An in-depth survey of American children reveals that one in five children under age 18 has a learning, emotional, behavior or developmental problem that researchers say can be traced to the continuing dissolution of the two-parent family.

The federally sponsored survey of more than 17,000 children nationwide showed that by the time youngsters enter their teenage years, one in four suffers from one or more of these problems, and for male teenagers it is nearly one in three.

Emotional and behavioral problems have become the "new morbidity of childhood," affecting 10 million children, said Nicholas Zill, a psychologist and executive director of Child Trends Inc., a Washington-based organization that studies social changes affecting children. Zill headed the study for the National Center for Health Statistics.

Two factors in contemporary society are contributing to the increase in childhood psychological disorders, he said.

The most important involves family dynamics: the increasing number of children who experience their parents' divorce, are born outside of marriage, or are raised in conflict-filled families or low-income, low-education, single-parent households, Zill said.

Emotional and behavioral problems were two to three times higher among children in single-parent homes or in families with one stepparent, while learning difficulties were nearly twice as high among children whose mothers had not completed high school as compared to those whose mothers had more than 12 years of education.

The second factor is the mental damage children suffer at birth or later on.

Although survival of extremely low-birthweight babies is becoming more routine, they have a higher risk of brain damage during the first weeks of life. Children also are affected by environmental pollutants, and more babies are being born to mothers addicted to drugs, especially crack cocaine, he said.

Despite the large number of youngsters with psychological problems, many are not diagnosed and those who are do not receive adequate help, he said. The survey, which involved interviews with one parent of each of the 17,000 children, found that only a fourth of the children with emotional or behavioral problems received any kind of special educational help in schools.

The problems are particularly acute in the black and Hispanic communities where many children with emotional and behavioral disorders go untreated, only to be dealt with eventually by the criminal justice system, Zill said.