What makes for great relationships.

There's an old Spanish proverb: Tell me who you're with, and I'll tell you who you are.

I now see the great power in that statement, having immersed myself in the lives of 10 women who grew up together in Ames, Iowa. My book about them, “The Girls from Ames,” traces their 40-year friendship.

Now in their mid-40s, the Ames girls have a lifetime of memories in common, some evocative of their generation, and some that will resonate with any woman who has ever had a friend. Here are 11 lessons on friendship that can be learned by these relationships that have spanned four decades.

1. You Can Make a New Friend But Not an Old Friend

Women who nurture long-term friendships can find profound comfort recounting shared moments, good and bad. Its okay if some of those moments make them wince or leave them sad.

Whatever the memory, its a gift to have other people who were there with them. Among the Ames girls, no one needs to say it, but they all feel it: On the entire planet, only the rest of you can remember certain things I remember.

2. Old Friends Help You Hold on to Your Younger Self

One of the Ames girls, Cathy, is now a make-up artist in Los Angeles. Her L.A. friends are intrigued that she remains close with her old Iowa friends. What do you still have in common with them? they ask.

Cathy's response: What keeps me going back to them? I think its this: We root each other to the core of who we are rather than what defines us as adults by careers or spouses or kids. There's a young girl in each of us who is still full of life. When were together, I try to remember that.

3. Friendships Are Most at Risk During the Busy Years

Studies show that in their late 20s and 30s, women have a harder time staying in touch with old friends. Those are the years when theyre busy starting careers and raising children. But at around age 40, women start reconnecting.

Before the 1990s, researchers assumed this was because women had more time for friendship in their 40s, as their children became self-sufficient. But now researchers consider this middle-aged focus on friendship to be a life stage; as women plan the next chapter of their lives, they turn to friends for guidance and empathy.

Women who are still friends in their 40s, like the Ames girls, are almost certain to be lifelong friends. Female friends show us a mirror of ourselves, one researcher told me.

4. Lifelong Friendships Make You Happier and Healthier

The Ames girls haven't tracked the latest scientific studies on friendship, the ones showing that having close friends helps people sleep better, improve their immune systems, boost their self-esteem, stave off dementia, and actually live longer. The Ames girls just feel the benefits in their guts.

In fact, researchers say a woman who wants to be healthier and more psychologically fit in her old age is better off having one close friend than a half-dozen grandchildren.

5. Good Friendships Have Ground Rules, Even If Unspoken

Over four decades, the Ames girls have had their share of fights and hurt feelings. But now that they've reached their 40s, they've come up with unspoken or barely acknowledged ground rules that seem to work, including:

They don't brag about their husbands, jobs or incomes. They talk about their children's achievements, but not in a gloating way. They root for each others kids, just as they root for each other. They make every effort to be with each other for key events in their lives: weddings, serious illnesses, funerals.

6. Strong Friendships Can Improve Your Marriage

One explanation is that women who are good at intimacy with friends are good at intimacy with husbands. But researchers also say that women with close friends don't burden their husbands with all of their emotional needs.

The Ames girls husbands, like a lot of men, tend to show their love and concern by being solution-oriented. They want to be fixers. When one of the Ames girls tells her husband shes having issues with her mother, the husband is apt to recommend strategies: Here's what you ought to do But a female friend is more apt to say, I have trouble with my mother, too. And no matter what your mother says, I think you're terrific.

Because the Ames girls can talk to each other, sending constant reply all emails within the group, they don't have to lean on their husbands for all of their emotional support.

7. Friends Can Offer Great Solace in Moments of Grief

One of the Ames girls, Karla, lost her 12-year-old daughter, Christie, to leukemia in 2002. The Ames girls offered Karla comfort and support during Christie's illness, and they were there for her when Christie died.

After the memorial service, they joined 100 friends and relatives back at Karla's house. At one point, the 10 Ames women ended up together in Karla's bedroom. Someone closed the door and there they were. They noticed they were touching each other; everyone had a hand on a shoulder, an arm, a hand, as Karla talked about Christie's final hours.

In that moment, gathered together on Karla's king-sized bed, the Ames girls saw that true friendship means a willingness to share both joy and complete despair.

As one of the Ames girls later described it: Outside that door, grief was waiting to envelope Karla. But in that bedroom, for that half hour, a profound sisterly love was keeping it all at bay.

8. There Are Differences Between Men's and Women's Friendships

Researchers say women's friendships are face to face: They talk, cry together, share secrets. Men's friendships are side by side: They play golf. They play poker together. Men often resist going too deep.

Don't assume, however, that men's friendships are inferior to women's friendships. If we use a woman's paradigm for friendship, were making a mistake, says Geoffrey Greif, a professor at the University of Maryland's School of Social Work, who studies male friendships. Men might not be as physically or emotionally expressive as the Ames girls, he says, but they derive great support from their friendships.

Men turn to other men to escape from their problems. If they want to open to about emotional issues, they often prefer to talk to wives, mothers, sisters and platonic female friends.

9. Forgiveness Is a Part of Friendship

One of the Ames girls, Sally, was kicked out of the group in eleventh grade. Some of the others gathered at a slumber party, told her what they didn't like about her, and sent her home crying. It was devastating and painful for Sally, but at the same time, it was liberating and life-changing. Sally realized she was happy with the person she was, and she wasn't going to let mean girl comments stop her.

The other Ames girls eventually welcomed her back to the group, and have apologized for the incident repeatedly ever since. And Sally, now a teacher, feels that what happened to her at age 16 has made her more empathetic with her children and her students.

Readers of "The Girls from Ames" have written to say they admire Sally for finding it in her heart to forgive her friends. "I've forgiven everything," Sally recently told a crowd at a bookstore in Minnesota. "I mean, it happened such a long time ago. It happened thirty years, six months, five days, and six hours ago, not that I'm keeping track."

It was the perfect laugh line, and a reminder of the role forgiveness must play in our friendships.

10. Friendships Thrive When People Calibrate Their Expectation

Friends can strengthen their bonds if they raise some expectations and lower others.

The Ames girls have come to expect loyalty and good wishes from each other, but not constant attention. If one of them doesn't return an email or phone call, they realize, it doesn't mean shes angry or backing away from the friendship; she's likely just exhausted from her day.

11. It's Comforting to Have Friends Along as You Age

Friends such as the Ames girls, who've traveled the time-line together, tend to have more empathy for each others ailments. They knew one another when they were younger and stronger, and they've watched their bodies change.

Gerontologists say longtime friends are often more understanding about health issues than family members are. Friends are more apt to acknowledge each others ailments without dwelling on them the way a parent or spouse might.

A friend might offer a litany of health issues, especially as she ages, but then shell say: Lets forget about the pain were feeling today and have fun.

Jeffrey Zaslow is columnist for The Wall Street Journal and the bestselling author of The Girls From Ames.