There have been two prospective full parent-offspring adoption studies that have followed children from around the time of birth for a decade or more. The first and landmark parent-child longitudinal adoption study of this type was the Colorado Adoption Project (CAP; Plomin & DeFries, 1983). Initiated in 1975, infants were placed with adoptive families when they were an average age of 29 days old. The final parent sample includes 286 birth mothers, 60 birth fathers, 242 adoptive mothers, and 237 adoptive fathers. In addition, a one-to-one matched control group of nonadoptive parents rearing their biological children was ascertained. Adoptees in CAP have been assessed at regular intervals from infancy into adulthood. The study has yielded evidence of increasing heritability on cognitive abilities from early childhood to adolescence (Rhea et al., 2013) and GxE interactions on social behaviors during later childhood (e.g., Hershberger, 1994) and has been instrumental in furthering the understanding of the role of the environment on development.
