You want to  be happier . You want to feel more fulfilled . You want to  live a longer, healthier life . 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, carefree, 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. —Jeff Haden This article originally appeared on Fast Company ’s sister website, Inc.com.   Inc. is the voice of the American entrepreneur. We inspire, inform, and document the most fascinating people in business: the risk-takers, the innovators, and the ultra-driven go-getters that represent the most dynamic force in the American economy.