People who perceive themselves as less active than their peers are at greater risk of death—whether they're accurate or not.

(Photo: Sydney Jackson/Unsplash)

Consider, for a moment, how physically active you tend to be on any given day. Then think about your friends and neighbors around the same age. Do you exercise more, or less, than they do?

If your answer is "less," you're in trouble—whether or not your perception is accurate.

That's the conclusion of a newly published study, which pinpoints a possible dark side to the well-known placebo effect. Among a large group of Americans who were tracked for up to two decades, the death rate was far higher among those who viewed themselves as less active than their peers.

Intriguingly, this held true even "after adjusting for actual levels of physical activity." This suggests that, while spending too much time on the sofa is clearly bad for your health, there are dangers in simply thinking of yourself as couch potato.

"For any given level of physical activity, people may perceive themselves as more or less active, fit, and healthy, depending on what they believe is the 'right' type and amount of activity," Stanford University researchers Octavia Zahrt and Alia Crum write in the journal Health Psychology. "We present suggestive evidence that such perceptions meaningfully affect health outcomes."

The study featured data on 61,141 American adults who participated in either the 1990 National Health Interview Survey, or the National Health and Nutrition Examination Survey. The researchers noted which of the participants had died by December 31st, 2011.

"Perceiving everyday activities as good exercise is almost as important as doing the activities in the first place."

Upon enrolling in their survey, all participants provided demographic information, and were asked whether they are "physically more active, less active, or about as active as other persons your age."

To measure their actual level of exertion, they were presented with a long list of physical activities, and asked to indicate whether they had engaged in it in recent weeks. (If the answer was yes, they reported how frequently, and for how long.) A subset of nearly 9,000 participants wore accelerometers, which provided a precise record of their physical activity, for seven days.

The key result: "The less active individuals perceived themselves to be, as compared with other people their age, the more likely they were to die in the follow-up period," the researchers report.

"Individuals who perceived themselves as less active than other people their age had an up to 71 percent higher mortality risk than those who perceived themselves as more active."

"Most important," they add, "this result held when controlling for actual amounts of activity, either as reported in the detailed questionnaires or through accelerometer data."

Zahrt and Crum offer several possible explanations for their findings, including the aforementioned placebo effect—or, to be precise, its unwanted opposite. If you believe you're basically sedentary and know that's not good for you, bad health could become a self-fulfilling prophecy.

There is also the fact that "perceptions can affect motivation," the researchers note. A 2015 study found that people "who perceive themselves as unfit compared with their friends are less likely to exercise a year later, even after controlling for their current level of activity."

In addition, fears about living a sedentary lifestyle can themselves contribute to stress or depression—both of which can negatively affect health.

Whatever combination of factors may be at play, the results suggest health-conscious interventions may have to be re-thought. "Finding the right balance may be a challenge," the researchers admit, "but carefully crafted campaigns that promote behavior change while simultaneously instilling positive perceptions are likely to be the most effective."

So, give yourself credit for moving around more than you might think—and then use that foundation to build a more rigorous exercise regimen.

"Many Americans think that the only healthy physical activity is vigorous exercise in a gym or on a track," Crum said in announcing the findings. "Our research suggest that perceiving everyday activities as good exercise is almost as important as doing the activities in the first place."

So long as you don't credit yourself for physical exertion when reaching for the bag of Cheetos.