Public health campaigns typically focus on motivating people to change their behavior: exercise more, eat healthier and stress less. But they often fail to consider an important variable: people’s own ideas about how healthy or unhealthy they are.

These beliefs about how active or inactive you are — your perception of your own physical fitness — affect your health and longevity, according to a new Stanford University study. Those who see themselves as less active than others their age are likely to die before their peers. This risk of earlier death holds true even for those people whose actual physical activity levels are similar to others in their age bracket.

Feeling positive about the health benefits you get from everyday activities such as taking the stairs, walking or biking to work, or doing housework, is an easy first step toward greater health benefits.

“Our findings fall in line with the growing body of research suggesting our mindsets, in this case how much exercise we think we are getting relative to others, can play a crucial role in our health,” senior author, Alia Crum, said in a statement.

Crum, an assistant professor of psychology at Stanford, and her colleague, Octavia Zahrt, analyzed the responses of over 60,000 American adults who were surveyed in 1990 about their physical activity and general health. Next they looked at the 2011 death records for the group, 21 years after the first survey was taken. People who believed they were less physically active than others in 1990 were up to 71 percent more likely to die during the 21-year follow-up period than those who believed themselves to be reasonably physically active.

There are a couple of possible explanations for the effect mindsets and perceptions have on health. The first is that our mindsets can affect our motivation and stems from a study of hotel maids Crum reported in 2007.

Many of the hotel employees had believed they were too sedentary until researchers told hotel maids their physical activity at work met recommended levels of exercise. Once they had the mindset that they were getting enough exercise at work, the workers were motivated to exercise even more. They lost weight, and their body fat levels and blood pressure decreased.

Mindsets can also create a placebo effect. People who believe they are getting enough exercise may experience more physiological benefits from physical activity than those who believe they do not exercise enough. “Placebo effects are very robust in medicine. It is only logical to expect they would play a role in shaping the benefits of behavioral health as well,” said Crum.

“Most people know that not exercising enough is bad for your health,” says Zahrt, a doctoral candidate at Stanford's Graduate School of Business. “But most people do not know that thinking you are not exercising enough can also harm your health.”

Changing your mindset from the idea that you need a tough gym workout to reach your exercise goals to feeling positive about everyday activities such as taking the stairs, walking or biking to work, or doing housework, is an easy first step toward greater health benefits.

“It’s time that we start taking the role of mindsets in health more seriously,” says Crum. In the pursuit of health and wellness goals, healthy thoughts can be as important as healthy goals.

The study is published in Health Psychology.