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Turn your mind for a moment to a friend or family member you cherish but don’t spend as much time with as you would like. This needn’t be your most significant relationship, just someone who makes you feel energized when you’re with them, and whom you’d like to see more regularly.

How often do you see that person? Every day? Once a month? Once a year? Do the math and project how many hours annually you spend with them. Write this number down and hang on to it.

This article is adapted from Waldinger and Schulz’s new book.

For us, Bob and Marc, though we work closely together and meet every week by phone or video call, we see each other in person for only a total of about two days (48 hours) every year.

How does this add up for the coming years? Bob is 71 years old. Marc is 60. Let’s be (very) generous and say we will both be around to celebrate Bob’s 100th birthday. At two days a year for 29 years, that’s 58 days that we have left to spend together in our lifetimes.

Fifty-eight out of 10,585 days.

Of course, this is assuming a lot of good fortune, and the real number is almost certainly going to be lower.

Since 1938, the Harvard Study of Adult Development has been investigating what makes people flourish. After starting with 724 participants—boys from disadvantaged and troubled families in Boston, and Harvard undergraduates—the study incorporated the spouses of the original men and, more recently, more than 1,300 descendants of the initial group. Researchers periodically interview participants, ask them to fill out questionnaires, and collect information about their physical health. As the study’s director (Bob) and associate director (Marc), we’ve been able to watch participants fall in and out of relationships, find success and failure at their jobs, become mothers and fathers. It’s the longest in-depth longitudinal study on human life ever done, and it’s brought us to a simple and profound conclusion: Good relationships lead to health and happiness.