The book "Aging Well" goes into detail about the men themselves, the study perimeters, and the interviews they conducted:

https://www.amazon.com/Aging-Well-Surprising-Guideposts-Deve...

None of these guys led perfect lives. Alcoholism, abuse, death, divorce. But as they grew older the things they most often cited in hours of interviews as objectively making them happier, were the relationships they forged and kept along the way.

This article breaks down the Grant Study at Harvard Medical School, which is one of the longest-running longitudinal studies ever (at over 75 years)[1]. The primary researcher has written several books[2][3] on the topic, and it's a fascinating study.

One thing to remember is that happiness is relative and changes over your lifetime. What makes you happy does not make others happy, and what makes you happy at 20 may not make you happy at 30 or 40. It's easy for HNers to read an article like this and strive to achieve happiness in the boxes defined in this article, but living authentically in what gives you happiness is just as (if not more) important than the tenants that are outlined in this article.

[1] https://en.wikipedia.org/wiki/Grant_Study

[2] http://www.hup.harvard.edu/catalog.php?isbn=9780674004146

[3] http://www.amazon.com/Aging-Well-Surprising-Guideposts-Devel...