Illustration by David Cutler

Part of that plasticity relates to responding to those around us. "We all have the experience of being comforted by someone else's smile. And once you start to smile, you start to feel better. You will have associated reduced stress and reduced heart rate. We've evolved some really sophisticated brain systems for picking up on other people's emotions."

Pelphrey mentions studies documenting that, neurologically, feelings arising from observed events are communicable. That observation-feeling nexus activates a particular area of the brain. And when another person observes the result—for example, a facial expression suggesting pleasure or pain—the same area of the brain is activated in the observer. So when we watch someone, say, pick up an object on a table, the same part of our brain will fire up as that of the person performing the action. In some neurological sense, then, we are mirroring the thought patterns of those in our company.

Over time, Pelphrey says, "there is a kind of melding" among individuals in a tight network, "and you become more and more alike." We all know that confidants can finish each others' sentences; in conversation, they imitate each others' mannerisms on a subconscious level. "Neuroscientists are just starting to understand how you put yourself in someone else's shoes."

If we're walking alone more than we're putting ourselves in others' shoes, that hurts society overall, says Harvard public-policy professor Robert Putnam, who calls the new Duke-based study convincing—particularly, he notes, because the study's authors were initially skeptical of his findings. His popular book, Bowling Alone, documented a decline in informal social activities like dinner parties and, of course, bowling leagues. In the book, he argues that the fabric of American communities has frayed badly. The new study, he says, has implications for individuals; for example, social isolation is as big a risk factor for premature death as smoking. It has larger implications as well. "Schools don't work as well where parents and community members aren't as involved in community life. The crime rate is higher in communities where people are more socially isolated. The economy doesn't work as well, because there is less trust, and so productivity growth is low. The political culture doesn't work as well. Bureaucracies don't work as well."

One thing that is working well—if relentlessness is an indicator of working well—is communication by e-mail. But ease of communication doesn't constitute depth of friendship, according to Putnam. "If you get focused on the contrast between purely virtual and purely face-to-face connections, you miss what I think is the most interesting thing, which is what I call 'alloys.' These are networks that are partly virtual and partly real. E-mail is a perfect example. Very few people engage in e-mail with total strangers; the huge majority of e-mail is with people who we actually also know offline."

Facebook.com, a popular social-networking site, was invented by the roommate of one of Putnam's former students. With more than 12 million registered users, Facebook began as a "social utility" for college students. Now it's made up of multiple networks—high schools, companies, and regions among them—each of which is independent and closed off to non-affiliated users. Putnam has had a profile on the site since its beginnings. "As it has become detached from real places—that is, as it has become no longer campus-based and therefore more anonymous in a way—I have become more skeptical about whether it is serving the purpose that it used to serve," he says.

"Now I get requests every day to be someone's friend. And it's crazy, because these aren't my friends; it's a complete abuse of the term 'friend,' a complete abuse to think that somebody who was assigned my book in some freshman course thinks it would be neat to say, 'I'm a friend of Bob Putnam.' What is the real meaning of that? Will they bring me chicken soup if I get sick?"

They may hold little promise for delivering chicken soup, but relationships across cyberspace have redefined the meaning of friendship, according to Nan Lin, a professor of cultural anthropology at Duke. Cyberspace has "enlarged our sense of community to an extent that's unprecedented," he says. When blogs and online networks blur the lines between private and public information, it may not seem so important to have a small network of close confidants. At the same time, if friendships are defined in terms of the hundreds of people tied together in an online network, it's impossible to talk about intimate matters with all of them.

Friendships, Lin says, never develop merely as expressions of admiration or affection; they are also instrumental or purposeful. "In the past, we've tended to argue that friendship is important, that it promotes our sense of stability and our feelings of belongingness," Lin says. "What's happened is that we see people connecting with others we would consider familiar strangers; we're willing to share and interact with people about whom we have very little knowledge." Membership in an online community can be an avenue for receiving support, exerting influence, or inspiring individual activities toward a larger goal—all benefits of friendship.

But that doesn't mean cyberspace is the best space for friendship. Even in an era of online connectivity, "We want instantaneous feedback, one-on-one and face-to-face, not just through text messages," Lin says. "We want to see faces and gestures, to hear the tone of someone's voice. It's comforting to have that."

A Chronicle columnist who has written about the Facebook phenomenon, senior James Zou, refers to real-life interactions as "friendship capital." He says, "The more special the shared experience is, the more capital you get. Obviously the capital diminishes through time unless there is a new experience between the two of you. And I think the capital diminishes fast if you have just these mundane, once-a-week, instant-messenger interactions or Facebook interactions. When you're sharing a space with someone, you're replenishing your stock of friendship capital."

"Some of my close friends—my actual, physical friends on campus—have massive social networks on places like Facebook," Zou says. "Maybe 400, 500 friends." To what end, then, do students keep adding virtual friends? Social status is one reason, Zou says. "I mean, it looks nice that you have a thousand friends, whereas someone else has 200 friends. There's definitely a kind of competitive urge to have as many friends as possible, especially when it's so easy to make friends online, as opposed to making friends in real life."

One avid communicator who is less enthusiastic about constant connectivity is Sam Wells, dean of Duke Chapel. "The increasing number of ways that we communicate with one another probably means that we communicate less rather than more," he says. In his new book God's Companions, he refers to friendship as steeped in "the simple sharing of life"—the sharing that sustains religious belief. Friendship "offers a bridge between the somewhat lonely pursuit of a personal vocation and the somewhat self-denying participation in a community's common life," he writes. "In short, a friendship may offer a more intimate, focused, rewarding experience of what pursuit of a personal vocation or a participation in community may offer in a more challenging or less intense way."

"I see friendship as something where I say to you, 'I am going to be changed by knowing you,' " says Wells. Friendship has been a frequent theme in his sermons and his writing. "One of the early theologians of the church says, 'The glory of God is a human being fully alive.' That means to be fully alive isn't just to stand independently on your own two feet and not need anybody else. It's to be involved in life-giving relationships with all different kinds of people."

A friend is there in part "to keep you true to your vocation," Wells says. "I remember one very frustrating point in my life when I was quite miserable doing the job that I was doing. It was an ordained role, and I felt trapped. And one friend of mine just said, 'Where's your faith?' That was a real experience of friendship for me. It was a real jolt. I was just moaning, really, and he was saying, 'But what about the whole frame of reference that this conversation is a part of?' It was very challenging of him to say that. Most people were just saying, 'Okay, Sam, I hope you feel better tomorrow.' "

Wells says that students, reflecting as they do a fast-paced communications culture, may harbor a cheapened concept of friendship—reducing it to calling someone on a cell phone to ask, "Hey, what're you doing?" He says, "If I were to coin a phrase that summed up many students' approach to friendship, it would be, 'Might catch you later.' Most cell-phone conversations end with those words. That is, 'I'm not committing my evening to you; I might get a better offer. But if I don't get a better offer, I may be back in touch, because you may be part of my evening's entertainment.' Everything becomes provisional."

"We have this phrase, 'keeping in touch,'" Wells adds. "When we say 'keeping in touch,' we mean cell phones, possibly even a letter or an e-mail, none of which actually involves touching anybody. One of the reasons I talk about sharing meals with people as being so important is that in sharing meals, you do touch one another, or at least you touch the table together. The committed physicality of staying in the same place with somebody else for an hour it takes to eat the meal, rather than eating on the run and talking to someone on the cell phone at the same time, creates a different kind of relationship."