In 1938, Harvard researchers embarked on a decades-long study to find out: What makes us happy in life?

The researchers gathered health records from 724 participants from all over the world and asked detailed questions about their lives at two-year intervals.

Contrary to what you might think, it's not career achievement, money, exercise, or a healthy diet. The most consistent finding we've learned through 85 years of study is: Positive relationships keep us happier, healthier, and help us live longer. Period.

The No. 1 key to a happy life: 'Social fitness'

Relationships affect us physically. Ever notice the invigoration you feel when you believe someone has really understood you during a good conversation? Or a lack of sleep during a period of romantic strife?

To make sure your relationships are healthy and balanced, it's important to practice "social fitness."

We tend to think that once we establish friendships and intimate relationships, they will take care of themselves. But our social life is a living system, and it needs exercise.

Social fitness requires taking stock of our relationships, and being honest with ourselves about where we're devoting our time and whether we are tending to the connections that help us thrive.

How to take stock of your relationships

Humans are social creatures. Each of us as individuals cannot provide everything we need for ourselves. We need others to interact with and to help us.

In our relational lives, there are seven keystones of support:

1. Safety and security: Who would you call if you woke up scared in the middle of the night? Who would you turn to in a moment of crisis?
2. Learning and growth: Who encourages you to try new things, to take chances, to pursue your life's goals?
3. Emotional closeness and confiding: Who knows everything (or most things) about you? Who can you call on when you're feeling low and be honest with about how you're feeling?
4. Identity affirmation and shared experience: Is there someone in your life who has shared many experiences with you and who helps you strengthen your sense of who you are?
5. Romantic intimacy: Do you feel satisfied with the amount of romantic intimacy in your life?
6. Help (both informational and practical): Who do you turn to if you need some expertise or help solving a practical problem (e.g., planting a tree, fixing your WiFi connection).
7. Fun and relaxation: Who makes you laugh? Who do you call to see a movie or go on a road trip with who makes you feel connected and at ease?

Below you'll find a table arranged around the seven keystones. The first column is for the relationships you think have the greatest impact on you.

Place a plus (+) symbol in the appropriate columns if a relationship seems to add to that type of support in your life, and a minus (-) symbol if a relationship lacks that type of support.

Remember, it's okay if not all — or even most — relationships offer you all these types of support.

Think of this exercise like an X-ray — a tool that helps you see below the surface of your social universe. Not all of these types of support will feel important to you, but consider which of them do, and ask yourself if you're getting enough support in those areas.

Looking at the gaps on the chart, you might realize that you have plenty of people you have fun with, but no one to confide in. Or maybe you only have one person you go to for help, or that a person you take for granted actually makes you feel safe and secure.

Don't be afraid to reach out to the people in your life. Whether it's a thoughtful question or a moment of devoted attention, it's never too late to deepen the connections that matter to you.

Robert Waldinger, MD, is a professor of psychiatry at Harvard Medical School, director of the Harvard Study of Adult Development, and director of Psychodynamic Therapy at Massachusetts General Hospital. He is a practicing psychiatrist and also a Zen master and author of "The Good Life." Follow Robert on Twitter @robertwaldinger.

Marc Shulz, PhD, is the associate director of the Harvard Study of Adult Development, and a practicing therapist with postdoctoral training in health and clinical psychology at Harvard Medical School. He is also the author of "The Good Life."

Don't miss:

• I'm a psychology expert in Finland, the No. 1 happiest country in the world—here are 3 things we never do
• Here's the No. 1 phrase used in successful relationships, say psychologists who studied 40,000 couples
• A 67-year-old who 'un-retired' shares the biggest retirement challenge 'that no one talks about'

Sign up now: Get smarter about your money and career with our weekly newsletter