The MCTFR twin-family sample was ascertained through Minnesota birth records. The adoptive-family sample was ascertained from infant placements made by the three largest private adoption agencies in Minnesota. The non-adoptive controls were ascertained through Minnesota state birth records and selected to have a pair of siblings of comparable age and gender to the adoptive sibling pairs. Eligibility requirements for the adoptive families included having, at the time of the intake assessment, an adopted adolescent between the ages of 11 and 21 who had been placed permanently in the adoptive home prior to the age of 2 years and a second adolescent in the home who was not biologically related to the adopted adolescent and who was no more than 5 years different in age. The second child could be biologically related to one or both of the parents or could, like the first child, have been adopted and placed prior to the age of 2 years. Additional eligibility requirements, which applied to all subjects, included living within a day drive of the University of Minnesota laboratory and not having any physical or mental disability that would preclude completing the day-long, in-person intake assessment. To be included in this analysis, the subject must additionally have been willing to make a blood or saliva donation.