There are very few studies like the Harvard Grant Study. Started in 1938, it has been following its approximately 200 participants ever since, analyzing their physical and mental health and assessing which factors are correlated with healthy living and healthy aging. One of the psychiatrists of the study is George E. Vaillant, who was a young man in 1966 when he joined the research group, and has now written Triumphs of Experience: The Men of the Harvard Grant Study (Harvard University Press, 2012). This fascinating book relates how the participants have changed over the course of their lifetimes (yes, Dr. Vaillant claims, people can change) and highlights the factors correlated with both happiness (e.g. warm childhoods, close relationships) and misery (e.g. alcoholism). Some of the findings are what you would expect, but this longitudinal study also holds some surprises, even as its participants reach their 90s and beyond.
The Harvard Brief