The key to a fulfilling life has nothing to do with getting ahead at work, making money, or traveling the world. As Fast Company reports, according to a 75-year Harvard study, living your best life and creating meaning is all about one thing: relationships.

The Grant and Glueck study of adult development has been running at Harvard since 1938, and is now on its second generation of participants—the children of the original study’s subjects. The study is actually made up of two different longitudinal research projects. One, originally called the Grant study, recruited 268 participants from Harvard’s classes of 1939 to 1944. The other, the Glueck study, recruited 456 men growing up in working-class neighborhoods in Boston. Over decades, the Harvard research team collected data about their lives, including their physical and mental health, marital status and quality, career happiness, and more.

They found that the most important factor in how happy and healthy these men were over time was their relationships. In other words: Finding fulfillment in life is all about the people you love.

The Grant and Gluck research doesn’t only encompass romantic partnerships and marriage, though. Quality, close relationships are important whether they are in the context of romantic partnerships or intimacy between friends or family members. The kind of relationship you have is less important than how close you feel with them.

The study has one big caveat: It only included men, and there are notable gender differences in how people experience relationships. Some research has suggested that men may benefit more from marriage than women. As a group, men also tend to have a harder time maintaining friendships; surveys have found that men, particularly as they get older, are more likely than women to say they have no one to discuss important subjects with. So it's possible that having close relationships throughout their lives might affect men differently than women.

However, the findings line up with empirical research on the effects of loneliness, which studies have found can drastically impact your health. People who are socially isolated have a greater likelihood of heart attacks and strokes, higher blood pressure, reduced immunity, and chronic inflammation. That’s not to mention the obvious mental health effects. Loneliness has become an important enough topic in the public health world that Great Britain has appointed a government minister dedicated entirely to the topic.

Unfortunately for the youngest generations among us, recent surveys have found that young Americans are lonelier than older generations. That will likely have a big impact on how healthy and fulfilled people feel throughout their lives.

[h/t Fast Company]