People who think they exercise less than their peers die earlier, regardless of their actual activity levels

According to the World Health Organization (WHO), physical inactivity is the fourth-leading risk factor for global mortality, causing an estimated 3.2 million deaths around the world annually. Readers of this blog need no convincing that it's important to be active every day. But is spending more time on it enough to reduce the risk of early death? Not necessarily. How we perceive this activity turns out to be just as important. We learn of this from the authors of an intriguing study in Health Psychology devoted to physical activity and mortality.

Octavia Zahrt and Alia Crum at Stanford University were inspired by an earlier experiment involving hotel room attendants who completed a 20-minute intervention informing them that their daily work satisfied exercise recommendations and highlighting the benefits of this active lifestyle. This intervention not only shifted room attendants' perceptions, but also resulted in health improvements including lower blood pressure and reductions in weight and body fat.

Zahrt and Crum examined data from 61,141 Americans (selected to be representative of the general population) to determine whether the way we think about our own physical activity could be of major and long-term significance for health. The data came from the US National Health Interview Survey and the National Health and Nutrition Examination Survey, which included questions assessing how much exercise individuals think they get compared with their peers. The surveys also asked respondents detailed questions about actual physical activity they had undertaken, and in some cases participants wore an accelerometer to measure their activity objectively. The researchers cross-checked these survey data against the National Death Index records as they stood 21 years after the exercise surveys had been completed.

As the researchers expected, perceived physical activity relative to peers was closely associated with risk of dying. Even after adjusting for actual levels of physical activity, individuals who perceived themselves as less active than others were up to 71 per cent more likely to die in the follow-up period than those who perceived themselves as more active. One can say with confidence that individuals' perceptions about their level of physical activity were strongly related to their longevity, even after accounting for the effects of actual physical activity and other known determinants of mortality such as smoking or obesity.

There are a few potential explanations. A convincing one is that perceptions can affect motivation. For instance, a room attendant's awareness that she is getting exercise at work might increase her confidence and commitment to a healthy lifestyle, and motivate her to act on this "active" identity. Conversely, longitudinal research shows that individuals who perceive themselves as unfit compared with their friends are less likely to exercise a year later.

Another potential mechanism is that perceptions can have emotional consequences. Public health messages often warn of the "life-threatening consequences" of physical inactivity. A person's perception that she/he is inactive might thus lead to fear and stress about not getting enough exercise, with harmful consequences for health.

Still another explanation is that our positive beliefs and expectations can directly induce physiological responses, even following inert treatments, as shown by the placebo effect (similarly, negative expectations can lead to a harmful "nocebo effect"). Following this logic, participants in the current research who failed to realise that they were getting good exercise may not have experienced its full physiological benefits. Conversely, negative expectations related to the belief that one is not getting enough exercise could have become self-fulfilling because of nocebo-like effects.

Today we are not able to give a conclusive answer to the question of which of these mechanisms is most important. Perhaps a constellation of them? Or maybe there is another, unknown process involved.

Whatever the mechanisms at play, what do these results mean for mere mortals, and for those who are involved in promoting an active lifestyle? Instead of working out, should we stand in front of the mirror and repeat to ourselves "I'm an active and physically fit person"? To be clear, the authors warn that their findings don't mean exercise is unimportant. Separate from the influence of our perceptions, physical activity continues to be a crucial determinant of health. However, a more thorough understanding of these results could help us optimise public health messages, finding the happy medium between highlighting that people need to exercise more, but not to the extent that they become downhearted about the exercise they do get.

Further studies will doubtlessly bring us more answers, but before that happens, let's get up, walk away from the desk, and look at our activity with friendlier eyes than before.

About the author

Dr Tomasz Witkowski is a psychologist and science writer who specialises in debunking pseudoscience in the field of psychology, psychotherapy and diagnosis. He has published over a dozen books, dozens of scientific papers and over 100 popular articles (some of them in Skeptical Inquirer). In 2016 his latest book Psychology Led Astray: Cargo Cult in Science and Therapy was published by BrownWalker Press. He blogs at https://forbiddenpsychology.wordpress.com/.