What makes a happy, healthy life?

How can you live your best life? One of the world's longest-running health studies seeks to find the answer.

In 1938, the Harvard Study of Adult Development began following two groups of young men - some from inner-city Boston and others who studied at Harvard, including President John F. Kennedy and Washington Post editor Ben Bradlee. What began as a study about adult development transformed into a 75-year examination of what keeps men happy and healthy.

"What we realized was that we had an amazing resource - we could look at their health, relationships their work lives - and all of that added up show how happy they were in their lives," said Dr. Robert Waldinger, the fourth director of the study. "And so we started looking at the whole package."

The researchers followed 724 men, conducting questionnaires every two years, interviews at different intervals and collecting health information every five years. They looked at everything from their personalities, to drinking habits, and even skull shape and size.

"In the 1930s, they thought some of those things made a big difference in who you were, what your personality was like, what your happiness levels were," Waldinger explained. "Turns out they don't make much of a difference and now we study very different things."

What did make all the difference were the men's relationships with others.

"Our men found that good, close relationships predicted not only that they would stay happier, but that they would stay physically healthier," Waldinger said.

The study found men who are lonely were less happy, experienced an earlier decline in health and brain function and lived shorter lives. The health benefits of affectionate relationships with family, friends and community were "as important" as avoiding cigarettes and excessive alcohol.

Men's relationships with their mothers and siblings were particularly important. Men who had warmer relationships with their mothers had more successful marriages, and having at least one close relationship with a sibling made it less likely that you'd be depressed later in life.

"We spend a lot more time with our siblings than most of us think about, and those are some of the earliest training grounds for how we are in relationships with each other," Waldinger explained.

Waldinger - who is also a Zen Buddhist priest - said giving people your "full, undivided attention is probably the most valuable thing you have to offer" and key to being happy and healthy.

"Simply watch what you are doing each day and who you are with and (see) if you can pay more and more careful attention to the people you're with," Johnson advised. "Put aside all your preconceptions and just be there with somebody. It'll make a huge difference."

The study is still not over. The researchers continue to track the 60 surviving members. They will now also look at the almost 2,000 children of all the men they have followed, to see what their childhoods were like and how that predicts their aging.

"If you grew up in a particular stressful environment, your health breaks down sooner. So what we are going to be able to do is figure out how that works and whether there are particular ways that we can help people who are struggling with difficult childhoods."