Every child responds to divorce differently. But psychologist Judith Wallerstein, executive director of the Center for the Family in Transition in Corte Madera, Calif., has found some common threads based on her 10-year study of 131 psychologically healthy youngsters.

In the study, the longest evaluation of divorce's impact, Wallerstein interviewed children aged 3 to 18 at the time their parents divorced and again five and 10 years later. Almost all of the children studied were in the custody of their mothers.

Now she worries that even the problems identified by her study -- which was funded by the San Francisco Foundation, Zellerbach Family Fund in San Francisco and the Kenworthy Swift Foundation in New York -- do not show how distressing divorce can be for American youngsters. Preschoolers

These young children seemed the most upset and frightened when their parents first left each other. All showed behavioral and mood changes. Regression was common, frequently in their most recent achievement -- in toilet training or going to nursery school. Some resumed thumb sucking or returned to their security blankets. Many were worried about being abandoned by both parents.

Eighteen months later, Wallerstein found that almost half of the preschool children looked even more distressed. Boys seemed significantly more troubled than many of the girls, who seemed to be on the road to recovery.

Ten years after the divorce, younger children seem to recover, and in general, they fare better than their older brothers and sisters. Almost as though their young age protected them, most claimed no memories of their early intact family. Memories of stressful or violent periods had largely been repressed. Happiness seemed strongly related to what was happening in their new lives.

For a third of the preschool children, however, the divorce remained a central, disruptive event in their lives. It evoked strong feelings, tears and profound sadness as they spoke of their loneliness and continued sense of deprivation within the divorced or remarried family.

Wallerstein found that the children still yearned for their noncustodial father, even if his place had been taken through remarriage. The need for the absent father became more intense as the youngsters, especially the girls, reached adolescence.

The most positive sign that this group weathered the divorce was that many of them looked forward to an enduring marriage and to having children. School-Age Children

Children in the 5- to 8-year-old group were the most likely to show grieving. Sometimes they sobbed openly. They feared being abandoned or replaced by someone or something else, even if their father had not been a caring or attentive parent. Girls often refused to believe that the divorce would last and wove elaborate fantasies of their father's return. Some were preoccupied with trying to rescue and restore the lost family. This age group tended to feel rejected by the parent who left.

In contrast, children in the 9- to 12-year-old group felt intensely angry at both parents, especially the one they blamed for causing the divorce. In addition, they suffered grief, anxiety, a sense of loneliness and powerlessness.

These youngsters were old enough to form a close relationship with one parent aimed at humiliating the other. They also could be quite capable of responding with care and love to a parent's need for companionship.

Health problems also began to appear, usually stomachaches and headaches. Some became involved in minor delinquency. School performance and relationships with friends declined in roughly 50 percent of the children in this age group.

By 10 years, Wallerstein found that many of these children had lost their intense anger and forgiven their parents. However, a subgroup preferred to embrace a more traditional view. They remained critical of their parents' past behavior, despite the passage to time. Teen-Agers

About 33 percent of the adolescents studied appeared more troubled one year after the divorce than they had been at the time of the marital separation. They tended to become anxious over the new vulnerability of their parents. They also worried about their own entry into adulthood. They expressed concerns about their future marriage, afraid that it would fail. Aware of their own sexuality, many became upset about their parents' suddenly visible sexual needs. They preferred to think of their parents as old and beyond sex.

Because this group could spend time away from home, they also were most likely to act out their anger through school problems or delinquency. Overall, 68 percent of youngsters in the study had engaged in some kind of illegal activity during their adolescence or young adulthood. The young women turned almost exclusively to drugs or alcohol abuse. The young men tended toward more serious offenses -- arson, drug dealing, theft, forgery and serious traffic violations. Sexual activity was common, particularly as parental controls lessened.

At 10 years, the most striking aspect of this age group was the vividness and freshness of their memories, especially of emotional pain. They agreed that they had sustained an important loss and that their childhood and adolescence had been significantly burdened.

Most also had spent uncounted hours thinking about how to select a marital partner more carefully than their parents had. Most agreed that marriage should not occur early and that impulsive marriages should be avoided. They had an intense wish to avoid divorce for the sake of their children.

"Despite their adulthood," Wallerstein said, "this group identified with these projected unborn children. In a profound way they continued to see themselves as children of divorce."

Future research will focus on what triggers divorce, the role of children in divorce and why some people never return to being normal, healthy individuals after they have been through a divorce.

"It's ironic that divorce for many today isn't resulting in the remedy for which it was intended," she says. "Too freqently the result is a fragile post-divorce family marked by greater unhappiness and unforeseen instability for the children.