Enter Lewis Terman with an answer.
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.
Then 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 I did in my 20s actually made me happy in my 70s unless you catch me at both stages of life.)
So who tended to live the longest, most fulfilling lives? The 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.
According to The Longevity Project, actually achieving your lifelong dreams doesn't matter. Pursuing those 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 definitely 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 don't thrive.
Persistent, conscientious people thrive.
Of course, other things matter as well. Other research shows good relationships make you happier and healthier. Terman's study shows that 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. 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 that truly matters.
Make sure you're delighted with the results of that study.