In June 2009, The Atlantic published a cover story on the Grant Study, one of the longest-running longitudinal studies of human development.

The project, which began in 1938, has followed 268 Harvard undergraduate men for 75 years, measuring an astonishing range of psychological, anthropological, and physical traits — from personality type to IQ to drinking habits to family relationships to “hanging length of his scrotum”—in an effort to determine what factors contribute most strongly to human flourishing.

Recently, George Vaillant, who directed the study for more than three decades, published Triumphs of Experience, a summation of the insights the study has yielded.

Among them: “Alcoholism is a disorder of great destructive power.” Alcoholism was the main cause of divorce between the Grant Study men and their wives; it was strongly correlated with neurosis and depression (which tended to follow alcohol abuse, rather than precede it); and — together with associated cigarette smoking — it was the single greatest contributor to their early morbidity and death.

Above a certain level, intelligence doesn’t matter. There was no significant difference in maximum income earned by men with IQs in the 110–115 range and men with IQs higher than 150.

Aging liberals have more sex. Political ideology had no bearing on life satisfaction—but the most-conservative men ceased sexual relations at an average age of 68, while the most-liberal men had active sex lives into their 80s. “I have consulted urologists about this,” Vaillant writes. “They have no idea why it might be so.”

But the factor Vaillant returns to most insistently is the powerful correlation between the warmth of your relationships and your health and happiness in old age. After The Atlantic’s 2009 article was published, critics questioned the strength of this correlation. Vaillant revisited the data he had been studying since the 1960s for his book, an experience that further convinced him that what matters most in life are relationships.

For instance, the 58 men 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) than the 31 men who scored lowest; the former were also three times more likely to have achieved professional success worthy of inclusion in Who’s Who.

And, in a conclusion that surely would have pleased Freud, the findings suggest that the warmth of your relationship with Mommy matters long into adulthood. Specifically:

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.

On the other hand, warm childhood relations with fathers correlated with lower rates of adult anxiety, greater enjoyment of vacations, and increased “life satisfaction” at age 75 — whereas the warmth of childhood relationships with mothers had no significant bearing on life satisfaction at 75.

Vaillant’s key takeaway, in his own words: “The seventy-five years and twenty million dollars expended on the Grant Study points … to a straightforward five-word conclusion: ‘Happiness is love. Full stop.’ ”