Want to be truly happy? Here's the only thing Harvard researchers say you absolutely need

It isn't money, success, or power. It's something more basic and rewarding.

In just a second, we'll share the research-backed secret of happiness. But first, even though it's March, let's think about an old Christmas movie: It's a Wonderful Life.

You'll remember Jimmy Stewart's character, George Bailey--and you might even remember the book inscription at the very end that sums up the movie's theme:

Here's a clip:

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Bonus content: 9 Things Great Leaders Say Every Day (free infographic).

So why are we talking about a Christmas movie when it's just shy of St. Patrick's Day?

Because it sums up perfectly one of the key findings of a 75-year (and counting) study of human development at Harvard University called the Grant Study:

That's Dr. Robert Waldinger, the Harvard psychiatrist who has been running the Grant Study since 2003 (the fourth person in charge in its history). The study, you might know, followed the lives of graduates of Harvard University beginning in 1938, including men like (they were all men) future President John F. Kennedy, and Ben Bradlee, who would go on to become editor of The Washington Post during Watergate.

As of the 1970s, the study was paired with another research program that had followed men from inner-city Boston, beginning in the 1940s. (H/T to Colby Itkowitz of The Washington Post, whose article got me thinking about this.)

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About 60 of the 724 men who began the program are still alive. They're all in their 90s now. Most served in World War II, all were white. Some achieved great success like Kennedy and Bradlee; others failed to live up to their potential.

And while there were many aspects of the men's lives that were striking, Waldinger says there are three key findings about relationships that predicted how happy and healthy the men were as they grew older.

1. Loneliness kills.

"People who are more isolated than they want to be from others find that they are less happy, their health declines earlier in midlife, their brain functioning declines sooner and they live shorter lives than people who are not lonely. And the sad fact is that at any given time, more than one in five Americans will report that they're lonely," Waldinger said.

2. Quality of relationships matters more than quantity.

"It turns out that living in the midst of conflict is really bad for our health," Waldinger said. "High-conflict marriages, for example, without much affection, turn out to be very bad for our health, perhaps worse than getting divorced. And living in the midst of good, warm relationships is protective.

3. Good relationships protects the brain.

"People who are in relationships where they really feel they can count on the other person in times of need, those people's memories stay sharper longer," Waldinger said. "And the people in relationships where they feel they really can't count on the other one, those are the people who experience earlier memory decline."

So what are the takeaways?

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Well, Waldinger suggested, instead of just "leaning in" when it comes to professional life, it might involve leaning in to relationships.

You can see Waldinger's TED talk (viewed more than 6.5 million times) here. Also, don't forget about the free bonus content, 9 Things Great Leaders Say Every Day (free infographic).

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