In 1938, researchers at Harvard set out to learn what makes a person thrive.

They recruited 724 participants, a combination of students at Harvard College and low-income teenage boys in Boston. All were willing to let the researchers track their lives, from childhood troubles to first loves to final days.

Every five years, the researchers gathered health records from the participants. They asked detailed questions about their lives at two-year intervals, and, in later years, took DNA samples and performed brain scans. Twenty-five of the participants even donated their brains to the study after their deaths.

Now, 85 years later, the Harvard Study of Adult Development has expanded to three generations and more than 1,300 descendants of the original subjects; it is, according to the researchers, the longest-running in-depth study on human happiness in the world.

From all the data, one very clear finding has emerged: Strong relationships are what make for a happy life. More than wealth, I.Q. or social class, it’s the robustness of our bonds that most determines whether we feel fulfilled.

How to build your social bonds for more joy

In a new book, “The Good Life: Lessons From the World’s Longest Scientific Study of Happiness,” Dr. Bob Waldinger, a professor of psychiatry at Harvard Medical School and the study’s fourth director, and Marc Schulz, an associate director of the study and a psychology professor at Bryn Mawr College, have distilled the study’s insights.

If you’re going to do one thing this year to ensure your own health and happiness, the authors maintain, find the time to nurture and develop relationships. To help get you started, Dr. Waldinger and I have created this challenge — seven simple exercises, starting with taking today’s quiz.

Dr. Waldinger and Dr. Shultz have coined a term for the process of assessing and treating the health of our relationships: “social fitness.” It’s just as crucial as physical fitness, said Dr. Waldinger, who added that neglected relationships can atrophy, like muscles. “Our social life is a living system, and it needs exercise,” he said. “It’s a choice you make to invest in, week by week, year by year — one that has huge benefits.”

Why ‘social fitness’ matters

The Harvard study is far from the only one to have found a link between our relationships and happiness. Ample research shows that people who are more socially connected live longer and are more protected against stress, depression and declines in memory and language.

Loneliness, on the other hand, damages our physical health. “I believe loneliness is one of the defining public health concerns of our time,” Surgeon General Vivek Murthy told me in an email. While the pandemic exacerbated loneliness, he added, “it also helped many people take stock of their lives and reflect more deeply on how important their relationships are. That means taking steps in our day-to-day lives to invest in them.”

It’s not just your bonds with friends and family that are crucial to happiness. It’s your relationships with romantic partners and community groups. It’s the friendly conversation with your mail carrier or the acquaintance you see at the dog run.

Take the quiz

Today, you will identify the areas of your life in which you would like to be more connected. “Maybe you’ve got lots of people to have fun with, but you don’t have anyone to confide in about really personal, private things,” Dr. Waldinger said. “Or maybe you’ve got lots of people to confide in but don’t have anyone to have fun with.”

A few things before you begin:

There is no right number of friends. Don’t get hung up on numbers, Dr. Waldinger said. It’s the quality of your relationships, not the quantity, so try to discern what is most fulfilling for you (our quiz will help give you some perspective).

You don’t need to be an extrovert to improve your social fitness. While adult friendships require effort, happiness is not out of reach for you if you are shy or introverted, Dr. Waldinger said. You can engage with others in quieter settings around things that you care about. Try small, controlled activities such as a knitting group, a computer programming class, hiking or working in a community garden.

It’s never too late. People often assume that it’s too late for them to build relationships, Dr. Waldinger said, but that’s never the case. He points to many stories in “The Good Life” about people who made connections later in life, like a lonely 68-year-old who joined a gym after he’d retired. Three months later, he had amassed more friends than he’d ever had in his life.

From the Friendship Files: Amy and Al

Each day of the challenge, we’ll be sharing stories of meaningful friendships collected from readers across the country. We’d love to hear yours — tell your own tale of friendship here.

Amy Pechukas met her friend Al in 2018 when she rented the apartment under his in Northampton, Mass. They didn’t connect immediately. Amy, now 42, worked four jobs and thought Al, 76, was a curmudgeon with questionable boundaries. He helped look after their two-family home and would frequently enter her apartment to check on her two cats and two dogs.

But Al’s peculiar brand of kindness grew on her. “He often pops in for a conversation spontaneously, at times when I need a lift, and we end up talking for hours,” she said. “We go for evening walks and argue about the route.”

Covid brought them even closer. During lockdown, they would meet in the driveway to talk about the virus or politics. Amy made a Thanksgiving meal, which they ate outside on their porch with electric blankets on their laps. They have celebrated the holiday together ever since.

Al can still be overbearing. He has firm ideas about the way things should be done around the home, like the “right” way to rake the leaves. Every summer, he frets that Amy’s elderly cat, who grows lazy in the heat, is on the verge of death.

But Amy feels deep gratitude for their unexpected friendship, and for the constant, unselfish care Al has shown her and her pets. “When my dog got very sick a year ago and needed me to do round-the-clock care for her, I would come home on occasion to find Al in my kitchen doing my dishes,” she recalled. “‘You can’t do everything, Amy,’ he’d say. ‘You’re doing a great job.’”

Though Al does not say it outright, Amy knows he worries she might move out. She recently interviewed for a position out of state, and Al told her several times that it sounded terrible — reminding her that there were other jobs closer by.

“We just have a lot of fun,” she said. “We like to quote movie lines endlessly, we’ll do that for, like, two hours straight. Last winter we went ice skating in the cemetery because it was flooded. Al’s just a good person.” — Catherine Pearson