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Back in 1938, researchers started tracking the physical and emotional health of 268 Harvard sophomores and 456 men living in Boston’s inner city in the hopes of unlocking the secret to a long, happy life. What they found was something that anyone’s mother—or The Beatles—could have told them.

For over 75 years, Harvard’s Grant and Glueck study has tracked the physical and emotional well-being of its participants, following along as the men graduated school and went on to become lawyers and businessmen and, in the case of Harvard student John F. Kennedy, president of the United States. Over the many, many years of the research, under the careful watch of generations of researchers, the changing technology of blood sample analysis and brain scans, and thousands of self-reported surveys and interviews with the subjects, the researchers found something out—when it comes to a life of happiness, love is all you need.

According to George Vaillant, the Harvard psychiatrist who directed the study from 1972 to 2004, the secret to a long and happy life is love. Specifically, finding love and making sure not to push love away.

“The surprising finding is that our relationships and how happy we are in our relationships has a powerful influence on our health,” said Robert Waldinger, the fourth director of the study, who is a professor of psychiatry at Harvard Medical School. “Taking care of your body is important, but tending to your relationships is a form of self-care too. That, I think, is the revelation.”

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The researchers found that relationships with friends, and especially spouses, were a major indicator of longevity. “When we gathered together everything we knew about them at age 50, it wasn’t their middle-age cholesterol levels that predicted how they were going to grow old,” said Waldinger in his popular TED Talk titled “What Makes a Good Life? Lessons from the Longest Study on Happiness”. “It was how satisfied they were in their relationships. The people who were the most satisfied in their relationships at age 50 were the healthiest at age 80.”

The health benefits weren’t just for those people in stable marriages or those with the most friends on Facebook or in real life. Instead, according to Waldinger, “It’s the quality of your close relationships that matters.” The people in the strongest relationships whether friends or family or those in between were protected against chronic disease, mental illness, and decline in memory– even if those relationships had many ups and downs or were full of bickering. “Those good relationships, they don’t have to be smooth all the time,” said Waldinger in his TED Talk. “Some of our octogenarian couples could bicker with each other day in and day out, but as long as they felt that they could really count on the other when the going got tough, those arguments didn’t take a toll on their memories.”

Why do quality relationships make people live longer? The study demonstrates that having someone to rely on helps your nervous system relax, helps your brain stay healthier for longer, and reduces both emotional as well as physical pain. It also shows that people who feel lonely are more likely to see their physical health decline earlier and die younger.

While the original study solely looked at men (Harvard was an all-male school at the time) over the years it expanded to include wives and children of both genders. In part of a recent study, researchers found that women who felt securely attached to their partners were less depressed, happier in their relationships, and had better memory functions than those with frequent marital conflicts.

“Over and over in these 75 years,” Waldinger said, “our study has shown that the people who fared the best were the people who leaned into relationships with family, with friends and with community.”

If you already have strong relationships, the other thing that the study makes clear is that you want to age well the second most important thing you can do is avoid smoking. That might be easier than finding love.