Participants were members of the longitudinal Minnesota Twin and Family Study (MTFS)25 or Sibling Interaction and Behavior Study (SIBS)25,26, both of which utilized a family design that included the mother, father, and two siblings. Offspring from the MTFS were either monozygotic (MZ) or dizygotic (DZ) twins, while the SIBS included non-twin biological or adoptive offspring. The mean age of placement for the adoptive offspring was 4.7 months (SD = 3.4). The total sample included 1999 families: 902 families with MZ twins, 480 families with DZ twins, 208 families with two non-twin biological offspring, 124 families with one biological and one adoptive offspring, and 285 families with two adoptive offspring. All twin pairs were same-sex; non-twin biological and adoptive siblings were a mix of same-sex (61%) and opposite-sex (39%). The mean age was 44.0 years (SD = 6.2) for mothers (n = 1993), 45.4 years (SD = 8.2) for fathers (n = 1790), and 26.2 years (SD = 3.8, range 16.2 to 32.4) for offspring (n = 3877; 53.3% female) at their most recent assessment. Over 90% of the combined MTFS-SIBS sample reported European American ancestry, though a large number of the adoptive offspring reported Korean ancestry (n = 433).
