The idea that “thin is good, fat is bad” has become so ingrained in Western culture that the thought experiment I’m about to suggest will require some serious imagination.

Suppose you live in a country that celebrates amplitude. Celebrities in magazines are enormous; their photos barely fit on the page. In glossy ads for top-shelf perfume, waists are altered by computer to look thicker; necks are manipulated to make way for five or six chins. Co-workers share their exasperation at lunch that no matter how hard they try, they can’t seem to gain weight. The movers and shakers who get ahead in professional spheres are nearly as wide as they are tall. Amid the scrawny untouchables, a small but growing movement insists that one can be beautiful and petite; the hulking elite humors these squirts, but in private those defiant skinny minnies are derided as merely trying to make the best of a bad hand. For that matter, the repellently slight draw a hint of opprobrium. They clearly lack the self-mastery to polish off a rack of ribs in a sitting. They have no self-respect. Meanwhile, little girls dream of the day that they won’t fit through the door.

Hold it — we’re not finished. Now picture the medical authorities in this country announcing that it turns out that being huge isn’t good for you. The diseases to which the prestigiously plump have proved mysteriously prone have at last been linked to their delectable mass. The health of the population would be vastly improved if everyone slimmed down, which would save the nation a bundle in hospital bills to boot.

Yet this declaration changes nothing about the culture’s standard of beauty. Film ingenues are still bursting from their dresses. Fat is still sexy. Fat is still a key advantage in every occupation. Everyone will still admire you more if you are fat than if you are pitiably thin. But your compatriots do now begrudgingly acknowledge that they would live longer, cost less and have fewer health problems if they purposefully made themselves ugly, thus sending their social status into free fall and becoming objects of scorn.

So: Do you lose weight, or not?

This hypothetical scenario is simply an attempt to tease out the tangled issues in our discussions about obesity. We pay a great deal of lip service to the potential health consequences of being overweight, and by now most people can recite the litany of maladies that excessive bulk may invite: Type 2 diabetes, heart disease, hypertension, circulatory disorders; the list goes on.

Nevertheless, what motivates most people to diet? The desire to look, and feel, more attractive.

Since the release of my novel “Big Brother” — about a sister who risks her marriage to rescue her morbidly obese older brother from the potentially fatal consequences of his compulsive eating — I have become one of those annoying instant experts on this subject. But then, by now, we’re all experts on this stuff.

Admittedly, there are exceptions to my claim that most people slim down out of vanity. Patients suffering from heft-related physical complaints that put them in pain may indeed attempt to lose weight largely for health reasons. I have a friend whose knees are shot and for whom it’s become excruciating to ascend a flight of stairs. She has already dropped 45 pounds. Being overweight contributed to the deterioration of her joints to begin with, and lightening their load both alleviates some of her pain and forestalls the need for knee replacements — although the side effect of making herself more fetching in modern American terms must also be welcome.

Yet for most of us, “health” is abstract. Until it isn’t. Only once an unpleasant ailment materializes in the here and now does it generate any serious motivational power. In other words, health warnings about obesity really work only when they’re too late.

The medical, aesthetic and moral aspects of this issue are so intertwined that big people have every reason to be suspicious when they’re badgered about their increased vulnerability to diabetes. Expressing concern about other people’s health is socially acceptable, since you’re ostensibly worried on their account.

But given the judgment that now attaches even to the word “fat”— on “The Diane Rehm Show” recently, a listener castigated both me and the host for using the F-word — all this blah-blah about health is often a disguise for visceral, cosmetic disgust. The medical tsk-tsking is an expedient blind for prejudice.

For the neurotically narrow, the heavyset seem to present either an affront or a threat — an “after” picture to which slender people are subconsciously terrified they too will succumb. But what is the fear of, really? Of being prone to illness? Of “contracting” obesity, which the American Medical Association declared a disease in June? Uh-uh. Of being unattractive.

Given what a feeble driver of behavior is the threat of its vague “health” consequences, which may or may not manifest themselves in a future that is also abstract, maybe it’s fortunate that we’re not in the situation I described in my thought experiment. After all, even the decline of smoking may be less inspired by an immediate terror of lung cancer, more by the broad social-exile status that smokers have assumed — a demonization much akin to our widespread denunciation of the flabby. Smoking has become unattractive.

Up to a point, doctors and glossy magazines are delivering the same message. I say “up to a point” because cadaverously thin models, whose images are sometimes retouched to portray a tapering that’s biologically impossible, promote no less deadly an archetype than Gabourey Sidibe did in the film “Precious.” Granted, roughly 300,000 American deaths per year are attributed to obesity, the country’s second-leading cause of preventable death. Yet 20 percent of anorexics die from complications of their eating disorder, and only about a third ever fully recover. It’s medically convenient: If people won’t lose weight to prevent heart disease, at least they’ll drop a few pounds to look svelte at their weddings. Leaving aside outliers such as anorexics and fat fetishists, the cultural and therapeutic ideals are crudely in sync.

But how fascinating that mere aesthetics provide more powerful inspiration than the prospect of disability and chronic illness, when poor health has the capacity to put us in blinding agony, to curtail what we most love to do in life and even to take that life from us altogether.

Take exercise. How many fitness freaks go for grueling runs, push themselves through monotonous calisthenics or churn lap after lap in swimming pools purely to care for their hearts? Oh, the aerobic effect is a pleasant little extra, and it certainly makes for an acceptable, plausible excuse. But why do the abundance of amateurs work out? Really, really? To look good. To feel good, sure, but also to feel good because they look good. To burn calories so they look good. To have firm muscles so they look good.

Of course, you can do things for more than one reason. I exercise a fair bit myself, and my diet is pretty deliberate. On both fronts, my habits are so ingrained that I rarely question why I perpetuate them. The rationales are multiple: My routines give me energy and ensure I sleep well. Roughly the same for decades, my dimensions and fitness level are doubtless integral to my sense of self (so a car accident or paralyzing arthritis could bring on an identity crisis). Oh, somewhere in the mix is a hazy concern for my health, and I’m relieved that the aims of well-being and tolerable appearance aren’t at odds. But also? Face it: I want to look good.

Thus I draw the line at the suggestion that whatever the state of our bodies or however we choose to take care of them is to any considerable degree a moral matter. The fact that keeping in decent shape is primarily a mechanical concern is why fitness and nutrition fanatics can be so off-putting — by their seeming to flaunt an unjustified air of virtuousness, nobility and self-sacrifice. As the narrator in “Big Brother” tells her husband, a manic cyclist, regarding her ballooning older brother, “His weight makes him a social pariah. It reduces the likelihood he’ll remarry. It has grave implications for his health. But it isn’t evil. Just like all that exercise of yours has nothing to do with being good. I know you think it does. It makes you feel good, and feel good about yourself, and feel superior to people who slob around all day. But it’s mostly a waste of time that doesn’t do anything for anybody else but you.” And, yes, every once in a while a fictional character does indeed voice her creator’s views.

If the obese cost our health-care system inordinately, we can certainly allow that they may be consuming more than their share of social resources. But that’s the extent of the moral argument, which is often used as a cover for the more brutal, prejudicial assumptions we too commonly make about the supersized: that they are lazy, undisciplined and self-indulgent. Maintaining the body, the mobile home in which we travel, is like any other kind of housekeeping: often a chore, sometimes ignored, but nothing to do with righteousness and everything to do with enlightened self-interest.

As I age and my ambitions for my appearance slide wanly to “I want to disintegrate as slowly as possible,” that “health” motivation for eating well and remaining active is getting subtly more pronounced. By the time I grow elderly, “I want to look good” is apt to slide still further — to “I want to be able to walk.” In which case, I may not only remain ambulatory; I will finally have become wise.

Shriver is the author of 11 novels including “So Much for That” and “We Need to Talk About Kevin.”