The SIBS sample includes 409 adoptive and 208 non-adoptive families, each consisting of a pair of adolescent siblings and their rearing parents. Adoptive families were recruited from records of the three largest adoption agencies in Minnesota, and non-adoptive families were recruited from Minnesota birth records. At least one offspring in every adoptive family was not genetically related to other family members, but many adoptive families had a biological offspring of one or both of the rearing parents. All offspring in non-adoptive families are full biological siblings. Among those families that were eligible to participate, 63% of adoptive and 57% of non-adoptive families completed an intake assessment. There are minimal differences in socioeconomic status and in offspring mental health between participating and non-participating but eligible families (McGue et al., 2007). Unlike the MTFS, it was not logistically possible to link SIBS assessments with specific targeted ages. Rather, on average offspring in SIBS families were in mid-adolescence at intake, late-adolescence at their first follow-up and early adulthood at their second follow-up. The first follow-up of the SIBS sample is complete (with a 94.2% participation rate) and the second follow-up is in progress.
