What if there was a study dedicated to unearthing the secrets to a happy and purposeful life? It would have to be conducted over the course of many decades, following the lives of real people from childhood until old age, in order to see how they changed and what they learned. And it would probably be too ambitious for anyone to actually undertake.

Only, a group of Harvard researchers did undertake it, producing a comprehensive, flesh-and-blood picture of some of life’s fundamental questions: how we grow and change, what we value as time goes on, and what is likely to make us happy and fulfilled.

The study, known as the Harvard Grant Study, has some limitations -- it didn’t include women, for starters. Still, it provides an unrivaled glimpse into a subset of humanity, following 268 male Harvard undergraduates from the classes of 1938-1940 (now well into their 90s) for 75 years, collecting data on various aspects of their lives at regular intervals. And the conclusions are universal.

We spoke to George Vaillant, the Harvard psychiatrist who directed the study from 1972 to 2004 and wrote a book about it, in order to revisit the study’s findings. Below, five lessons from the Grant Study to apply to your own pursuit of a happier and more meaningful life.

Love Is Really All That Matters

It may seem obvious, but that doesn’t make it any less true: Love is key to a happy and fulfilling life. As Vaillant puts it, there are two pillars of happiness. "One is love," he writes. "The other is finding a way of coping with life that does not push love away."

Vaillant has said that the study's most important finding is that the only thing that matters in life is relationships. A man could have a successful career, money and good physical health, but without supportive, loving relationships, he wouldn't be happy ("Happiness is only the cart; love is the horse.").

It’s About More than Money and Power

The Grant Study's findings echoed those of other studies -- that acquiring more money and power doesn't correlate to greater happiness. That’s not to say money or traditional career success don’t matter. But they’re small parts of a much larger picture -- and while they may loom large for us in the moment, they diminish in importance when viewed in the context of a full life.

“We found that contentment in the late 70s was not even suggestively associated with parental social class or even the man’s own income," says Vaillant. "In terms of achievement, the only thing that matters is that you be content at your work.”

Regardless of How We Begin Life, We Can All Become Happier

A man named Godfrey Minot Camille went into the Grant study with fairly bleak prospects for life satisfaction: He had the lowest rating for future stability of all the subjects and he had previously attempted suicide. But at the end of his life, he was one of the happiest. Why? As Vaillant explains, “He spent his life searching for love.”

Connection Is Crucial

"Joy is connection,” Vaillant says. "The more areas in your life you can make connection, the better."

The study found strong relationships to be far and away the strongest predictor of life satisfaction. And in terms of career satisfaction, too, feeling connected to one's work was far more important than making money or achieving traditional success.

"The conclusion of the study, not in a medical but in a psychological sense, is that connection is the whole shooting match," says Vaillant.

As life goes on, connections become even more important. The Grant Study provides strong support for the growing body of research that has linked social ties with longevity, lower stress levels and improved overall well-being.

Challenges –- and the Perspective They Give You -- Can Make You Happier

The journey from immaturity to maturity, says Vaillant, is a sort of movement from narcissism to connection, and a big part of this shift has to do with the way we deal with challenges.

Coping mechanisms -- “the capacity to make gold out of shit,” as Vaillant puts it -- have a significant effect on social support and overall well-being. The secret is replacing narcissism, a single-minded focus on one's own emotional oscillations and perceived problems, with mature coping defenses, Vaillant explains, citing Mother Teresa and Beethoven as examples.

“Mother Teresa had a perfectly terrible childhood, and her inner spiritual life was very painful," says Vaillant. "But she had a highly successful life by caring about other people.

Creative expression is another way to productively deal with challenges and achieve meaning and well-being.