I have no friends. At least, that's what it feels like. To be accurate, when I say "no," I really mean "a paltry few." And though life is often so hectic that I forget how unpopular I am, little incidents occur that tend to give me a jarring reminder.

Take a morning a few weeks ago, when I tried to make small talk with another mother while waiting to pick up my kids at school. "Yes, it is unseasonably warm," she replied tersely to my comment about the weather. The conversation stopped there. I figured she was socially impaired. A few minutes later, more people arrived. She ditched me and quickly became the center of a chatty clique. She was a talker, only not with me. Ouch.

Hours later, I got great news about a project I'd been slaving over. Thrilled, I called my husband, then my parents. I started to dial a friend next but realized she wasn't around, and I couldn't think of anyone else with whom I felt comfortable confiding the vulgar financial details. I went from elation to depression in seconds.

Reach out and thrive

You've likely heard how important friendships are to your health, happiness and longevity: Back in 1988, a landmark study at the University of Michigan at Ann Arbor found that people who lead isolated lives have an increased risk for mortality that's on par with cigarette smokers'. Scores of more recent studies have also shown that lonely folks are more likely to develop dementia, get high blood pressure, sleep poorly and have weakened immunity. People who feel connected, on the other hand, "are simply healthier—they have better cardiovascular health and report a higher level of life satisfaction," says Shelly Gable, Ph.D., professor of social psychology at the University of California at Santa Barbara. "The happiest 10 percent of self-described happy people all say they have positive, close relationships."

Indeed, every psychologist I interviewed rhapsodized about the benefits—physiological and otherwise—of strong social connections. "Friendships give us access to unconditional support as well as practical help in times of stress," says Linda Sapadin, Ph.D., a psychologist and author of Now I Get It!: Totally Sensational Advice for Living and Loving (Outskirts Press). "They also expand our lives and let us take on alternate roles apart from coworker, mom or spouse, giving us the chance to grow."

In college, I had scores of friends. "When you're in school, you're involved with people your own age, in the same setting, with similar points of reference. It's easy to befriend just about anyone," Sapadin explains. As an undergraduate, I was mainly friends with English majors, Deadheads and the student-newspaper crowd. Not surprisingly, I was an English major and a Deadhead, and I wrote for the student newspaper. When I left school, I stopped being those things and within three months, 90 percent of my college friendships were on the brink of extinction. Instead, I roamed New York City with new friends I'd met at my job in publishing, going to parties, concerts, restaurants, bars and clubs. My phone rang constantly with invitations, which I always accepted. I had wing women for bar crawling and guy friends to console me when my latest relationship went bust. I had friends who were always available for a movie or takeout at home, friends who were great for shopping, friends who always seemed to have scads of entertaining gossip. Whatever my social need, I had a friend (or several) to meet it.

The incredible shrinking circle

That was then. Now about the only time I feel as if my friendship cup runneth over is on Facebook, where, at last count, I had 317 "friends." Yet unlike online friendships, those in real life require more than a clever status update to maintain. Time must be spent. And therein lies the problem. My social network started contracting in my mid-20s, when I started dating someone seriously. Then one by one, my single friends paired off themselves, and we seemed to fade from one another's lives.

Things got even worse after Glenn and I married and had kids. We were among the first in our crowd to become parents, and when our daughter Maggie was born, we noticed, with sadness and resentment, that a lot of our pals seemed annoyed. ("I'm sorry the baby's sick, but this is the third time you've canceled.") We could no longer indulge in wine-soaked, talk-until-3-A.M. dinner parties or drive 100 miles to a concert on a whim. More chums, casual and close, drifted away. Then we had Lucy, baby number two, and I quit my job to be a freelance writer. Suddenly, my seemingly strong work bonds began to disintegrate. One former buddy, in a painful phone call, accused me of putting the kids' needs before hers. She was right, of course. I couldn't be there for her the way I had been when I was childless and carefree.

That's the case for many women: The older (and busier) we get, the fewer friends we have. But it turns out that this downward trend is quite natural—and not necessarily a bad thing. "In early adult life, there's more benefit to having a wide range of friends. You need a large social circle to meet as many potential mates as possible," says Jennifer Lansford, Ph.D., a developmental psychologist at Duke University in Durham, North Carolina. "But as people get older, they choose to have fewer but more fulfilling connections." In fact, the average person's network (including casual and close friends) shrinks over time, according to a 2003 study from the University of Sussex in Brighton, England. For 16- to 18-year-olds, the average number of contacts is 59. For those between 25 and 35, it's closer to 30. In their 50s? Most people average 21.

That may seem like a perilous plunge, but in Lansford's study of how friendships change with age, she discovered that 63 percent of people older than 60 describe themselves as satisfied with their number of connections, compared with only 50 percent of 20-year-olds. "As you get established, you have less time to maintain friendships, but also less of a need to put yourself out there," she explains. "And as extraneous relationships are discarded, established connections become stronger, and their impact is more deeply felt."