A research study lead by Margaret Keyes, of University of Minnesota, found that adopted children were significantly more likely to suffer from a psychiatric disorder, compared to those who were not adopted.

Comparing a random sample of 540 Minnesota-born adolescents who were not adopted with a representative sample of 700 adoptees placed by the three largest adoption agencies in Minnesota, they found 14 percent of the adopted kids had attention deficit-hyperactivity disorder and 19 percent of them had oppositional defiant disorder, compared to 10 percent of the non-adopted children. Of the latter group, 514 were foreign adoptions and 178 were domestic. Researchers interviewed the subjects, as well as their parents and teachers. They performed psychiatric assessments on all adolescents.

ADHD is regarded as a chronic disorder, or a set of “neurobehavioural developmental disorders,” which affect about 3-5% of the world’s population under the age of 19 and about 4% to 8% of American children ages 4 to 17; the symptoms include hyperactivity, as well as forgetfulness, poor impulse control, impulsivity, distractibility, or difficulty with concentration and focus.

"Despite the popularity of adoption, there is persistent concern that adopted children may be at a heightened risk for mental health or adjustment problems," the study's authors write in a report released Monday.

"When you have all the information, you're better prepared to make decisions for your family," Keyes said. "You have information that your adopted child might be at a slightly increased risk, so you can be aware of that and can you use the social services agencies with which you already familiar through the process of adopting."

More than 120,000 children are adopted annually in the United States and about 1.5 million adopted American children are under the age of 18.