SOUTHAMPTON, England -- Not long after moving to the University of Southampton, Constantine Sedikides had lunch with a colleague in the psychology department and described some unusual symptoms he'd been feeling. A few times a week, he was suddenly hit with nostalgia for his previous home at the University of North Carolina: memories of old friends, Tar Heel basketball games, fried okra, the sweet smells of autumn in Chapel Hill.
His colleague, a clinical psychologist, made an immediate diagnosis. He must be depressed. Why else live in the past? Nostalgia had been considered a disorder ever since the term was coined by a 17th-century Swiss physician who attributed soldiers' mental and physical maladies to their longing to return home -- nostos in Greek, and the accompanying pain, algos.
But Dr. Sedikides didn't want to return to any home -- not to Chapel Hill, not to his native Greece -- and he insisted to his lunch companion that he wasn't in pain.
"I told him I did live my life forward, but sometimes I couldn't help thinking about the past, and it was rewarding," he says. "Nostalgia made me feel that my life had roots and continuity. It made me feel good about myself and my relationships. It provided a texture to my life and gave me strength to move forward."
The colleague remained skeptical, but ultimately Dr. Sedikides prevailed. That lunch in 1999 inspired him to pioneer a field that today includes dozens of researchers around the world using tools developed at his social-psychology laboratory, including a questionnaire called the Southampton Nostalgia Scale. After a decade of study, nostalgia isn't what it used to be -- it's looking a lot better.
Nostalgia has been shown to counteract loneliness, boredom and anxiety. It makes people more generous to strangers and more tolerant of outsiders. Couples feel closer and look happier when they're sharing nostalgic memories. On cold days, or in cold rooms, people use nostalgia to literally feel warmer.
Nostalgia does have its painful side -- it's a bittersweet emotion -- but the net effect is to make life seem more meaningful and death less frightening. When people speak wistfully of the past, they typically become more optimistic and inspired about the future.
"Nostalgia makes us a bit more human," Dr. Sedikides says. He considers the first great nostalgist to be Odysseus, an itinerant who used memories of his family and home to get through hard times, but Dr. Sedikides emphasizes that nostalgia is not the same as homesickness. It's not just for those away from home, and it's not a sickness, despite its historical reputation.
Nostalgia was originally described as a "neurological disease of essentially demonic cause" by Johannes Hoffer, the Swiss doctor who coined the term in 1688. Military physicians speculated that its prevalence among Swiss mercenaries abroad was due to earlier damage to the soldiers' ear drums and brain cells by the unremitting clanging of cowbells in the Alps.
A Universal Feeling
In the 19th and 20th centuries nostalgia was variously classified as an "immigrant psychosis," a form of "melancholia" and a "mentally repressive compulsive disorder" among other pathologies. But when Dr. Sedikides, Tim Wildschut and other psychologists at Southampton began studying nostalgia, they found it to be common around the world, including in children as young as 7 (who look back fondly on birthdays and vacations).
"The defining features of nostalgia in England are also the defining features in Africa and South America," Dr. Wildschut says. The topics are universal -- reminiscences about friends and family members, holidays, weddings, songs, sunsets, lakes. The stories tend to feature the self as the protagonist surrounded by close friends.
Most people report experiencing nostalgia at least once a week, and nearly half experience it three or four times a week. These reported bouts are often touched off by negative events and feelings of loneliness, but people say the "nostalgizing" -- researchers distinguish it from reminiscing -- helps them feel better.
To test these effects in the laboratory, researchers at Southampton induced negative moods by having people read about a deadly disaster and take a personality test that supposedly revealed them to be exceptionally lonely. Sure enough, the people depressed about the disaster victims or worried about being lonely became more likely to wax nostalgic. And the strategy worked: They subsequently felt less depressed and less lonely.
Nostalgic stories aren't simple exercises in cheeriness, though. The memories aren't all happy, and even the joys are mixed with a wistful sense of loss. But on the whole, the positive elements greatly outnumber the negative elements, as the Southampton researchers found by methodically analyzing stories collected in the laboratory as well as in a magazine named Nostalgia.
"Nostalgic stories often start badly, with some kind of problem, but then they tend to end well, thanks to help from someone close to you," Dr. Sedikides says. "So you end up with a stronger feeling of belonging and affiliation, and you become more generous toward others."
A quick way to induce nostalgia is through music, which has become a favorite tool of researchers. In an experiment in the Netherlands, Ad J. J. M. Vingerhoets of Tilburg University and colleagues found that listening to songs made people feel not only nostalgic but also warmer physically.
That warm glow was investigated in southern China by Xinyue Zhou of Sun Yat-Sen University. By tracking students over the course of a month, she and colleagues found that feelings of nostalgia were more common on cold days. The researchers also found that people in a cool room (68 degrees Fahrenheit) were more likely to nostalgize than people in warmer rooms.
Not everyone in the cool room turned nostalgic during the experiment, but the ones who did reported feeling warmer. That mind-body link, Dr. Wildschut says, means that nostalgia might have had evolutionary value to our ancestors long before Odysseus.
"If you can recruit a memory to maintain physiological comfort, at least subjectively, that could be an amazing and complex adaptation," he says. "It could contribute to survival by making you look for food and shelter that much longer."
Finding a Sweet Spot
Of course, memories can also be depressing. Some researchers in the 1970s and '80s suggested that nostalgia could worsen a problem that psychologists call self-discontinuity, which is nicely defined in "Suite: Judy Blue Eyes," by Stephen Stills: "Don't let the past remind us of what we are not now." This sense of loss and dislocation has repeatedly been linked to both physical and mental ills.
But the feeling of discontinuity doesn't seem to be a typical result of nostalgia, according to recent studies. In fact, people tend to have a healthier sense of self-continuity if they nostalgize more frequently, as measured on the scale developed at Southampton. To understand why these memories seem reassuring, Clay Routledge of North Dakota State University and other psychologists conducted a series of experiments with English, Dutch and American adults.
First, the experimenters induced nostalgia by playing hit songs from the past for some people and letting them read lyrics to their favorite songs. Afterward, these people were more likely than a control group to say that they felt "loved" and that "life is worth living."
Then the researchers tested the effect in the other direction by trying to induce existential angst. They subjected some people to an essay by a supposed Oxford philosopher who wrote that life is meaningless because any single person's contribution to the world is "paltry, pathetic and pointless." Readers of the essay became more likely to nostalgize, presumably to ward off Sartrean despair.
Moreover, when some people were induced to nostalgia before reading the bleak essay, they were less likely to be convinced by it. The brief stroll down memory lane apparently made life seem worthwhile, at least to the English students in that experiment. (Whether it would work with gloomy French intellectuals remains to be determined.)
"Nostalgia serves a crucial existential function," Dr. Routledge says. "It brings to mind cherished experiences that assure us we are valued people who have meaningful lives. Some of our research shows that people who regularly engage in nostalgia are better at coping with concerns about death."
Feeding the Memory Bank
The usefulness of nostalgia seems to vary with age, according to Erica Hepper, a psychologist at the University of Surrey in England. She and her colleagues have found that nostalgia levels tend to be high among young adults, then dip in middle age and rise again during old age.
"Nostalgia helps us deal with transitions," Dr. Hepper says. "The young adults are just moving away from home and or starting their first jobs, so they fall back on memories of family Christmases, pets and friends in school."
Dr. Sedikides, now 54, still enjoys nostalgizing about Chapel Hill, although his range has expanded greatly over the past decade. He says that the years of research have inspired strategies for increasing nostalgia in his own life. One is to create more moments that will be memorable.
"I don't miss an opportunity to build nostalgic-to-be memories," he says. "We call this anticipatory nostalgia and have even started a line of relevant research."
Another strategy is to draw on his "nostalgic repository" when he needs a psychological lift or some extra motivation. At such moments, he tries to focus on the memories and savor them without comparing them with anything else.
"Many other people," he explains, "have defined nostalgia as comparing the past with the present and saying, implicitly, that the past was better -- 'Those were the days.' But that may not be the best way for most people to nostalgize. The comparison will not benefit, say, the elderly in a nursing home who don't see their future as bright. But if they focus on the past in an existential way -- 'What has my life meant?' -- then they can potentially benefit."
This comparison-free nostalgizing is being taught to first-year college students as part of a study testing its value for people in difficult situations. Other experiments are using the same technique in people in nursing homes, women recovering from cancer surgery, and prison inmates.
Is there anyone who shouldn't be indulging in nostalgia? People who are leery of intimate relationships -- "avoidant," in psychological jargon -- seem to reap relatively small benefits from nostalgia compared with people who crave closeness. And there are undoubtedly neurotics who overdo it. But for most others, Dr. Sedikides recommends regular exercises.
"If you're not neurotic or avoidant, I think you'll benefit by nostalgizing two or maybe three times a week," he says. "Experience it as a prized possession. When Humphrey Bogart says, 'We'll always have Paris,' that's nostalgia for you. We have it, and nobody can take it away from us. It's our diamond."
Correction: July 8, 2013, Monday
This article has been revised to reflect the following correction: An earlier version of this article incorrectly rendered the nickname of the University of North Carolina basketball team as one word. The team is the Tar Heels, not the Tarheels.
This article originally appeared in The New York Times.