The 1 thing you should do to strengthen any relationship, according to a Harvard expert on happiness

• A Harvard study has been examining what makes people happy for three generations.
• Good relationships are key — they not only make us happier, they also protect our health.

Dr. Robert Waldinger has spent years researching one of life's most important questions: what actually makes people happy?

The psychiatrist and professor is the fourth director of the 85-year old Harvard Study of Adult Development, a research project that spans across three generations and includes 2,024 participants.

Over decades of painstakingly detailed data collection and analysis gathered year after year, Waldinger and his colleagues have arrived at one sharp, simple, scientific conclusion.

"Good relationships keep us happier, healthier, and help us live longer," Waldinger and his co-author, associate study director Marc Schulz, write in their new book "The Good Life: Lessons from the world's longest scientific study of happiness."

"This is true across the lifespan, and across cultures and contexts, which means it is almost certainly true for you, and for nearly every human being who has ever lived," they wrote, referencing other studies like their own that have examined a wide variety of diverse populations around the world, and gathered similar results.

Their book is filled with the stories of people who've spent many years as the living data points leading to this humble truth. A lawyer named John who was "one of the more professionally successful members of the Study," they wrote, "was also the least happy," while Leo, a high school teacher who never pursued his dream of becoming a writer, "is generally considered to be one of the Study's happiest men."

A simple way to strengthen relationships

Because Waldinger knows how much our relationships matter, he is deliberate about cultivating them as often as he can — both for himself, and for others.

"Sometimes when I give talks I'll say, 'okay, now everybody take out your phone, think about somebody you want to connect with, and send a text,'" he told Insider.

He suggests prompts to his audiences, like: "Thinking of you and wanted to say hi," or "just wanted to say hi and reconnect." You can also ask the person if they'd like meet up, maybe for coffee, a walk, or a beer.

Often, he said, people are surprised at how quickly and eagerly their recipients respond. Waldinger remembers one woman who said her friend texted back "immediately, saying 'he was so glad I'd reached out.'"

"He'd just had surgery and was really grateful to hear from me," the woman said.

Waldinger offers another similar, but more intimate, step-by-step exercise that is meant to stimulate and deepen relationships of all kinds — even our ties to people we're already close to — in the very last lines of the book:

• Think of one person who is important to you.
• It could be your significant other, a friend, a coworker, or even a coach or a teacher from your youth.
• This person could be sitting beside you — or halfway across the world.
• Next, the book suggests, "Think about where they stand in their lives. What are they struggling with? Think about what they mean to you, what they have done for you in your life. Where would you be without them? Who would you be?"
• What would you want to thank them for if you were never going to see them again?
• And finally, "at this moment — right now — turn to them. Call them. Tell them."