"Why no Sisters Karamazov?" demands Elizabeth Fishel, only half in jest. In 1971, while she was a junior at Radcliffe, Fishel set out to write a magazine article about her intense and complex relationship with her sister Annie, younger by five and a half years. "That relationship, "she says, "has been one of the most important of my life." Fishel was surprised at how little information existed about the psychology of sisterhood. She let the subject percolate while working toward a master's in English and creative writing at Stanford University in 1974. Then, drawing up a detailed questionnaire and searching both fiction and science for insight, she wrote Sisters, published last year by Morrow and now a Bantam paperback. About 150 women across the country answered Fishel's questionnaire after she placed ads in Ms. magazine and the newsletter of the Mothers of Twins Club. About half of the women also had brothers, the rest had sisters only. They ranged in age from 18 to 82 (the average was 32), and were equally divided between working-class and middle-class backgrounds. Fishel, 30, herself grew up in what she describes as "an extremely stable" New York family—her father runs the Fairfax advertising agency and her mother is a potter. "Annie and I got a lot of attention and encouragement, " Fishel recalls. She and Annie, a 24-year-old graduate student, keep in touch by phone and see each other twice a year. Fishel teaches creative writing at Berkeley and shares a sunny apartment in Oakland with husband Robert Houghteling, a private school phys ed instructor. She discussed her findings on sisterhood with Dianna Waggoner of PEOPLE.

Is a sister's influence as strong as a mother's?

In many areas, it is. Sisters are our peers, the voice of our own times. A sister's experience carries the persuasiveness of the contemporary, the hip, while parental styles are rooted in the past and are always a trifle suspect. In addition, the physical proximity is almost overwhelming. We play with our sisters, go to school with them and often sleep in the same room with them. The last voice you hear for 15 or 20 years before falling asleep can be pretty powerful indoctrination. A sister is both your mirror—and your opposite.

Isn't the same true of brothers?

Not quite. Years of social conditioning have taught women to be more open about their feelings. Brothers are taught to be stoic. By and large, they just don't share the same kind of intimacies sisters do.

How do parents affect the relationship between sisters?

They can label one baby "the easy one" and another "the difficult one," and these differences may dog the sisters their entire lives. Parents can drive sisters apart by talking about one behind her back to the other, by repeating confidences, by holding up. one too often as a model to the other. Comparison is a death knell to sibling harmony.

How close were the women you surveyed?

The feelings ran the gamut. Some sisters were terribly estranged and full of bitterness toward each other. Some reported nothing but pleasure in each other's company. Most women were in the middle, describing ambivalence and love-hate, and questioning why things weren't better.

How do sisters learn from each other?

They take all kinds of cues from each other, absorbing lessons about how to walk, talk on the telephone, think, dress and respond to the world—what to fear and what to embrace.

Where do they look for role models?

An amazing number of them remember Little Women as a powerful influence, despite its age and overt sentimentality. The book is still as central to the mythology of most American girlhoods as slumber parties and fantasies of Mr. Right. Of the four sisters in the story, the most popular heroine is Jo—the spirited, lovably sharp-tongued renegade and tomboy.

Why has Little Women endured?

It has clearly delineated female characters, which are relatively rare in young people's fiction, and it imposes reassuring order on the chaos of growing up. Maybe more important is the appeal of seeing sisters pulling together. It reinforces the message that sisters are more powerful together than separate.

What happens when one sister is more successful?

There is, understandably, some resentment. We grow up expecting everything to be doled out equally, but it doesn't turn out that way. Carly Simon, 35, and her two older sisters—Joanna, 40, and Lucy, 37—have formed a strong unit despite their competition. Joanna, a soprano with the New York City Opera, is very successful in her own field. Still, she admitted, "The financial aspect is a little staggering, because I work terribly hard and make not even one-tenth the money Carly makes." Lucy, like Carly a songwriter and pop singer, feels that Carly's success put "a kind of distance between us."

How does Carly feel about it?

She's experienced guilt—that "Why me?" feeling. She handles it by minimizing the pluses of success and maximizing the drawbacks, the nuisance of being in the public eye. Ironically, Carly was once so jealous of older sister Lucy that she wrote a song about it called Oh Lord, Won't You Let Me Be Her for Just One Day?

What makes sisters angriest at each other?

Again and again women said that it was when their sisters borrowed or ruined their clothes. It's not just a matter of taking material goods, but of daring to slip into someone else's carefully planned persona and palming it off as one's own. That's a deep threat to adolescent identity. My great-grandfather tried to stamp out the fire among his five daughters by forbidding them to borrow each other's clothes.

What arouses the most jealousy?

It almost always focuses on food and their bodies. I got countless comments like "My sister can eat anything and stay slim, but I go on Weight Watchers and don't lose a pound." What society teaches all women, sisters internalize in the family. To be acceptable, they feel they have to be the thinner one.

Is sibling rivalry among sisters different than among brothers?

Very. Brothers experience incredible rivalry, but there is a socially acceptable outlet for it: sports. Girls have no sanctioned outlet. Instead, all that jealousy and competition must be dealt with at home. By confronting their real feelings, girls break through to levels of intimacy boys rarely achieve.

Then quarreling is healthy?

It can be. The worst thing parents can do, I think, is force their girls to smooth over their differences in the interest of being "ladylike." I heard from a lot of women who remained in an embattled state internally for years because they-had never been allowed to express any anger as kids.

During adolescence, are boys a source of jealousy?

It's common for a few years. Joanna Simon remembers stealing away Lucy's boyfriend, only to throw him over after winning his heart—the sport was gone. But around 17 or 18 a line is drawn. Then it becomes taboo to flirt with your sister's man.

Do women without sisters try to find one in a friend?

I expected to find just such a pattern. Instead I discovered that some women who had no sisters had no need for them. But women who have had intense relationships with sisters try to re-create that with other women.

Are they able to find adequate sister-substitutes?

Some are. Playwright Ntozake Shange, who wrote For Colored Girls Who Have Considered Suicide, is a good example. She and her friend Thulani met as freshmen at Barnard College and immediately felt like twins in every way, even physically. Ntozake says the relationship is easier than if they really were sisters, because they're not carrying around the old emotional baggage—the unsettled scores and bloody knees—of childhood.

What happens as sisters age?

Generally, the relationship improves. The death of parents often knits sisters together, as does the fading of outside friendships, the failure of marriages or the dashing of youthful dreams. Margaret Mead summed it up perfectly. She said, "Being sisters is probably the most competitive relationship within the family; but once the sisters are grown, it becomes the strongest. On the whole, sisters would rather live with each other than with anyone else in their old age."

Is that what Mead did?

Well, she traveled frequently almost right up to her death in 1978. But her younger sister, Elizabeth Mead Steig—an artist and art teacher—kept a room for Margaret in her Cambridge, Mass. apartment and they spent a great deal of time together. Elizabeth once told me—and I think it amounts to the bottom line on sisterhood—"If you tell your sister to go to hell in 12 different languages and then you need a quarter, you can say, 'I need a quarter.' And she'll give it to you."