Growing up in Maryland, Sonja Lyubomirsky could see that her mother was unhappy. When Sonja was 9, her parents moved the family from Moscow, where her mother taught literature at a high school, to the United States, hoping to offer their children more opportunities. In their new country, Sonja’s mother could no longer teach, so she cleaned houses to help the family get by. She missed her old career; she longed for her home country; she was frequently teary. She was unhappy on a Tolstoyan scale. Sonja understood her nostalgia and frustrations, which were compounded by a miserable marriage, but she still wondered: Were Russians just less happy than Americans? Was her mother destined to be unhappy anywhere, or was this a result of life circumstances? What, if anything, might make someone like her mother happier, if not wholly content?

In 1985, Lyubomirsky left for college at Harvard, where, her adviser reminded her years later, she frequently brought up the topic of happiness, even though his expertise was in the social psychology of the stock market. At the time, the study of happiness was far from the wellness mega-field it has become today. In the ’60s, a researcher making a rare foray into the subject noted that very little progress on the theory of happiness had been made since Aristotle weighed in two millenniums earlier. That paper concluded that youth and modest life aspirations were key components of happiness (findings later called into question).

Many scientists at the time believed that happiness was essentially random: It was not something to cultivate, like a garden, or to reach for, by setting and achieving meaningful goals. It was something that happened to people, by virtue of their genes, their circumstances or both. “It may be that trying to be happier is as futile as trying to be taller and therefore is counterproductive,” the authors of a 1996 study concluded.

When Lyubomirsky arrived at graduate school for social psychology at Stanford in 1989, academic research on happiness was only beginning to gain legitimacy. Ed Diener, a psychologist at the University of Illinois at Urbana-Champaign who would eventually be known for his work in the field, waited until he was granted tenure before tackling the subject, despite harboring a longstanding interest in it. Lyubomirsky, too, was wary of choosing happiness as a specialty — she was a woman in science eager to be taken seriously, and anything in the realm of “emotions” was considered somewhat soft. Nonetheless, on her first day of graduate school at Stanford, in 1989, following an energizing conversation with her adviser, she resolved to make happiness her focus.

Lyubomirsky began with the basic question of why some people are happier than others. A few years earlier, Diener published a survey of the existing research, which touched on the kinds of behaviors that happy people seemed inclined to engage in — religious observance, for example, or socializing and exercising. But the studies, which sometimes had conflicting findings, yielded no clear consensus. Lyubomirsky’s own research, over many years, pointed toward the importance of a person’s mind-set: Happy people tended to refrain from comparing themselves with others, had more positive perceptions of others, found ways to be satisfied with a range of choices and did not dwell on the negative.

But Lyubomirsky knew she couldn’t separate cause and effect: Did being happy encourage a healthy mind-set, or did adopting that mind-set make people happier? Were people like her mother doomed to live with whatever their natural level of happiness was — or could they take control of their mood, if they only knew how? Even if you could change your mind-set, that process seemed to take a long time — people spend years in therapy trying (and often failing) to do it — and Lyubomirsky wondered whether there were simpler, easier behaviors they could adopt that would quickly enhance their sense of well-being. She decided to put it to the test.

Lyubomirsky started by studying some of the habits and practices that were commonly believed to be mood boosters: random acts of kindness and expressions of gratitude. Each week for six weeks, she had students perform five acts of kindness — donating blood, for example, or helping another student with a paper — and found that they were happier by the end of that period than the students in her control group. She asked a separate group of students to contemplate, once a week, the things they were grateful for, like “my mom” or “AOL Instant Messenger.” They, too, were happier after doing so than a control group. The changes in well-being weren’t particularly large in either study, but Lyubomirsky found it remarkable that so small and low-cost an intervention could improve the quality of students’ lives. In 2005, she published a paper based on those studies arguing that people did have considerable control over how happy they were.

Lyubomirsky’s research came out just as the field of psychology was reconsidering its objectives and even its purpose. When Martin Seligman, a psychologist at the University of Pennsylvania, took the helm of the American Psychological Association in 1998, he expressed a concern that he and his colleagues had spent too much time focusing on dysfunction and not enough devoted to fostering life satisfaction; he encouraged his peers to pursue “the understanding and building of the most positive qualities of an individual: optimism, courage, work ethic, future‑mindedness, interpersonal skill, the capacity for pleasure and insight and social responsibility.” He called for a return of the field to its origins, “which were to make the lives of all people more fulfilling and productive.”

Psychologists heeded the call, bearing down on new fields of research, including well-being and happiness. They conducted thousands of happiness-intervention studies over the next 15 years: acts-of-kindness and gratitude studies, like Lyubomirsky’s, but also forced-smile studies, “looking on the bright side” studies, diet studies and meditation studies. Many of them seemed to show that people could, in fact, make themselves happier, but most of the effect sizes were small, the results were short term and the options seemed endless. Those aspiring to greater happiness might even have suffered from the paradox of choice: With the limited time they had, should they devote it to journaling? Practicing gratitude? Meditating? The public would have to wait another two decades for a more decisive answer to break through — delivered by a researcher who presided over the longest-running happiness study in the field’s history.

In 2003, the psychiatrist Robert Waldinger accepted a new job at Harvard, where he had long been affiliated, overseeing one of its most prized research projects. Waldinger, a psychoanalyst by training who would later be ordained as a Zen Buddhist priest, had always been preoccupied by questions “with an existential flavor,” which is why, when the university asked him to take on the longest-running wellness study in American history, he readily agreed. It was the rare study that surveyed people over the course of their lives, from young adulthood through old age, and it held clues about the choices and circumstances that lead people to look back on their lives with regret or satisfaction.

The study began in 1938, in an attempt to discern the habits of healthy and sound young men. Two doctors who tended to Harvard students were running it with a grant from a Midwestern retail mogul whose goal, Waldinger says he was told, was to figure out what made for a good department-store manager. The researchers’ goal, by contrast, was to reverse the usual trajectory of medical research, which was to study those who were ill in the hope of finding ways to make them well. Instead of examining patients after their troubles began, the doctors hoped to “attempt to analyze the forces that have produced normal young men.”

In search of young men who could, as one researcher put it, “paddle their own canoe,” the two doctors recruited a group of 268 Harvard undergraduates from the classes of 1939 through 1944, among them John F. Kennedy and the future Washington Post editor Ben Bradlee. The students (all of whom were white) were handpicked by their deans as exemplars of a kind. “All of us need more ‘do’s and fewer ‘don’ts,’” the doctors wrote in a news release announcing their aims.

The college students were studied from every conceivable angle. They spoke for more than 20 hours each to psychiatrists; their family histories were explored; their parents were interviewed about their childhood foibles, and they were given a barrage of physiological tests. They had their insulin tolerance assessed, along with their respiratory function and how their bodies responded when they were asked to run on a treadmill to the point of exhaustion. They were measured from head to toe in a pseudoscientific pursuit of a connection between body shape and personality. Once they left college, a majority of the men continued to submit to regular medical exams and filled out lengthy questionnaires that asked them about their lives and their state of mind; roughly once a decade, a researcher traveled to interview them in person.

In the 1970s, the researchers brought another group of men into the study — the so-called Glueck cohort, 456 men, mostly white, from the Boston area who came from disadvantaged backgrounds. Sheldon Glueck, a professor at Harvard Law School, and his wife, Eleanor, a social worker and researcher, had begun interviewing them when they were still boys in 1939, originally hoping to compare their fates with those of another group of young men from their community who had been labeled juvenile delinquents.

Some 30 years after the study first started, the psychiatrist and researcher George Vaillant had taken it over, switching the emphasis away from a search for the inherent qualities of those considered the best and brightest and toward deeper questions about how much people change over time and what makes them happy and healthy in the long run. The survey asked open-ended questions that captured the shifts in the men’s worldviews and sense of self. “I have a drive — a terrible one,” one Harvard student initially told the psychiatrist who interviewed him. “I’ve always had goals and ambitions that were beyond anything practical.” He added that he was suspicious of “sneaky liberals,” so much so that he had torn up “propaganda” from the Harvard Liberal Union. In his 30s, that same man said that his goal was no longer to “be great at science, but to enjoy working with people.” By 50, he had resolved, the researchers reported, that “the world’s poor were the responsibility of the world’s rich.”

Many of the study participants served in World War II; they went on to work in fields like marketing, bricklaying, banking, real estate development and furniture moving. Sixty-five years after they first submitted themselves to investigation, many from both the Harvard and Glueck cohorts found themselves on less-than-hospitable shores, while others continued to glide along in the sun. In 2001, when the men were in their late 70s and early 80s, Vaillant published some of his most significant findings: He found that for both cohorts, one of the best predictors of the men’s overall well-being in their old age was how happily married they were at age 50.

Waldinger, who worked as a therapist for many years, had always felt that his primary goal was to help his patients have more satisfying emotional lives by enabling them to sustain meaningful relationships. But he was fascinated to find that the Harvard study bore out his intuition so clearly. Asked to name a regret, one man answered: “Wish I’d spent more time with my wife. She died just as I’d begun to taper down with work.” When Waldinger came on board, one of his first initiatives was to expand the study to include the wives of the Harvard and Glueck men. One 80-year-old woman interviewed said she wished she had spent less time getting upset about “silly things” and instead focused more on “more time with my children, husband, mother, father.”

Waldinger knew that being married was associated with overall well-being, but he was intrigued by other, more recent studies that found that marriage alone wasn’t enough — how happy the marriage was mattered. Waldinger decided to do some research of his own: He followed 47 octogenarian couples from the study over an eight-day period, noting how much time each person spent with their spouse and with friends and family. He found that for those in happy marriages, socializing with others in their circle contributed to their happiness. If they were in pain or ill health, however, it was only spending time with their spouses that seemed to protect them from the mood-dampening effects of physical suffering. Other research he conducted found that the people who scored highest on measures of attachment to their spouses were also the ones who reported the highest levels of happiness.

Waldinger, the fourth steward of the Harvard study, was moved by the consistency of his own research and the work that preceded him — the thousands of questionnaires, saliva samples, genetic analyses, cholesterol reports, dental records, I.Q. tests, wide-ranging interviews and brain scans. Much of it added up to one key insight: “The clearest message that we get from this 75-year study is this: Good relationships keep us happier and healthier. Period,” he said in a TED Talk in 2015. Strong, long-term relationships with spouses, family and friends built on deep trust — not achievement, not fortune or fame — were what predicted well-being. Waldinger had worried that his big reveal was so intuitive that he would be laughed off the stage; instead, the talk is one of TED’s most watched to date, with more than 40 million views.

Waldinger’s work was building on other prominent research about happiness and relationships that had been drawing attention in the field: Ed Diener and Martin Seligman found that happy people spent less time alone every day than unhappy people, and a large study published in 2008 found that those who were more socially engaged — attending church, belonging to organizations — were consistently happier, as were those with large social networks.

And yet at the same time the field was also recognizing weaknesses in its methods. The Harvard study, like a lot of other happiness research, posed that same question of cause and effect that had bedeviled psychology for so long: It was hard to know, for example, if happy marriages made for happier people at the end of their lives — or if happy people were simply more inclined to have happy marriages. In much of the work that researchers like Lyubomirsky conducted, the sample sizes were often too small to yield meaningful findings, and critics within and outside the field charged that psychology journals allowed researchers too much discretion in how they analyzed their data. A new generation of psychologists started re-examining the field’s practices and trying to prove, using more rigorous methods and new statistical tools, some of its core findings.

Julia Rohrer, who arrived as a graduate student at the Max Planck Institute in Berlin in 2016, was part of that new cohort. Eager for her work to have real meaning, she tried to find a rigorous way of looking at the connection between happiness and social relationships. Three years after Waldinger’s TED Talk, Rohrer analyzed a survey that asked nearly 2,000 Germans to write down ways they thought they could make themselves happier, or at least as happy, in the future. She coded the answers into “nonsocial” answers (“get a better job”) or “social” ones (“spend more time with friends and family”). Rohrer found that people who proposed a social goal had taken more steps toward that goal and were happier a year later. She concluded that “socially engaged pursuits predict increases in life satisfaction,” as she put it in the prestigious journal Psychological Science.

Rohrer’s work was published around the same time that other researchers were finding, in high-quality and replicated studies, that even fleeting social interactions could improve happiness. Nicholas Epley and Juliana Schroeder, researchers both then at the University of Chicago, conducted an experiment in which they asked people to interact with strangers on public transit — to try to have a moment of connection — and found that the commuters seemed to get a mood boost from the exercise. Epley and Schroeder’s research and other studies have found that people underestimated both how much they would enjoy the experience and how open the strangers would be to it.

That work was important beyond the decision about whether to talk to a stranger on a train, says Waldinger, who considers these findings some of the most helpful in recent years. “We have this innate reluctance to socially connect, particularly with strangers — and then we’re happier when we make ourselves. I find it a really useful thing to know.”

Finding purpose in serving others, spending more time with others — it all points toward the same thing, Lyubomirsky says. “After all these years, it hit me,” she says. “The reason that all of these interventions are working is because they make people feel more connected to others. So when I write a gratitude letter to my mom, it makes me feel more connected to my mom. When I do an act of kindness, it makes me feel more connected to the person I’m helping, or just humanity as a whole. Yes, you could go running, and that would make you happier, and meditation doesn’t necessarily have to be about other people. But I would say that 95 percent of things that are effective in making people happy and that have been shown to be true through happiness interventions are because they make people feel more connected to other people.”

Although social media has come to be associated with negative moods, the research on its effects on happiness is actually more mixed, Lyubomirsky says, because it does provide a certain kind of connection. In her own research, Lyubomirsky has found that when people talk to someone — whether in person, by phone or video chat — those simple interactions seem to boost happiness equally, and that they are all preferable to texting. “Maybe it’s just because our brains are not wired for that,” she says. With the exception of passive scrolling on social media — behavior that often inspires the scrollers to compare their own lives unfavorably with those of the posters — she believes connecting with old friends or possible new ones on social media is better than making no connections at all.

That strong marriages and family relationships make people happier — yes, that’s intuitive, Lyubomirsky acknowledges. What she found more surprising was just how effective even having smaller points of connection throughout the day could be for happiness — and how achievable that is, if people could only overcome their own hesitation. “If someone were to ask me what’s the one thing you could do tomorrow to be happier, that’s my answer: having a conversation with someone — or a deeper conversation than you normally do,” she says.

Talking to strangers — on trains, in a coffee shop, at the playground, on line at the D.M.V., in the waiting room at the doctor’s office — could be dismissed as an exercise that simply makes the time pass. But it could also be seen as a moving reflection of how eager we all are, every day, to connect with other humans whose interiority would otherwise be a mystery, individuals in whose faces we might otherwise read threat, judgment, boredom or diffidence. Talking to strangers guarantees novelty, possibly even learning. It holds the promise, each time, of unexpected insight.

A few weeks after I first talked to Waldinger by phone, I flew down to Florida, where he was spending a month with his wife in the borrowed home of a dear friend. I was struggling to come out of a low mood that came on in recent weeks after an injury. I had greatly underestimated the length of the drive from my hotel to the house, and I was reminded of research from about a decade ago finding that for every 10 minutes of extra commuting time, the likelihood of having symptoms of depression increases by 0.5 percent. By the time I arrived, I was irritable and my back was hurting, so much so that for the first 10 minutes of our conversation, I felt a dual consciousness: I was both listening and thinking about my pain, monitoring its level, worried it would only increase.

On a small patio by a very small pool, Waldinger and I talked about the rise of the happiness industry — the countless podcasts, conferences, best-selling books — and his own role in it. He gives considerable thought to maintaining his own happiness in the face of becoming a kind of influencer, someone called on to travel around the world to speak about happiness at conferences, sometimes to crowds of very wealthy people, repeating the same turns of phrase and giving the same advice about deep relationships.

As a Zen priest, someone accustomed to reckoning with his place in the world, Waldinger is acutely aware of the tension between achieving status and doing work that demands humility. Before becoming the steward of the Harvard study, he walked away from a high-profile job as the director of training and education at the Massachusetts Mental Health Center, after deciding that the prestige of the role didn’t offset his lack of enthusiasm for the administrative work it demanded. At age 45, he started over, taking a major pay cut to pursue work he found more fulfilling: working under the guidance of Stuart Hauser, a psychiatrist recognized for his work in adolescent development. That professional step, of course, led Waldinger to the Harvard study and the work that has catapulted his visibility far beyond that of his previous career.

He reflected with honesty about how much thought he gives to keeping his newfound fame in perspective. “I grapple with the feeling that it’s important,” he told me, as we sat over turkey sandwiches his wife had made; ordinarily, the two of them have lunch together, a small moment of connection they started sharing during the pandemic. The work is meaningful, he said; it was the feeling of ego gratification that he struggled with. “It feels important,” he said. “But it’s really not. I work at a hospital where every water fountain is named after someone who was once maybe famous. But now no one knows who they are.” The badges of achievement — that’s the least important part of who he is, he tries to remind himself. Because otherwise who would he be when the calls from The New York Times, from Aspen, from TED, stopped coming?

Even knowing that Waldinger was a Buddhist priest, I felt somehow surprised by how quickly our conversation had moved past the discussion of research and deepened into something that felt bracingly and reassuringly honest. When we finally said goodbye after a few hours of talking, mostly in the sun, I left feeling that I had connected with someone who was, just a few hours earlier, a stranger. I noticed, as I got in the car and remembered my concerns about my back, that it was incontrovertible: I felt better.