"ACCENTUATE the positive,” the 1944 song by Johnny Mercer and Harold Arlen cheerfully implored us. From Benjamin Franklin’s 1750 Poor Richard’s Almanack (which advised readers that “sorrow is good for nothing but sin”) to today’s parade of motivational speakers, Americans have long embraced an optimistic, “can-do” attitude toward life. Plug “positive thinking” into Amazon.com, and you will find a never-ending supply of products designed to help us see life through rose-colored lenses, including a “Power of Positive Thinking” wall calendar and an “Overcoming Adversity with Encouragement and Affirmation” poster series.

In fact, however, positivity is not all it is cracked up to be. Although having an upbeat attitude undoubtedly has its benefits, gains such as better health and wealth from high spirits remain largely undemonstrated. What is more, research suggests that optimism can be detrimental under certain circumstances.

Pluses of Pessimism

Despite the popular emphasis on positive thinking, academic psychology was for many decades centered on the negative. Even today a perusal of the typical psychology textbook reveals a predominance of topics dealing with the dark side of life—mental illness, crime, addiction, prejudice and the like—probably reflecting an aim to remediate these personal and social problems.

Then, in the late 1990s, a cadre of prominent psychologists led by University of Pennsylvania psychologist Martin E. P. Seligman established a field called positive psychology. This burgeoning discipline explores the causes and consequences of happiness, character strengths and virtues, resilience, and other important aspects of psychological adaptation and health. Not all positive psychologists push cheerfulness at any cost—in a 1990 book Seligman warned that optimism “may sometimes keep us from seeing reality with the necessary clarity.” But many do advocate a perspective that implies that positive thinking is good for all of us, all of the time, noted Bowdoin College psychologist Barbara Held in a 2004 article.

In fact, much of the data supporting solid benefits from positive thinking is weak. According to a 2010 review by Cornell University psychologist Anthony Ong, although most studies show that optimistic people tend to be physically healthier than others and they may also live longer, these findings come from correlational studies, which examine statistical associations between positive thinking and life outcomes but cannot tell us about cause and effect. Thus, thinking positively might make us healthier, but being healthier may instead lead us to think positively. Another interpretation of the same results: positive thoughts and good health are the result of a third factor—being highly energetic, say—that was not measured in most of these studies. The same ambiguity plagues most studies purporting to show that optimism can lift depressed moods or boost job performance.

Even if more optimistic results about optimism eventually surface, a rosy outlook is unlikely to benefit everyone. Defensive pessimists, for example, tend to fret a great deal about upcoming stressors such as job interviews or major exams, and they overestimate their likelihood of failure. Yet this worrying works for these individuals, because it allows them to be better prepared. Work by Wellesley College psychologist Julie Norem and her colleagues shows that depriving defensive pessimists of their preferred coping style—for example, by forcing them to “cheer up”—leads them to perform worse on tasks. Moreover, in a 2001 study of elderly community participants, Seligman and Brandeis University psychologist Derek Isaacowitz found that pessimists were less prone to depression than were optimists after experiencing negative life events, such as the death of a friend. The pessimists had likely spent more time bracing themselves mentally for unpleasant possibilities.

Another study calls into question the healing power of positive affirmations—those ubiquitous fixtures of pop psychology parodied by former comedian Al Franken as counselor Stuart Smalley (“I’m good enough, I’m smart enough, and doggonit, people like me”). In a study published in 2009 University of Waterloo psychologist Joanne Wood and her colleagues found that for participants with high self-esteem, repeating a positive affirmation (“I am a lovable person”) multiple times indeed resulted in slightly better moods right afterward. But among those with low self-esteem, the positive affirmations backfired, resulting in worse moods. Wood and her colleagues conjectured that statements like Smalley’s ring hollow in the minds of individuals with low self-esteem, serving only to remind them of how often they have fallen short of their life goals.

Too Much of a Good Thing?

Another potential hitch in the positive-thinking movement is that a sanguine attitude may be unhealthy when taken to an extreme, because it can become unhinged from reality. In a 2000 article University of Michigan psychologist Christopher Peterson, a founder of the positive psychology movement, distinguished realistic optimism, which hopes for the best while remaining attuned to potential threats, from unrealistic optimism, which ignores such threats.

A 2007 study by University of Virginia psychologist Shigehiro Oishi, University of Illinois psychologist Ed Diener and Michigan State University psychologist Richard Lucas reinforces Peterson’s concerns. Using analyses from several large international samples, they found that although extremely happy people are the most successful in close interpersonal relationships and volunteer work, moderately happy people are more successful than extremely happy people financially and educationally and are also more politically active. Admittedly, Oishi and his colleagues measured happiness rather than optimism per se, although the two tend to be fairly closely associated. Still, their findings raise the possibility that although a realistically positive attitude toward the world often helps us to achieve certain life goals, a Pollyannaish attitude may have its costs—perhaps because it fosters complacency.

Positive thinking surely comes with advantages: it may encourage us to take needed risks and expand our horizons. But it has downsides as well and may not be for everyone, especially those for whom worrying and kvetching come naturally as coping mechanisms. Moreover, positive thinking may be counterproductive if it leads us to blithely ignore life’s dangers. Finally, as journalist Barbara Ehrenreich warns in a 2009 book, the pervasive assumption that positive attitudes permit us to “think our way out of” illnesses such as cancer has an unappreciated dark side: it may lead people who fail to recover from these illnesses to blame themselves for not being more chipper.