Happiness can be hard to quantify, because it can mean something different to everyone. But let's say you could change one thing in your life to become a happier person, like your income, a job, your relationships or your health. What would make the biggest difference?

That's the question that Dr. Robert Waldinger has been investigating for decades as the director of the world's longest-running scientific study of happiness. Waldinger says it began as a study of what makes people "thrive."

"We've spent so much time studying what goes wrong in life. And so, this was a study of how people take good paths as they go through life," said Waldinger.

The study followed people through the decades, consulting with their parents and now their children, who are mostly of the baby boomer generation. And Waldinger notes that there are different kinds of happiness.

"We do like that sugar rush high, that 'I'm having fun right now at this party' kind of high. And then there's the happiness that comes from feeling like, 'I'm having a good life, a decent life, a meaningful life," Waldinger explained. "We all want some of both, but some of us really prioritize one kind over the other kind."

So, if people could change one thing in their lives to be happier, what does the data say they should choose?

"They should invest in their relationships with other people."

His study has shown that the strongest predictors for people to maintain their happiness and health throughout the course of their lives were people who described their relationships as having satisfying levels of quality and warmth. And that applies to a wide breadth of interactions in your daily life, from spouses, close friends and colleagues to the barista who makes your morning coffee or the person delivering your mail.

"We get little hits of well-being in all these different kinds of relationships," Waldinger added.

He points to relationships acting as stress regulators in our everyday lives. Chronic stress is linked to a variety of negative health impacts, and can take a toll on people's physical and mental health. Having an effective outlet, like a good friend to rant to after a long day, can help alleviate that pressure. You don't have to be an extrovert to reap those social benefits, either. Waldinger says as long as you feel comfortable and connected, your relationships are benefitting you in many ways.

Waldinger emphasizes the importance of putting effort into friendships, saying that many valuable relationships can wither away from neglect. And even if you find yourself realizing that you may not have the connections you seek, today's as good a day as any to start forming those bonds.

"You know, we've tracked these lives for eight decades. And the wonderful thing about following these life stories is we learn it's never too late," he added. "There were people who thought they were never going to have good relationships, and then found a whole collection of good close friends in their 60s or 70s. There were people who found romance for the first time in their 80s. And so the message that we get from studying these thousands of lives is that it is never too late."

So if you've been prioritizing your well-being lately, and perhaps meaning to reach out to a friend, family member or loved one, it's never too late to send a quick message and catch up.

This article was adapted for the web by Manuela Lopez Restrepo.