A burgeoning science of mood is here to disabuse you of that belief (and perhaps give solace to those with mood disorders who find the effects of their moods anything but wimpy). Not only can moods, even ones that are at odds with our typical emotional state, leave lasting imprints on our mental and physical health; they also influence how we perceive the world and learn from experience—a function that, new research suggests, may explain the mystery of why we even have moods and why we don’t always want to immediately shake off negative ones. When people feel down, they don’t necessarily want to cheer up right away, as the seeming paradox of sadness and sad music shows: Most of us aim for happiness (whatever our personal definition of that), yet when feeling down we swipe through our iTunes downloads for a most heartstring-tugging tune (Eric Clapton’s “Tears in Heaven”? Itzhak Perlman’s rendition of the theme from Schindler’s List?). Researchers at Ireland’s University of Limerick asked scores of people why they did so. One reason was a desire for connection, for a sense that someone else (the composer, other listeners) had experienced grief such as theirs—a realization that keeps a sad mood from feeling isolating. Other participants in the 2013 study, published in Psychology of Music, said they “wanted to stay with those emotions for a while until I was ready to let go of them,” as a twenty-something woman explained. “I didn’t want music that would cheer me up.” Others said that sad music helped them experience that mood longer and more intensely, a pre-requisite for leaving it behind. One woman said she played sad music “in order to cry a little and then feel relieved and move on,” while another said it “encourage[d] me to feel the pain . . . plus allow me to have a good cry,” adding that “it probably did not make me feel better at the time, but may have helped me cope overall.” Those reasons for wanting to deepen, even sharpen, one’s sad mood are among the many hints that all moods, up as well as down, have a role to play in helping us navigate the world. Indeed, among scholars of human behavior, it’s a bedrock assumption that when an experience is extremely common it must serve a purpose. In evolutionary biology terms, it’s adaptive. Mood is so prevalent in our lives, lying just beneath the surface of any moment, that it’s ridiculously easy to trigger one. Music, weather, news, traffic, sporting events, thinking about ourselves, making facial expressions, fleeting interactions with strangers (why did that couple take up so much of the sidewalk that I had to walk in the gutter?)—all affect our mood. And that can have powerful consequences. When people feel happy, enthusiastic, or excited they tend to take more risks, including financial ones. Feeling down, by contrast, makes us more likely to choose the safe bet. (Keep that in mind next time you have to make decisions about investing for retirement or setting aside money for medical expenses.) Sadness and disappointment make us more likely to pay attention to and be affected by negative information. When you’re feeling low, all news is bad, all friends act selfishly, and every stranger is cutting in line. Moods are so powerful they can shape how we feel about something as basic as our age. People felt older on days when they experienced more bad moods, researchers reported last year in Psychology & Health. Low moods tip us toward thinking more analytically than creatively and intuitively. Being in a cheerful mood makes us think we’re particularly empathic, better able to, for instance, judge the emotional tone of a speaker (even though that confidence outstrips our actual capability), a 2014 study in PLOS ONE found. Nothing shows the power of mood more dramatically than how it affects something supposedly stable, if not fixed: personality. In a 2014 study, researchers gave 98 volunteers a standard personality test and had them watch a 10-minute video—neutral or upbeat (families reunited after the fall of the Berlin Wall) or sad (scenes from the movie Philadelphia, where the Tom Hanks character dies of AIDS). They then repeated the personality test. After watching sad scenes, volunteers scored notably higher on one of the Big Five personality traits, neuroticism, and somewhat lower on two others, extraversion and agreeableness, compared to their scores when their mood had not been manipulated, the researchers reported in BMC Psychology. Why would our brains have evolved not only to experience bad moods at something as trivial as a loss by our favorite sports team, but also to be so deeply affected by these reactions? Because—say researchers led by Yael Niv of Princeton University in a 2016 paper in Trends in Cognitive Sciences—moods “bias how we perceive outcomes.” Moods both positive and negative “serve an important role” of helping us learn from experiences. Our brains get a hedonic hit not from plain old rewards and good outcomes, but from rewards and outcomes that exceed our expectations. The old dogma was that dopamine was released in the brain when we experienced something positive, such as food or sex or a promotion. But now scientists know that dopamine is actually released when outcomes exceed our expectations. (Hence my own preferred outlook on life—expect everything to turn out terribly. But I digress.) “Happiness depends not on how well things are going,” Niv and her colleagues explained, “but whether they are going better than expected.” As we learn from experiences, we adjust our expectations accordingly. If things turn out better than expected, the dopamine hit and the resulting good mood encourage us to try for more of it. For instance, making surprise gains in the stock market improves a trader’s mood, leading her to take more risks, thereby taking advantage of a rising market. The reverse is also true: Sustaining a loss triggers negative emotions; those make us back away, protecting us from worse to come. In both cases mood pushed us toward the optimal behavior. The idea that even negative moods can be adaptive, leading us away from repeating some stupid behavior, might seem at odds with the longstanding belief that being in a bad mood is bad for health. Those who are frequently in an angry, anxious, or sad mood do tend to have worse health, a classic 1989 study found, perhaps because those moods are stressful and stress can take a physical toll. But negative moods don’t adversely affect everyone. People who see meaning and value in bad or sad moods tend to suffer less from them, a 2015 study published in the journal Emotion found. Enjoy the sadness. Embrace disappointment. Find empowerment within grouchiness. Your mind evolved to be moody. Don’t deny it.