Adecade after most couples divorce, only one partner appears to benefit from the break-up, a 10-year study of divorced families concludes. The other partner is likely to be just as unhappy or even worse off than during the failed marriage.

Conducted by California psychologist Judith Wallerstein, the study was one of numerous research projects probing the effects of divorce on families presented last week at a two-day workshop sponsored by the National Institute of Child Health and Human Development (NICHD). Divorce is a wrenching experience for families, not just during the initial periods of break-up and separation, but often for years following the legal dissolution of the family, the research suggests.

Wallerstein presented new findings from an intensive follow-up study of 60 middle and upper-middle income California couples who divorced a decade ago. At the 10-year follow-up point, men in the study ranged in age from 35 to 75; women were between 33 and 65. More than 100 children also participated in the study. They ranged in age from about 12 to the late twenties.

The partner who seeks a divorce is generally the one whose life improves, Wallerstein reports. About two of every three people who initiated the divorce were happier 10 years later. "So these people were right in seeking a divorce," says Wallerstein, executive director of the Center for the Family in Transition in Corta Madera, Calif. "But by the same token, those who opposed the divorce were also right. Ten years later, they were worse off."

Among her other findings:

* Overall quality of life improved demonstrably for both former partners in only 10 percent of the divorced couples in the study. "Even more sadly, in one fifth of the families, both of the former marital partners were living in a signficantly worsened situation."

* Divorce is "psychologically and socially a more significant event for women than for men." About 55 percent of women had "notably improved the quality of their lives" in the 10 years since the divorce, compared with only 32 percent of men.

* Women's ability to thrive following divorce depends largely on their age and profession at the time the marriage dissolved. Women under 40 fared far better than women over 40, based on psychological and financial measures. Often these younger women were able to begin successful new careers and embark on new marriages. Seventy percent of the women under 40 improved their financial status after the divorce. Yet 40 percent of the women over 40 saw a decline in income. None of the older women remarried, and they often still lived with one of their children.

* A decade after divorce, few people took responsibility for the break-up of their marriage. More than half the women felt they bore no responsibility at all. About 20 percent of women felt they were partially at fault. Men were more willing to accept blame, with 30 percent assuming full responsibility and about 25 percent accepting partial blame.

* Despite often worsened living conditions, very few people wished to reconcile with their former spouses. Of the 60 families studied, only two couples remarried, and one of those couples later divorced a second time. At the 10-year mark, 90 percent of the women and 70 percent of the men still felt that the divorce was the right decision.

Intense anger may be the main reason most former spouses have no interest in reconciliation, Wallerstein discovered. Although the families studied broke up under fairly amicable conditions -- none battled in court over child custody or visitation -- Wallerstein still found that anger remained at high levels between the former partners even a decade later.

"It is striking that the incidence of very angry adults at the 10-year mark does not differ markedly from that at the marital rupture, when we reported that 44 percent of the women and 20 percent of the men were intensely angry," she says. Ten years later, 40 percent of the women and 28 percent of men were still "intensely angry at their former partner." Even when study participants had satisfying and successful second marriages, they still harbored a great deal of anger against their former spouse.

Women often felt this anger more strongly than men, a fact that Wallerstein found "most troubling, since the conflict with former husbands by angry women spilled over into high stress and intense conflict over the children's relationships with their fathers."

Children also suffered in other ways. "It is strikingly clear that five and 10 years after the marital rupture, the divorce remains for many children and adolescents the central event of their growing-up years and casts a long shadow over these years," Wallerstein said. "The child of divorce faces a special set of challenges and carries an added burden.

"Overall we have been startled with the long-time endurance of vivid memories and feelings stemming from the divorce . We have been startled to find young adults who are intensely preoccupied with a parent's infidelity 10 years earlier, or other youngsters, aged 2 or 3 at the time of the marital separation, who wept and seemed unable to leave our office where they cried for an intact family they had hardly known, for close contact with a father they had encountered over the years as a capricious or disinterested parent."

In particular, divorce and step family research is beginning to show some differences in the way boys and girls react to a family's dissolution. Studies in the last several years have pointed to behavior, discipline and learning difficulties for young boys of divorced parents. But new findings suggest that divorce may exert a "sleeper effect" on girls, which is not really evident until they reach adolescence and young adulthood.

Wallerstein's study found that "a whole group of girls are being derailed" by divorce. As they progressed into adolescence and adulthood, these girls became very anxious. They had difficulty establishing and keeping relationships with men. They expressed a "conscious fear of betrayal," she reports, "which they connected directly with their parents' divorce . . . We're not finding that degree of anxiety among the young men."

Girls also may fare differently from boys when their parents remarry. Recent studies by Temple University psychologist Eulalee Brand of 60 so-called "blended families" suggests some intriguing differences between stepdaughters and stepsons. Unlike intact families and families with a stepfather, tensions rose between stepdaughters and stepmothers as marital satisfaction increased between the remarried couple. When a stepdaughter lived with her father and stepmother, relations worsened when the daughter saw her biological mother often. Self-esteem also dropped for the daughter.

In contrast, stepdaughters get better treatment from their stepfathers when their biological fathers visit them frequently, Brand found. There was no such association for boys.

In families with stepfathers, family happiness depended on "overall marital satisfaction between the husband and wife," a finding Brand says "is consistent with studies of intact families." The better the relationship between husband and wife, the better the stepfather and stepchildren got along -- a finding directly opposite the results from stepmother-stepdaughter families.

In families with stepmothers and stepsons, the better the relationship between husband and wife, the better the stepson and stepmother got along. And the more positive the relationships of this blended family as a whole, the less aggression and other behavior problems were expressed by the sons.

"We're only beginning to understand the effects of remarriage," says psychologist James Bray of Texas Woman's University in Houston.

Patience seems to be a particularly important virtue for blended families. "It may take two to four years for the marriages to stabilize and for children to accept their stepparents," Bray told the NICDHD workshop. In several studies Bray found that it took youngsters aged 6 to 18 an average of 4.5 years to really adjust to their new family situation.

During these transition years, blended families often "face a bit more stress that intact families," Bray says. Blended families must cope with discipline problems, more family disorganization and confusion about parental responsibilities than families headed by biological parents.

An increasing number of children live in blended families. A new national survey on child health, conducted by the federal government and analyzed by psychologist Nicholas Zill of Child Trends, a research organization based in the District, reports that some 5.5 million children 17 years and younger resided in blended families in 1981. Another 13 million children lived with just one parent.

While divorce rates have dipped slightly in the past two to three years, the number of marriages that break up in the United States remains high. White children born in 1980 "have a 70 percent chance of spending time in a single-parent family by age 18," says Dr. Duane Alexander, acting director of NICHD. These children will spend about one third of their childhood in a single-parent home, as a result of divorce, separation or being born out of wedlock. For black children born in 1980, the chance is 90 percent that they will live in single-parent home by age 18. On average, 60 percent of their early years will be spent with just one parent.

The rising number of blended families has significance for future mental health problems, researchers say. "Just because some of these things have become more prevalent, doesn't mean that they are easier for people," says Child Trends' Zill. "The assumption is that everyone is getting divorced, so it won't be a problem for children. But since the late 1960s we have seen an increase in the use of psychotherapy by adolescents that is related to changing family structures."