If you want to study happiness, you first have to define what you mean by the word.

There are the momentary pleasures of a great glass of wine, a smile on a child's face, or a soft couch after a long day. But there are also deeper meanings of the word. The ancient Greek word for happiness, eudemonia, conveys not fleeting good feelings but rather the sum total of a life well lived, including a sense of meaning and virtue.

To figure out how to best achieve this higher form of happiness, you can't ask what cheers people up or brings them down, day to day. You have to follow them over decades to determine what really makes them flourish and feel fulfilled as they approach the end of their days.

Which is a tall order for a study, but amazingly, this research has actually been done. Starting in 1939, a team of scientists began tracking 268 male Harvard students, gathering extensive data about their mental and physical health every year up to the present day. It's the largest study of its kind ever done, and despite its limitations (all male, all Harvard, all white), it provides some of the strongest evidence yet of what constitutes a truly happy life.

The main takeaway of the research isn't hard to summarize. The study's original director, George Valliant, summed it up succinctly in all of five words: "Happiness is love. Full stop."

Short, sweet, and perhaps the most important lesson a human being will ever learn (or fail to learn), not much can top this conclusion for pithy good sense. But that doesn't mean it's the only useful takeaway of the research. A TED Ideas post recently dug deeper into the findings, uncovering some other big lessons of the decades-long research.

Several of these concern the value of a happy childhood and will mostly be of interest to parents or those trying to come to terms with a less-than-blissful start to life (the study shows a traumatic childhood doesn't necessarily doom you to unhappiness), but several other conclusions are helpful to pretty much anyone trying to make their way through life with dignity and good cheer.

1. Learn how to cope with stress.

No one can avoid life's slings and arrows, but you do get to choose how you deal with them. The study found there are more or less effective strategies for coping with stress.

"Among the adaptive coping methods they examined are sublimation (example: you feel unfairly treated by your employer, so you start an organization that helps protect workers' rights), altruism (you struggle with addiction and help stay sober by being a sponsor for other addicts), and suppression (you're worried about job cuts at your company but put those worries out of mind until you can do something to plan for the future). Maladaptive coping strategies include denial, acting out, or projection," notes the post.

Why are these adapative methods so much more effective? It all goes back to the central takeaway of the study -- that relationships are all important. "Subjects who dealt with stress by engaging in adaptive methods had better relationships with other people," which "had a cascade of beneficial effects," claims the post.

2. The best cure for the blues is other people.

See a theme emerging? Yup, here's the same conclusion viewed from a slightly different angle. When the scientists asked what was the best way to weather life's rougher moments, they found the same answer yet again -- relationships are all important.

"Looking back on their lives, people most often reported their time spent with others as most meaningful, and the part of their lives of which they were the proudest. Spending time with other people made study subjects happier on a day-to-day basis, and in particular, time with a partner or spouse seemed to buffer them against the mood dips that come with aging's physical pains and illnesses," they concluded.

Given these results, maybe it's time you reconsider how you measure success, to give more weight to the quality of your relationships and less to worldly accomplishments.