So what have we learned? What are the lessons that come from the tens of thousands of pages of information that we’ve generated on these lives? Well, the lessons aren’t about wealth or fame or working harder and harder. The clearest message that we get from this 75-year study is this: Good relationships keep us happier and healthier. Period.

I’ve written extensively on the topic of happiness. The single greatest reason why we do anything, when you boil it all down, is that we think it will make us happy. We think we’ll be happy when we buy that house. We think we’ll be happy when we’re successful or maybe we need to lower our expectations. The pursuit of happiness is both universal and wildly perplexing.

This is where science steps in, if only with a glimmer of remarkable insight. Robert Waldinger is the fourth director of a study that tracked some 724 men over the course of 75 years, asking them about nearly every aspect of their lives. The researchers asked them about their home life, their work life, their health, their spouses and more. Now, they’re following the lives of their children, over 2,000 of them.

And you have to recognize that the “children” of these men are now easily in their 60s and 70s. Only about 60 of the original participants are still alive and most of them are in their 90s. Part of the original cohort consisted of Harvard College students, while the other part came from Boston’s poorest neighborhoods. This all started in 1938 and the study is still ongoing.

So, what does it take to be happy? Not just today and not just tomorrow, but over the span of several decades? When the participants were asked as young men, the overwhelming majority pointed toward becoming rich and successful. Intuitively, we all believe this to be true. If we do well at our jobs and rake in tons of dough, we’ll be happy. Right?

Except that didn’t turn out to be true at all. As we have learned before, after wealth and income attain a certain minimum threshold, more money won’t actually make us any happier. Instead, it’s about relationships… and we’re not just talking about traditional marriage either. It’s about having strong relationships with our friends, our family members, our neighbors, and our colleagues.

Once we had followed our men all the way into their 80s, we wanted to look back at them at midlife and to see if we could predict who was going to grow into a happy, healthy octogenarian and who wasn’t. And 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. 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. And good, close relationships seem to buffer us from some of the slings and arrows of getting old.

As Robert Waldinger and these hundreds of men show us, happiness isn’t good only for our mental well-being. Absolutely, the intrinsic value of being a happier person cannot be understated. At the same time, it turns out that happy people tend to be healthier too. We may not want to jump to any causal conclusions, but when faced with similar ailments and health-related challenges, happier people tend to fare better. And live longer.

You don’t have to sacrifice meaning in your life in order to be happy. You do, however, need to exercise gratitude regularly and that’s a heck of a lot easier to do when you’re surrounded by people you love (and who love you). Even the most introverted of introverts could use a friend.

A friend who just might happen to be a fellow octogenarian.