The gravitational pull of individual friendships can have an enormous cumulative effect on the quality of our lives. With growing numbers of people living alone, either by choice or circumstance, friendships can occupy the emotional space that other people fill with spouses or significant others. Friends can link us to broader social networks and help enrich our lives. At the end of the day, a friend can be the emotional oasis that makes all the difference.

"Friends are what make us uniquely human," says James Fowler, professor of medical genetics and political science at the University of California at San Diego. "There is no other species that interacts so widely with other members of their species. So right away, you know that when you're studying these relationships with friends, what you're really doing is studying what makes us unique."

After a career studying different types of relationships and their impact on well-being and health, Harvard relationship expert Lisa Berkman has developed a broad view of the relationships people need in order to thrive. There is no optimal mix of friends and family, or of intimate and more casual friendships. "You can substitute these things," she says. "People who have a lot of friends may not need a lot of family ties." Religion and other group interests also can provide tremendous emotional support and human contact that fulfills our need for human companionship and reinforcement.

Solid friendships provide needed validation that a person is valuable and of interest to other people. "Relationships help people feel that they're worthy, that they are capable, that they can set goals and accomplish them, and that they can control their life," says Toni Antonucci, a professor of psychology at the University of Michigan. Antonucci has developed a structure of friendship represented by three concentric circles that she describes as very close, close, and not-so-close but still meaningful personal ties. The rings can play different roles, with strong and emotional ties serving some functions and less-intimate friendships filling other needs.

A person's inner circle usually includes close family members and friends who are thought of as family. Rosemary Blieszner, a specialist in aging and adult development at Virginia Tech, notes how common it is to hear someone say, for example, "My brother and I are so close, we are best friends," or "My girlfriend and I are so close, we're just like sisters."

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Close friendships display strong support and affection, Blieszner says. A close friend fills an invaluable role as a confidant, someone who listens and pays attention to you, is willing to help you, and has shared interests. There is give-and-take and often a balance that doesn't put too much weight on one party.

Women, it turns out, are often better friends to other women and also to men. Women also engage directly in shared activities and derive value by enjoying their friends' experiences. Men, by comparison, do not interact as much and tend to base enjoyment of sporting events and other shared activities on their own experiences. "Men confide in women, and women confide in women," Blieszner explains, "so I think women are perceived as better listeners." She borrows an observation from Paul Wright, a retired psychologist at the University of North Dakota, to describe how friends of the same sex interact differently.

Given the ability of friends to make you happier or sadder, Fowler says, it might be tempting to cull your network of friends to eliminate those who have a negative influence on you. "You might say, well, 'I'll just get rid of all my friends who aren't perfect,' and that's absolutely the wrong advice," he says. According to his research, dropping a bad friend actually raises the risk that your happiness will decline.

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"Every friend makes you healthier; every friend makes you happier," Fowler says. "We're not talking about your 500th friend on Facebook. We're talking about your dearest and closest friends, and these people are hard to find."

Antonucci has a different view. Human relationships have negative as well as positive consequences, and a friendship that has turned toxic is not worth keeping. "Nobody can drive you quite so crazy as someone who is near and important to you," she says. "One of the things people have to do is to learn when to give up on a relationship and how to do it."

The good news about friendships is that they get better with age, says Karen Fingerman, professor of human development and family science at the University of Texas. "It almost doesn't matter what relationship you're talking about. They get better when you get older." Older people are generally more happy and forgiving and less judgmental than younger people. They also are less driven by emotions and hormones and do a better job of controlling their behaviors.

The number and diversity of friendships tend to naturally decline in later years, and can lead to isolation and adverse effects on health and happiness. Psychologist Laura Carstensen, who directs the Stanford Center on Longevity, says people should consider paying attention to the diversity and ages of people in their circle of friends. This can minimize the serious impact of having all your friends die off, she says.

Developing and maintaining friendships requires continuous attention. "Give-and-take is important," Blieszner says. Other elements of solid friendships, she notes, include paying attention to what's going on in a friend's life, seeking out and participating in shared interests and activities, and being able to confide deeply to a friend.

"People should learn to value relationships," Antonucci concludes. "They will make them happier. And with longer life expectancies, they really have to think about the kind of life they want to lead ... I think we underestimate how important it is in our lives to have relationships."

This story comes from U.S. News World Report.

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