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. Schulz 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.”

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.”