JUST FRIENDS; The Role of Friendship in Our Lives. By Lillian B. Rubin. Harper & Row. 235 pp. $15.95.

WHEN I WAS growing up an only child, I wanted more than anything else to have sisters -- my favorite book was Little Women and my favorite singing group was The Lennon Sisters. Much of my life, I now see, has been spent trying to build just such a "family" through friendship. As I write these words, I am sitting at a friend's desk in Washington. She is away for a month, and I am staying in her house, overseeing repairs and providing such adult functions for her teenage son as nagging him to mow the lawn. This morning her 11-year-old, my adopted goddaughter, called to say she missed me; later her 15-year-old called to solicit my advice about a problem in getting along with her mother. (It is much easier to be a godmother, than a real mother, I notice.)

In recent years my closest friendships have survived, and been strengthened by, temporary estrangements, moves, psychoanalysis, disliked lovers, divorce, deaths of parents and of one friend's child. Now, at almost 40, a single parent as well as an only child, I find myself thinking a good deal about these friendships, sustained by them, aware, as I never was before, of the mutual responsibilities and obligations that do not merely go along with the pleasure but are, in fact, part of the pleasure of enduring relationships.

This is just one of the messages, though perhaps the most important one, of Lillian B. Rubin's Just Friends, a thoughtful and thought-provoking study of this often neglected aspect of modern life. Though many of us might agree with Aristotle that "Without friends no one would choose to live, though he had all other goods," friendship has rarely been the subject either of contemporary fiction or popular social science, particularly surprising, I think, in a society as mobile and fragmented as ours, in which such relationships take on heightened importance and often seem to suffer from some of the same problems as the family. Fortunately, Rubin, who has previously examined male-female relations in Intimate Strangers, as well as women at midlife and the working- class family, has stepped in to fill the gap.

THE BOOK is based on interviews with 300 men and women, ages 25 to 55, of varying backgrounds, and also draws on Rubin's experience as a practicing psychotherapist, focusing on such issues as the relationship between kinship and friendship, the differences between men's and women's friendships, how friendship affects marriage, and the concept of "best friends." Avoiding the jargon of too many pop psychologists, Rubin writes clearly and lucidly, providing a compassionate and convincing understanding -- based both on psychological theory and plain common sense -- of why we think and behave as we do. If what she found is not always surprising, what she says about it is well worth considering.

From the beginning of her research, Rubin discovered that it was harder to get a straight answer to questions about friends than to other personal questions about family life, including sexual relations. This may be, at least in part, because friendship in our society is not institutionalized, unlike relationships of marriage and family. We say that our friends are "like family" and yet we sometimes like our friends better than family members and feel freer to reveal ourselves to them. Nevertheless, Rubin points out, "The idea of kin is so deeply and powerfully rooted within us that it is the most common metaphor for describing closeness." Relationships with family members are often "loaded" -- precisely because they are so elemental and primordial, formed in our very earliest years and hence deep-seated and regressive. No matter how much we resent and dislike a sibling, for example, we expect our friends to step aside while we answer that sibling's demands. For this, Rubin says, we pay a heavy price, depriving both our friends and ourselves of "the sense of entitlement that is crucial to a secure and committed relationship." Indeed, many of Rubin's interviewees felt that just such a reciprocal commitment was missing from their friendships.

IT IS RUBIN's central thesis, however, "that friends are central actors in the continuing developmental drama of our adulthood . . . who provide a reference outside the family against which to measure and judge ourselves." It is our friends who often help us with issues of individuation and separation and who frequently offer us a kind of support and encouragement that the family, for whatever complex reasons, cannot.

Of course, how our friendships are experienced is often different for men and women, and some of the most interesting sections of the book deal with these differences. While women's friendships seem to rest on being -- shared intimacies and emotional support -- men's friendships most often center around doing -- shared activities, a difference with which both men and women express some dissatisfaction. Building on Nancy Chodorow's pioneering work in The Reproduction of Mothering, Rubin argues convincingly that "certain developmental requirements of early childhood are different for boys and girls, resulting in different structures of personality for men and women and, therefore, affecting the kinds of friendship they are likely to form in adulthood as well." These requirements arise because girls are raised primarily by their own sex while boys are not, and it is this that accounts for women's difficulty with separation and individuation and men's difficulty with intimacy, and which lies at the root of much of the conflict between the sexes. These patterns will remain with us, Rubin suggests, until the traditional family is modified to provide two parents who both act as primary nurturers of children, thus providing them with two figures with whom to make early and crucial identification.

SUCH DIFFERENCES often are responsible for the conflicts friendships cause in marriage -- the wife who complains of her husband's "nights out with the boys," the husband who says that his wife is always "yakking on the phone with her girlfriends." For both partners, but especially for women, Rubin found, "friends outside the marriage facilitate the acceptance of the limits inside it," limits inherent in every relationship, although some are particularly aggravated by the structure of the family and of society. It is not the recognition of these limits that is harmful or the turning to friends to fill the gaps that threatens a marriage, Rubin asserts, but rather the "efforts to transcend all limits," to insist that our partner measure up to some ideal of perfect unity.

What becomes infinitely clear from Rubin's analysis of her research, and from the moving personal experiences with which the book begins and ends, is that friendship, like any relationship, requires work and attention, the ability to change and to reorder a relationship if necessary so that it is built on reality instead of illusion. The reward is that we may be diligent enough, and lucky enough, to find ourselves with friendships that truly are "'like family,' combining commitment and obligation with the love that is reserved for honored family alone, moving finally from the secular realm to the sacred."