When people are asked, "What gives meaning to your life?" friendship figures at the top of the list. Yet the dynamics of friendship have remained mysterious and unquantifiable. Like , friendships were thought to "just happen." New research shows that the dance of friendship is nuanced—far more complex than commonly thought. With intriguing accuracy, sociologists and psychologists have delineated the forces that attract and bind friends to each other, beginning with the transition from acquaintanceship to friendship. They've traced the patterns of intimacy that emerge between friends and deduced the once ineffable "something" that elevates a friend to the vaunted status of "best." These interactions are minute but profound; they are the dark matter of friendship.

Entering The Friendship Zone

Years ago researchers conducted a study in which they followed the friendships in a single two-story apartment building. People tended to be friends with the neighbors on their respective floors, although those on the ground floor near the mailboxes and the stairway had friends on both floors. Friendship was least likely between someone on the first floor and someone on the second. As the study suggests, friends are often those who cross paths with regularity; our friends tend to be coworkers, classmates, and people we run into at the gym.

It's no surprise that bonds form between those who interact. Yet the process is more complex: Why do we wind up chatting with one person in our yoga class and not another? The answer might seem self-evident—our friend-in-the-making likes to garden, as do we, or shares our passion for NASCAR or Tex-Mex cooking. She laughs at our jokes, and we laugh at hers. In short, we have things in common.

But there's more: Self-disclosure characterizes the moment when a pair leaves the realm of buddyhood for the rarefied zone of true friendship. "Can I talk to you for a minute?" may well be the very words you say to someone who is about to become a friend.

"The transition from acquaintanceship to friendship is typically characterized by an increase in both the breadth and depth of self-disclosure," asserts University of Winnipeg sociologist Beverley Fehr, author of Friendship Processes. "In the early stages of friendship, this tends to be a gradual, reciprocal process. One person takes the risk of disclosing personal information and then 'tests' whether the other reciprocates."

Reciprocity is key. Years ago, fresh out of film school, I landed my first job, at a literary agency. I became what I thought was friends with another assistant, who worked, as I did, for an infamously bad-tempered agent. We ate lunch together almost every day. Our camaraderie was fierce, like that of soldiers during wartime. Then she found a new job working for a publicist down the street. We still met for lunch once a week. In lieu of complaining about our bosses, I told her about my concerns that I wasn't ready to move in with my boyfriend. She listened politely, but she never divulged anything personal about her own life. Eventually our lunches petered out to once a month, before she drifted out of my life for good. I was eager to tell her my problems, but she wasn't eager to tell me hers. The necessary reciprocity was missing, so our acquaintanceship never tipped over into friendship.

Why Some (And Only Some) Friends Stick

Once a friendship is established through self-disclosure and reciprocity, the glue that binds is intimacy. According to Fehr's research, people in successful same- friendships seem to possess a well-developed, intuitive understanding of the give and take of intimacy. "Those who know what to say in response to another person's self-disclosure are more likely to develop satisfying friendships," she says. Hefty helpings of emotional expressiveness and unconditional support are ingredients here, followed by acceptance, loyalty, and trust. Our friends are there for us through thick and thin, but rarely cross the line: A friend with too many opinions about our wardrobe, our partner, or our taste in movies and art may not be a friend for long.

When someone embodies the rules—instinctually—their friendships are abundant indeed. Kathy is one of my oldest friends; we were roommates in graduate school and have been through cross-country moves, divorces, deaths, and births together. Her ability to be a friend shines during a lousy breakup. She knows when to listen and make sympathetic sounds, when to act good and outraged at your ex's bad behavior, when to give you a hug, and when to tell you to stop obsessing and enjoy a glass of wine. She knows when to offer you her couch. It's this responsiveness that accounts for her having more friends than anyone I know—certainly more than the five our mothers told us we were lucky to be able to count on one hand over the course of a lifetime.

Compared to these emotional gifts, a friend's utility paled, Fehr found in her study. Study participants judged as peripheral the ability of a friend to offer practical help in the form of, say, lending 20 bucks or allowing use of a car. This fact often turns up as a truism in movies, where the obnoxious, rich kid can't understand why always picking up the tab never makes him popular. Money really can't buy love.

If anything, it's giving and not receiving that makes us value a friend more. It was the American statesman and inventor Ben Franklin who first observed the paradox, now called the Ben Franklin Effect: "He that has once done you a kindness will be more ready to do you another than he whom you yourself have obliged." In a nutshell, while material favors don't even come close to the emotional talents of our friends, we still want to validate our personal judgment by investing special qualities in those we select to help.

In one classic study, participants won "contest money" from a researcher. Later the researcher approached some of them and explained he'd actually used his own money and had little left; could he have the money back? Most agreed. Later, the researchers found, those asked to do the favor rated the researcher more favorably than those not approached. Psychologists concur that the phenomenon stems from a desire to reconcile feeling and action, and to view our instincts and investments as correct: "Why am I going out of my way to help this guy? Well, he must be pretty nice." The fondness we feel toward our yoga class buddy will continue to grow if one day she asks for a ride home and we go out of our way to give it to her.

Mirror, Mirror: The Truth About Best Friends

If closeness forms the basis of friendship, it stands to reason that your best friend would be someone with whom you enjoy supersized intimacy. If I confide that money is tight or my boyfriend's in the doghouse I might detail the money worries or give a blow-by-blow of the dramathon that led to the boyfriend's banishment. We have with our best friends a "beyond-the-call-of-duty" expectation. If we suffer an emergency—real or imagined—and need to talk, we expect our best friend to drop everything and race to our side.

But according to social psychologists Carolyn Weisz and Lisa F. Wood at the University of Puget Sound, in Tacoma, Washington, there's another component to best friendship that may trump even intimacy: social- support, the way in which a friend understands, and then supports, our sense of self in society or the group. If we view ourselves as a mother first and a belly dancer only on Saturday mornings at the local dance studio, our best friend is likely to be another mom because she supports our primary social-identity (as opposed to our personal identity as, say, someone who loves film noir or comes from the Bronx). Our social-identity might relate to our religion, our ethnic group, our social role, or even membership in a special club.

Weisz and Wood showed the importance of social identity support by following a group of college students from freshman through senior year. Over that period, the students were asked to describe levels of closeness, contact, general supportiveness, and social identity support with same-sex friends.

The results were revealing. Overall closeness, contact, and supportiveness predicted whether a good friendship was maintained. But when the researchers controlled for these qualities, only a single factor—social-identity support—predicted whether a friend would ultimately be elevated to the position of "best." Best friends often were part of the same crowd—the same fraternity, say, or tennis team. But Weisz and Wood found that friends offering such support could also be outside the group. Sometimes all a friend needed to do to keep the best friendship going was to affirm the other person's identity as a member of the given group ("You're a real Christian") or even the status of the group itself ("It's so cool that you play sax for the Stanford band!"). Reasons for the finding, say the researchers, may range from greater levels of intimacy and understanding to assistance with pragmatic needs to enhanced .

We become best friends with people who boost our self-esteem by affirming our identities as members of certain groups, and it's the same for both genders. Men who derive their most cherished identity through their role as high school quarterback, for instance, are most likely to call a former fellow teammate "best friend."

Our desire for identity support is so strong, Weisz found, that it may even make a difference for the addicted. In another study, she found people with problems were likelier to kick their habits after three months when they had felt more conflict between drug use and their social roles and sense of self. Those who felt socially in sync with the drug use were less likely to become substance-free. Indeed, our social identities are so important to us that we're willing to court disaster to preserve them. We stick with people who support our social identity and withdraw from those who don't. We may even switch friends when the original ones don't support our current view of ourselves.

Most of us would prefer to think that we love our friends because of who they are, not because of the ways in which they support who we are. It sounds vaguely , and yet the studies bear it out.

A corollary for many people is the impetus to change best friends when life throws us a curveball or alters us in basic ways. There's no better example than former members of breast cancer support groups whose diseases have been cured. Though the women no longer have breast cancer and have continued with family and careers, their social identity as survivors often remains so powerful that their primary bonds of friendship are with other survivors, the only people who can understand what they've been through and grasp their perspective on life. After such major life events as , , and divorce, we may easily switch up our best friend as well.

Built To Last: How To Stay Friends

From young adulthood onward, our notion of what makes a good friendship changes very little, but our capacity to maintain one does. It's a poignant reality; we know what it means to be and have friends, but after we graduate from college and go our separate ways—launching our careers, getting married, having children, getting , caring for aging parents—we're often unable to muster the time and energy to maintain friendships we profess to value. Like anything else in life, if we want to remain friends with someone, it requires a little work. Simply put, we must show up.

According to Marquette University psychologist Debra Oswald, who has studied the nature and complexity of high school "best" friendships, there are four basic behaviors necessary to maintain the bond. And they hold true whether we're 17 or 70.

Communication facilitates the first two essential behaviors: self-disclosure and supportiveness, both necessary for intimacy. We must be willing to extend ourselves, to share our lives with our friends, to keep them abreast of what's going on with us. Likewise, we need to listen to them and offer support.

Fortunately, studies show that physical proximity has little effect on the ability to keep a friendship in working order. Moving to another state is not the friendship death knell it once was, thanks to the Web. Between e-mail and cell phones with free long distance, we're able to stay close. Maintaining a lively e-mail correspondence may often be as good as being there.

Interaction is the third essential in tending to a friendship. You've got to write, you've got to call, you've got to visit. Find the nearest Starbucks and take time to catch up. "The specific activity doesn't matter," says Oswald. "The important thing is to interact."

The last and most elusive behavior necessary for keeping friends is being positive. Social psychologists tout the necessity of self-disclosure, but that doesn't mean an unrestricted license to vent. At the end of the day, the intimacy that makes a friendship thrive must be an enjoyable one, for the more rewarding a friendship, the more we feel good about it, the more we're willing to expend the energy it takes to keep it alive.

Eventually, my best friend, Olivia, went back to school to earn her master's in social work. She now has a full caseload of patients that consumes her time. Even though she lives within bicycling distance, we've resorted to e-mailing and talking on our cell phones whenever we take our daily walks, she in her neighborhood, me in mine, at whatever hour we can fit it in. And we do, always, make time to fit it in, which is how we stay good friends.