It stands to reason that if you know you’re overweight or obese, and you know your extra pounds are unhealthy, that you’ve made a stab at losing weight. Right?

Not so much anymore, new research shows.

The proportion of American adults who were either overweight or obese has been growing steadily for decades, rising from about 53 percent a generation ago to roughly 66 percent more recently.

But the share of these adults who had gone on a diet dropped during the same period, researchers reported in the Journal of the American Medical Association.

The study relied on data from the National Health and Nutrition Examination Survey, an ongoing project of the Centers for Disease Control and Prevention.

In the first survey period, between 1988 and 1994, about 56 percent of overweight or obese adults reported they had tried to lose weight in the last year. By the last survey period, between 2009 and 2014, the proportion of overweight or obese respondents reporting recent weight-loss attempts had declined to about 49 percent.

That trend was considered statistically significant in white women and white men. But it was most pronounced among black women, 55 percent of whom were overweight or obese in the final years of the study.

In the first survey period, about 66 percent of black women who were overweight or obese said they had tried to lose weight in the last 12 months. By the last period, 55 percent of overweight or obese black women said they had made weight-loss efforts.

The authors of the new report, a team from Georgia Southern University’s College of Public Health, offered a relatively simple explanation for this phenomenon, writing that “socially acceptable body weight is increasing.”

They pointed to a 2010 study in the journal Obesity that chronicled “a generational shift in social norms related to body weight” in which, effectively, fat has become the new normal. Between 1998 and 2004, that study showed, both men and women became less likely to classify themselves as overweight, even when their body mass index indicated that they were.

That shift, said the authors of the obesity study, may make people less likely “to desire weight loss than previously, limiting the effectiveness of public health campaigns aimed at weight reduction.” (On the other hand, those authors suggested, “there may be health benefits associated with improved body image.”)

But the researchers who produced the new report in JAMA acknowledged that there may be other faults in the chain of reasoning that goes, “if fat, then diet.”

Certainly, they wrote, it’s possible that “body weight misperception” may be reducing people’s motivation to engage in weight-loss efforts. And it may be that primary-care physicians, who are supposed to counsel obese patients to lose weight, are failing to do so.

But the authors also acknowledged another possibility: that many people have been overweight or obese for so long — and tried dieting so many times — that they have simply given up.

“The longer adults live with obesity, the less they may be willing to attempt weight loss, in particular if they had attempted weight loss multiple times without success,” they wrote.

It is likely that many Americans are just “letting themselves go.” But it’s also possible that some of the overweight and obese people who haven’t tried losing weight in the last year have heard from a growing chorus of experts — or discovered for themselves — that dieting may not be the most sustainable way to improve their health. While weight loss would be ideal, regular exercise can mitigate some of the effects of carrying excess weight.