Adolescents adopted as infants are twice as likely to have behavioral disorders as those who are not adopted, according to research published Monday that is the deepest analysis yet of the larger mental health burden carried by some adopted children.

Researcher Margaret Keyes, a University of Minnesota psychologist, stressed that adoptive parents or those thinking about adopting shouldn't be alarmed by her study, because rates of emotional problems are relatively low among all adolescents studied, but were higher among adopted kids.

For example, she said that among the 692 adopted kids in the study, 14 percent had attention deficit hyperactivity disorder, compared with 7 percent of the 540 kids in the non-adopted group.

Keyes found similar differences in other mental health conditions that manifest themselves outwardly in destructive or defiant behavior. But she found no significant differences in mental disorders such as anxiety or depression that are more internal.

In a finding that is contrary to what researchers expected, she also found that children adopted from the United States had somewhat higher rates of mental health problems than those who were adopted from other countries.

Adoption experts said that the study documents and quantifies what they have intuitively known for some time.

'A component of loss'

"All adoptions include a component of loss," said Jennifer Wilson, a adoption therapist and trainer for Children's Home Society in St. Paul. "We've learned so much about the impact of that loss" on adopted children, she said.

About 1.5 million children and teenagers under 18 in the United States are adopted. They are 2.5 to 6 times more likely than nonadopted kids to show up in counselors' offices for help, said Keyes, a research psychologist with the university's Minnesota Center for Twin and Family Research. In fact, Children's Home and other agencies provide specialized counseling programs for adoptive families to help them cope.

Adoption agencies often advise parents considering adoption that their children can encounter more severe emotional and psychological land mines, especially when they hit adolescence. That kind of parent education is a routine part of the adoption process.

"All adolescents struggle with identity. It's a little more intense in adoptees," Keyes said.

But it has never been clear whether their greater demand for mental health care is because adoptees are more likely to have psychological problems or because their pro-active, educated and often wealthier parents are more likely to seek help on their behalf, she said.

"Maybe adoptive parents were more sensitive to the behavior of their children," she said.

But according to her findings, published Monday in the Archives of Pediatric and Adolescent Medicine, that is not the case. She and her co-researchers interviewed both adopted and nonadopted Minnesota children and their parents, and obtained information about their behavior from the children's teachers. All of the children were adopted before the age of 1 and were interviewed when they were between the ages of 11 and 21. Their average age was 15.

Keyes found the adopted families with the help of the three largest adoption agencies in Minnesota -- Children's Home Society, Lutheran Social Service and Catholic Charities. Officials from all three agencies said that they contacted families and asked if they would like to participate in Keyes' research project.

Theories on why

No one understands why adopted children are more troubled, nor how often those emotional problems extend into adulthood, Keyes said.

She was unable to obtain information about the birth mothers. But maybe they experienced deprivations or had some unidentified genetic problem, she said.

She also said she could only theorize about the higher rates of mental health disorders among domestically adopted children. Perhaps those mothers were more likely to have used addictive substances or experienced other factors that affected the children prenatally, she said. Or perhaps the loss of a biological parent who is culturally and geographically closer is more painful and difficult for a child to accept, she said.

For those concerned about the study's findings, Keyes pointed out that being adopted is not the only risk factor for children's mental health. Boys have a higher incidence of such behavioral disorders, "but no one is overly concerned when they give birth to a son," she said.

The Chicago Tribune contributed to this report. Josephine Marcotty • 612-673-7394