The Grant Study is part of the Study of Adult Development at Harvard Medical School. It is a 75-year longitudinal study of 268 physically and mentally healthy Harvard college sophomores from the classes of 1939-1944. It has run in tandem with a study called "The Gluek Study," which included a second cohort of 456 disadvantaged non-delinquent inner-city youths who grew up in Boston neighborhoods between 1940 and 1945. The subjects were all male, white and of American nationality. The men continue to be studied to this day. The study, its methodology and results are described in three books by a principal investigator in the study, George Vaillant. The first book  describes the study up to a time when the men were 47 years of age, and the second book  to when the inner-city men were 70 years old and the Harvard group were eighty. In 2012, Valliant and Harvard University Press published Triumphs of Experience, sharing more findings from the Grant Study. 
The men were evaluated at least every two years by questionnaires, information from their physicians, and in many cases by personal interviews. Information was gathered about their mental and physical health, career enjoyment, retirement experience and marital quality. The goal of the study was to identify predictors of healthy aging.
The study is part of The Study of Adult Development, which is now under the direction of Dr. Robert J. Waldinger at Massachusetts General Hospital. The study included four members who ran for the U.S. Senate. One served in a presidential Cabinet, and one was President John F. Kennedy. The study is unique partly because of the long time span of the cohort, partly because of the high social status of some of the study participants.
Main Results of Experiment
George Vaillant, who directed the study for more than three decades, has published  a summation of the key insights the study has yielded:
- Alcoholism is a disorder of great destructive power.
- Is the main cause of divorce between the Grant Study men and their wives.
- Strongly correlates with neurosis and depression, which tended to follow alcohol abuse, rather than precede it.
- Together with associated cigarette smoking, was the single greatest contributor to their early morbidity and death.
- Financial success depends on warmth of relationships and, above a certain level, not on intelligence.
- Those who scored highest on measurements of “warm relationships” earned an average of $141,000 a year more at their peak salaries (usually between ages 55 and 60).
- No significant difference in maximum income earned by men with IQs in the 110–115 range and men with IQs higher than 150.
- Political mindedness correlates with intimacy: Ageing liberals have way more sex.
- The most-conservative men ceased sexual relations at an average age of 68.
- The most-liberal men had active sex lives into their 80s.
- The warmth of childhood relationship with mothers matters long into adulthood:
- Men who had “warm” childhood relationships with their mothers earned an average of $87,000 more a year than men whose mothers were uncaring.
- Men who had poor childhood relationships with their mothers were much more likely to develop dementia when old.
- Late in their professional lives, the men’s boyhood relationships with their mothers—but not with their fathers—were associated with effectiveness at work.
- The warmth of childhood relationships with mothers had no significant bearing on "life satisfaction" at 75.
- The warmth of childhood relationship with fathers correlated with:
- Lower rates of adult anxiety.
- Greater enjoyment of vacations.
- Increased “life satisfaction” at age 75.
- Main conclusion, warmth of relationships throughout life have the greatest positive impact on "life satisfaction".
- Valliant gives the five-word conclusion: ‘Happiness is love. Full stop.’
- Vaillant, G., Mukamal K. Successful Aging. American Journal of Psychiatry,2001:158:839-847
- Vaillant, George E. Adaptation to Life, 1977
- Vaillant, George E. Aging Well, 2002