You want to be happier. You want to feel more fulfilled. You want to live longer, healthier lives.

Hold that thought.

Lewis Terman, a Stanford University psychologist, was a pioneer in I.Q. testing. His revisions of the Stanford-Binet test helped it become a widespread tool for measuring general intelligence.

In 1921 he identified 1,500 children who had scored 135 or higher on the test and began one of the longest longitudinal studies ever conducted. (The New York Times calls Terman and his study of “Termites,” as the kids called themselves, the “grandfather of all lifespan research.”)

Terman’s study was guaranteed to outlive him, but that was the point: analyzing large groups of people over many decades allows researchers to uncover connections between cause and effect that short-term studies naturally miss. (It’s really hard to know if what you did in your 20s actually made you happy in your 40s and 70s unless the researchers catch you at all three stages of your life.)

Who tended to live the longest, most fulfilling lives?

People who actively pursued, and were highly engaged, in pursuing their goals. In fact, many of those who worked the hardest turned out to live the longest.

Even if they didn’t actually accomplish their goals. According to The Longevity Project, achieving lifelong dreams doesn’t matter. According to the authors, pursuing your dreams is what counts:

We did not find that precisely living out your dreams matters much for your health. It was not the happiest or the most relaxed older participants who lived the longest. It was those who were most engaged in pursuing their goals.

Those who were the most successful were the ones least likely to die at any given age. In fact, those men who were carefree, undependable, and unambitious in childhood and very unsuccessful in their careers had a whopping increase in their mortality risk.

Of course “success” means (and absolutely should mean) different things to different people.

That’s why determining what success means to you, and then actively working to achieve your definition of success, is the key. Living a laid-back, care-free, stress-free life may sound great… but, as the study shows, “happy-go-lucky” people tend not to thrive.

Persistent, conscientious, goal-oriented people thrive — again, even if they don’t always achieve their goals.

Of course, other things matter as well. Other research shows good relationships make you happier and healthier: Terman’s study shows kids who have greater willpower and perseverance tend to be more successful as adults, regardless of relative I.Q.

It’s not easy to change the quality of your relationships overnight, though. Nor is it easy to develop greater willpower and determination (although there are certainly ways you can increase your ability to resist temptation, stay focused and determined, and remain resolute in pursuit of your goals.)

But what you can do, starting today, is actively work toward achieving one of your goals. (A great double-dip goal would be to try to improve the quality of your relationships.)

Working toward a goal will make you happier. Working hard to achieve a goal will help you live longer. Actively pursuing a goal, even if you never quite achieve it, will make your life more fulfilling, both now and when you eventually look back on a life well-lived.

Because there’s only one longitudinal study of happiness that truly matters: yours.