Isolation from others is detrimental human health—a fact proven in study after study. In one study, researchers found that individuals who had no friends increased their risk of death over a 6-month period. Another study showed those who had the most friends over a 9-year period cut their risk of death by more than 60%.

For women, the famed Nurses’ Health Study from Harvard Medical School found the more friends a woman had, the less likely she was to develop physical impairments as she aged, and the more likely she would lead a fuller life. In fact, the results were so significant, the researchers concluded that not having a close friend and confidante was as detrimental to your health as smoking or carrying extra weight. There’s more: when the researchers looked at how well the women functioned after the death of their spouse, they found that those women who had a close friend were more likely to survive the experience without any new physical impairment or permanent loss of vitality. Those without friends were not as fortunate.

A landmark UCLA study conducted in 2000, suggests friendships between women are special. They shape who we are and who we will become. Female friendships help calm us, fill in emotional gaps, and help reinforce our personal identities. The study indicated that women respond to stress with a rush of brain chemicals that cause them to make and maintain friendships with other women.

Until this study was published, it was generally believed that when humans experience stress, it triggers a hormonal cascade that revs the body into a “fight or flight” response–an ancient survival mechanism left over from when humans faced predators daily.

The UCLA study indicated that women in particular, have a wider behavioral repertoire than just “fight or flight” says Dr. Laura Cousin Klein, an Assistant Professor of Bio-behavioral Health at Penn State University and one of the study’s authors. When the hormone oxytocin is released as part women’s stress response, it buffers the “fight or flight” response and encourages her to tend children and gather with other women instead. When she actually engages in this tending/befriending, the study suggested that more oxytocin is released, which further counters stress and produces a calming effect. This calming response does not occur in men, says Dr. Klein, because testosterone–which men produce in high levels when they’re under stress–seems to reduce the effects of oxytocin. The female hormone estrogen; she adds, seems to enhance it.

It will take time for new studies to reveal all the ways that oxytocin encourages women to care for children and hang out with other women, but the “tend and befriend” notion developed by Dr. Klein may help partially explain why women consistently outlive men.

If friends counter the stress that seems to affect so much of our life, if they keep us healthy and even add years to our life, why is it so hard to find time to be with them? That’s a question troubles researcher Ruthellen Josselson, PhD, coauthor of "Best Friends: The Pleasures and Perils of Girls’ and Women’s Friendships" (Three Rivers Press, 1998). “Every time we get overly busy with work and family, the first thing we do is let go of friendships with other women,” says Dr. Josselson. “We push them right to the back burner. That’s really a mistake, because women are such a source of strength to each other.”

So, when hustle and bustle of everyday life causes you to say “I’ll catch up with her later” when a friend calls, you should reconsider. In the words of an old song, “Make new friends but keep the old; one is silver and the other’s gold.”