The nocebo effect occurs when bad thoughts or associations produce bad results.

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Mass Psychogenic Disorders

Mass psychogenic disorder is a phenomenon that can be understood as resulting, in part, from the nocebo effect. Think of the nocebo effect as the opposite of the placebo effect. Instead of good thoughts or associations producing a positive outcome, bad thoughts and associations produce bad results. For example, in the early 1990s, a study showed that women who believed they were prone to heart disease were four times more likely to die than women who didn’t believe they were susceptible, even though both groups of women had similar risk factors [source: Washington Post]. The study showed that when people feel that they have been exposed to a contaminant or a disease -- or that they are predisposed to becoming sick -- they are more likely to develop symptoms.

If you think about it, this may not seem that unusual. We feel effects based on the power of the mind all the time, whether it’s the placebo effect or feeling sick after eating food you think is bad for you.

But the nocebo effect can be a difficult thing to study -- after all, who wants to make patients sick? However, it is an important thing to consider, both for better understanding the brain and because drug side effects, commonly associated with the nocebo effect, cost the U.S. health system tens of billions of dollars a year [source: Washington Post].

Why are drug side effects important? Let's consider another study, in which patients took either aspirin or another blood thinner. Two groups of patients were told that a common side effect of aspirin was gastrointestinal pain, while another group received no warning. In the end, the patients with the most information about possible problems were most likely to complain of gastrointestinal pain, even though examinations showed that side effects were evenly distributed among all three groups [source: Washington Post].

In a now-famous experiment from 1886, a woman who claimed to suffer allergies from roses was shown an artificial rose. Upon seeing the rose, she developed congestion, shortness of breath and other symptoms of an allergic reaction. When the woman was told the rose was fake, the symptoms went away, and days later, she no longer experienced an allergic reaction among real roses.

Even the color of pills has been shown to influence how people react to medication [source: Washington Post]. Red and orange pills are generally considered stimulating, while blue and green are seen as depressing. Perhaps not coincidentally, DayQuil and generic visions of the drug are usually red or orange, and NyQuil and its generic counterparts are usually green or blue.