5 percent. And within the top 5 percent, most of the gains have gone to the top 1 percent, and so on. 

Is this new pattern something to worry about? Many decry rising inequality because it makes those who’ve fallen behind feel impoverished. But it’s done much more than that. It has also raised the real cost to middle-income families of achieving many basic goals.

It’s done that through a process that I’ve elsewhere called “expenditure cascades.” The process begins with the completely unremarkable fact that top earners have been spending at a substantially higher rate than before. They’ve been building bigger mansions, staging more elaborate weddings and coming-of-age parties for their kids, buying more and better of everything.

Many social critics wag their fingers at what they perceive to be frivolous luxury spending. But that misses the point that all consumption norms are local. It’s not just the rich who spend more when they get more money. Everyone else does, too. The mansions of the rich may seem over the top to people in the middle, but the same could be said of American middle-class houses as seen by most of the planet’s 7 billion people.

The important practical point is that when the rich build bigger, they shift the frame of reference that shapes the demands of the near rich, who travel in the same social circles. Perhaps it’s now the custom in those circles to host your daughter’s wedding reception at home rather than in a hotel or country club. So the near rich feel they too need a house with a ballroom. And when they build bigger, they shift the frame of reference for the group just below them, and so on, all the way down.