Dr Matthew McGue is a behavioral geneticist at the University of Minnesota. He and Dr Bill Iacono started an ongoing research project in 1999. The project is called Sibling Interaction and Behavior Study (SIBS) and Research on Adoption.

Some results of the SIBS study was published in May 2008 in the Archives of Pediatric and Adolescent Medicine. Rick Weiss wrote up an article for the Washington Post about it. His article was published July 15, 2008 and was titled Adolescence Can Sting Adopted Kids.

According to Dr McQue:

SIBS is a study of 400 adoptive and 200 non-adoptive families, each consisting of a pair of adolescent siblings and their parents.

The primary goal of this study was to look at how much influence an older sibling had on a younger sibling in relationship to "adolescent substance abuse". The secondary objective was to explore the "psychological adjustment of adopted youth".

• 540 adolescences in the study weren't adopted.
• 514 adolescences in the study were adopted intercountry (internationally).
• 178 adolescences in the study were adopted domestically (within the United States).

The research team completed mental health assessments on 692 children and adults who were ages 11 to 21.

• 25% of domestically adopted children were diagnosed with ODD
• 20% of intercountry adopted children were diagnosed with ODD
• 12% of non-adopted children were diagnosed with ODD

• 29% of domestically adopted boys were diagnosed with ADHD
• 19% of intercountry adopted boys were diagnosed with ADHD
• 8% of non-adopted boys were diagnosed with ADHD

• 15% of domestically adopted boys were diagnosed with conduct disorder
• 8% of intercountry adopted boys were diagnosed with conduct disorder
• 6% of non-adopted boys were diagnosed with conduct disorder

Girls have about 1/3 fewer instances of ODD, ADHD and conduct disorder. But the same pattern of domestically adopted children receiving more diagnoses remains the same.

The Washington Post article stated that the study interviewed, "most of whom were adopted from Korea". This is a potentially weaknesses to the study if most of the intercountry adoptees were from Korea. The sociality conditions in other countries could potentially produce different results for adoptees.
 

Reference:

The Mental Health of US Adolescents Adopted in Infancy
Margaret A. Keyes, PhD; Anu Sharma, PhD; Irene J. Elkins, PhD; William G. Iacono, PhD; Matt McGue, PhD
Arch Pediatr Adolesc Med. 2008;162(5):419-425.