Last week, insurance giant Aetna announced a plan to cover the cost of depression screenings by primary-care physicians. Whether the move is a winner for workers, Aetna’s bottom line — indeed, the bottom line of all companies hoping to squeeze higher productivity from workers by spotting and treating depression early — such policies do a great deal to put depression on par with physical ailments traditionally covered by health insurance.

For decades, mental health professionals have fought for recognition of depression as a genuine disease. Meanwhile, scores of studies have claimed that depression is spreading like wildfire. According to some researchers, over ten times as many people in the U.S. and Europe suffer from depression than half a century ago. If we join the idea that depression is an honest-to-God disease with data that say one in six Americans has or will be afflicted, it becomes hard to resist the conclusion that we are undergoing an “epidemic” of depression. Martin Seligman, former president of the American Psychological Association, has argued just that.

The claim that we’re experiencing an historic plague of depressive illness has big social implications, way beyond insurance coverage.

“Americans and Europeans have more of everything except happiness,” writes Gregg Easterbrook in “The Progress Paradox,” citing depression figures. In “The American Paradox,” psychologist David G. Myers, noting similar studies, maintains that “never has a culture experienced such physical comfort combined with such psychological misery.”

The culprit? Myers posits “radical individualism” and “libertarianism,” both economic and civil. Swarthmore College psychologist Barry Schwartz argues our depression woes lay in “the proliferation of possibilities” afforded by contemporary commercial culture, leading us to reach beyond our grasp. His book? “The Paradox of Choice.”

While the “paradox” of misery amid plenty makes for catchy copy and tantalizing politics, the most likely explanation of skyrocketing depression numbers is mundane: we’re mis-measuring. The implication may be that there is no paradox after all — that we’re rich and happy.

In the penultimate edition of the Public Interest, Allan Horwitz, a renowned sociologist of mental health, and Jerome Wakefield, a top theorist of psychiatric classification, argue there is no plausible theory of depression that fits the numbers. “The explanation appears to lie in changes in the ways that physicians, mental-health professionals and people themselves characterize and diagnose their mental states,” they write.

In a fascinating account of the taxonomic career of depression in the bible of psychological diagnosis, the Diagnostic and Statistical Manual of Mental Disorders, Horwitz and Wakefield relate how the contemporary clinical conception of depression became almost uselessly lax, lumping the healthy sadness of a tough break-up together with the inexplicably chronic gloom of genuine dysfunction. The “epidemic” of depression, they argue, “is partly an artifact of a logical error,” which both trivializes the very real suffering of the truly ill and leads to a “one-dimensional public discourse that can undermine our capacity for making moral and political distinctions.”

So why don’t we just back up and fix the error? This is science, right? Yes, and it’s also big money. Thousands of mental health studies, hundreds of careers, and tens of millions in research funding, are wrapped up in this broken diagnostic category. Medical and mental health professionals who get paid by the likes of Aetna have an interest in keeping the category as permissive as possible. And don’t forget pharmaceutical companies who want to sell huge quantities of mood-enhancers in a society where people are taught to believe they shouldn’t take drugs unless they’re officially “sick.” Finally, will employers tolerate dolorous employees if they can be medicated into a higher gear of productivity?

Incentives matter, and the incentives in favor of continued diagnostic inaccuracy ensure that the real incidence of depression will continue to be overestimated, and our real success as a society in pursuit of happiness will continue to get short shrift. As John Maynard Keynes famously remarked, asserting the relative power of ideas over vested interests, “madmen in authority … distill … their frenzy from some academic scribbler of a few years back.”

So middlebrow best-sellers about the misery of market-based prosperity matter, too. It would be tragically depressing — paradoxical even — if madmen in authority undermined the institutions that have made us wealthy and happy on the false pretext that we are suffering an epidemic of misery. Our insurance doesn’t cover that.