In 1938, researchers at Harvard Medical School hit upon a crazy but visionary idea: They would sign up a bunch of men then studying at Harvard and follow them over their whole adult lives, until they died. They would question them every year along the way about their lifestyles, habits, relationships, work, and happiness. Even though the original researchers would all be dead and gone, in a few decades, new researchers would be able to see how what people do early in life relates to how well or poorly they age.

And thus, the Harvard Study of Adult Development was born. The original cohort of 268 men included people from many walks of life, including some who went on to become well-known, such as John F. Kennedy and Washington Post editor, Ben Bradlee. Still, over the decades, it was deemed too demographically insular, so it was paired with another dataset (called the Glueck Study) that had been tracking 456 disadvantaged youths from Boston starting at about the same time. Together, these data are updated continually for more than eighty years.

Fewer than sixty of the original participants are still alive, and the study is now tracking children and grandchildren of the first generation.

The result of this study is like a crystal ball of happiness: you look at how people lived, loved, and worked in their twenties and thirties, and then you can see how their lives turned out over the following decades. The longtime study director, Harvard psychiatry professor George Vaillant, wrote three bestselling books on the results. His successor, psychiatry professor Robert Waldinger, popularized the study even more with a viral TED Talk, “What Makes a Good Life? Lessons from the Longest Study on Happiness,” which has been viewed nearly forty million times.