Thinking that you’re less fit than others, even if you are actually just as fit as they are, can shorten you life. That’s the verdict from researchers at Stanford University, who analyzed surveys from more than 60,000 U.S. adults, documenting participants’ levels of physical activity, health and personal background, among other measures. A Stanford news release reports that the researchers were interested in one question in particular: “Would you say that you are physically more active, less active, or about as active as other persons your age?” The researchers studied death records from 2011, which was 21 years after the first survey was conducted. Controlling for physical activity and using statistical models that accounted for age, body mass index, chronic illnesses and other factors, they found that people who believed that they were less active than others were up to 71 percent more likely to die in the follow-up period than individuals who believed that they were more active than their peers. How does that work? No one knows, really, but the researchers suggest that perceptions can affect motivation, both positively and negatively, and people who are made aware of their healthy activity levels are more likely to build on them and exercise more. Conversely, those who deem themselves unfit are more likely to remain inactive, fueling feelings of fear, stress or depression that negatively affect their health. Wait, there’s more. The researchers also cite the influence of placebo effects, where patients who think they are getting a treatment experience physiological changes without receiving actual treatment. In the same way, people who believe they are getting good exercise may experience more physiological benefits from their exercise than those who believe they aren’t getting enough exercise.