Last month, a 20-year-long study published in Health Psychology found that when people think they're less active than similar-aged peers—even if they're equally or more active—they're more likely to die earlier. This is just the latest in a long line of research that's been mystifying psychologists as much as physicians for decades: How people feel about their health changes it.Our perceptions about our health can, in fact, have immediate effects. Naval cadets told that they're unlikely to experience seasickness are less likely to feel it than cadets told they probably will. Other research suggests that just thinking you slept well the night before—even if you didn't—improves both alertness and cognitive performance.What we think about our health also has long-term consequences: In one longitudinal study, people with positive self-perceptions about aging lived an average of 7.5 years longer than those who had less positive perceptions. How they felt about aging affected their longevity more than blood pressure, BMI, activity level, or cholesterol. Likewise, high self-rated health—even controlling for objective health—predicts great long-term health. Conversely, if you feel unhealthy, even if you're not, you're more likely to become unhealthy.But why? Harvard research psychologist Ellen Langer attributes these self-fulfilling health prophesies to the placebo effect. In one of her studies, she divided a group of housekeepers into two groups. Though these women were on their feet all day, they didn't see themselves as physically active. So Langer showed one of the groups how many calories their daily tasks burned and informed them that their physical activity at work met the surgeon general's recommendation for an active lifestyle. The second group was a control, and they learned nothing about their profession's potential health benefits.A month later, the first group had lower blood pressure and body weight on average, whereas the control group experienced no improvement. It's possible that Langer's housekeepers were simply exhibiting the Hawthorne effect, where we change our behavior in response to being watched. Maybe group one started taking the stairs more or really putting their back into it once they knew someone was watching their health.Alternatively, Langer hypothesized that the housekeepers experienced health changes in response to a placebo: because they thought their job was good for their health, their health improved. Indeed, researchers have found that placebo treatments can induce real and lasting physiological responses, like changes in heart rate, blood pressure, and pain tolerance.
More from VICE: Doctors Explain Why US Healthcare is So Expensive
The placebo effect may explain self-fulfilling health prophesies, but what explains the placebo effect? The simple answer: our behaviors change in response to what we think. For example, in one study, people who believed old age is lonely were more likely to be lonely eight years later than people who didn't. "Both stereotypes about, and expectation of, loneliness were positively associated with loneliness status later on," the researchers explain. It's easy to see how beliefs about loneliness in old age could become self-fulfilling; if you expect to be lonely, you're less likely to make an effort to put yourself out there and maintain social connections.This adjusting-your-reality-to-your-expectations thing is called the consistency effect: People instinctively try to verify or confirm whatever they already believe about themselves or, in this case, their health. Sometimes that means unconsciously changing behaviors.Here's another example of the consistency effect: teens who think they're overweight—even if they're not—are more likely to become obese in adulthood. Why? Perhaps because, as other research suggests, women who think they're overweight feel less capable of controlling their food intake than similarly-sized women who don't think they're overweight. So teens who think they're fat may eventually surrender their self-control and gain weight.Our behaviors may also change in response to what we think other people think about us. "There are a lot of anti-obesity programs that get people to realize they weigh too much," says Brenda Major, a psychologist at UC Santa Barbara. Unfortunately, the "admit you have a problem" routine can backfire, because once people realize that others see them as overweight, they're more likely to engage in the kinds of poor eating habits that confirm that perception. Indeed, in one study, exposure to weight-stigmatizing news articles caused women who thought they were overweight to consume more calories and feel less capable of controlling their eating than women who didn't see themselves as overweight or women exposed to irrelevant articles.Finally, the stress that accompanies certain health perceptions, like thinking you're getting old or fat, can also affect your health. If someone's stressed about aging because he has negative perceptions about it, for example, that stress itself may make him age faster. Likewise, when people stress about appearing overweight and undesirable, their stress may trigger emotional overeating, compromise impulse control and ultimately lead to weight gain. The stress of stigma may also help explain why perceived weight discrimination increases the health risks associated with obesity.Stress can affect our health outcomes more acutely, too. In one study, anxious teens who anticipated a painful recovery after surgery experienced more pain than calmer teens who expected smooth recoveries. Why? It's possible that their stress actually lowered their pain threshold.In short, when we expect the worst, our health suffers because we behave in ways that sabotage it. When we expect the best, our health benefits because we act in our best interest. Of course, health isn't all in your head. But we shouldn't forget that, when used right, our minds can be their own form of medication. Read This Next: The Cruel Truth About Burning Fat
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More from VICE: Doctors Explain Why US Healthcare is So Expensive
The placebo effect may explain self-fulfilling health prophesies, but what explains the placebo effect? The simple answer: our behaviors change in response to what we think. For example, in one study, people who believed old age is lonely were more likely to be lonely eight years later than people who didn't. "Both stereotypes about, and expectation of, loneliness were positively associated with loneliness status later on," the researchers explain. It's easy to see how beliefs about loneliness in old age could become self-fulfilling; if you expect to be lonely, you're less likely to make an effort to put yourself out there and maintain social connections.This adjusting-your-reality-to-your-expectations thing is called the consistency effect: People instinctively try to verify or confirm whatever they already believe about themselves or, in this case, their health. Sometimes that means unconsciously changing behaviors.Here's another example of the consistency effect: teens who think they're overweight—even if they're not—are more likely to become obese in adulthood. Why? Perhaps because, as other research suggests, women who think they're overweight feel less capable of controlling their food intake than similarly-sized women who don't think they're overweight. So teens who think they're fat may eventually surrender their self-control and gain weight.Our behaviors may also change in response to what we think other people think about us. "There are a lot of anti-obesity programs that get people to realize they weigh too much," says Brenda Major, a psychologist at UC Santa Barbara. Unfortunately, the "admit you have a problem" routine can backfire, because once people realize that others see them as overweight, they're more likely to engage in the kinds of poor eating habits that confirm that perception. Indeed, in one study, exposure to weight-stigmatizing news articles caused women who thought they were overweight to consume more calories and feel less capable of controlling their eating than women who didn't see themselves as overweight or women exposed to irrelevant articles.Finally, the stress that accompanies certain health perceptions, like thinking you're getting old or fat, can also affect your health. If someone's stressed about aging because he has negative perceptions about it, for example, that stress itself may make him age faster. Likewise, when people stress about appearing overweight and undesirable, their stress may trigger emotional overeating, compromise impulse control and ultimately lead to weight gain. The stress of stigma may also help explain why perceived weight discrimination increases the health risks associated with obesity.Stress can affect our health outcomes more acutely, too. In one study, anxious teens who anticipated a painful recovery after surgery experienced more pain than calmer teens who expected smooth recoveries. Why? It's possible that their stress actually lowered their pain threshold.In short, when we expect the worst, our health suffers because we behave in ways that sabotage it. When we expect the best, our health benefits because we act in our best interest. Of course, health isn't all in your head. But we shouldn't forget that, when used right, our minds can be their own form of medication. Read This Next: The Cruel Truth About Burning Fat