The secret to a happy life? It's not what you think.

What's the secret to a happy life? It's not wealth, fame, or even your preferred presidential candidate's standing in the polls. The single most important factor for a good life, according to a nearly 80-year-long study from Harvard, is good relationships.

"Good relationships keep us happier and healthier. Period," said Robert Waldinger, a clinical professor of psychiatry at Harvard Medical School, in a recent TED Talk. 

Dr. Waldinger is the current director of the Harvard Study of Adult Development, a research project that has tracked the lives of more than 700 men to explore which factors might determine whether a person is likely to age happily and healthily, or not.

Since 1938, researchers at Harvard have studied the lives of two different groups of men in Boston: 268 Harvard undergraduates, and 456 young men from some of Boston's poorest neighborhoods. Over the course of 78 years, researchers have collected data on various aspects of the study participants' lives at regular intervals. The men, the survivors of whom are now in their 90s, came from all walks of life. Some went on to become doctors, lawyers, and even a US president. Others succumbed to alcoholism and other illnesses.

The study's findings were both expected and surprising. The best thing one can do for physical health is to avoid smoking, according to a recent New York Times analysis of the study, which also found that alcohol was the primary cause of divorce among men in the study.

But the study also found that the single most important indicator of long term health and happiness is the strength of a person's relationships with family, friends, and especially spouses.

At midlife, the best predictor of who was going to grow old healthfully wasn't cholesterol levels, Waldinger explained. It was how satisfied they were in relationships. "The people who were most satisfied in relationships at 50 were the healthiest at 80," he said.

The quality, not quantity, of close relationships also matters. Living in conflict-ridden relationships, for example, is bad for health, according to the study, while living in good, warm relationships is protective.

And it turns out good relationships don't just protect our bodies, they protect our minds. If a person is in a relationship in which they feel they can count on their partner, their thinking stays sharper later into life, said Waldinger.

Of course that doesn't mean relationships need to be perfect to be protective.

“Those good relationships don’t have to be smooth all the time,” he explained. “Some of our octogenarian couples could bicker 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.”

In fact, the Harvard study's findings are nothing new. A 2005 study of longevity around the world found some of the cultures that enjoyed the longest, healthiest lives were those which emphasized family ties and social support.

An Islamic tradition holds that those who keep good relations with family will enjoy extended provisions and life span. Similar teachings also exist in Native American, Chinese, and Aboriginal cultures.

So how do Americans apply these teachings to modern life?

“The possibilities are endless,” Waldinger said in his Ted talk. “Something as simple as replacing screen time with people time, or livening up a stale relationship by doing something new together, long walks or date nights. Reach out to that family member you haven’t spoken to in years."