Twenty five years ago two young professors at the Institute for Behavioral Genetics began a longitudinal adoption study in Colorado.  However, John Defries and Robert Plomin probably did not imagine that a quarter century later they would be presiding over an ongoing study which has enrolled more than 2,400 individuals, tracking the development of children from infancy to adulthood.  At the time, they thought a prospective adoption study of normal development could resolve many questions about the relative roles of cultural and biological influence, but were uncertain about cooperation from adoption agencies and the likelihood of funding.  Fortunately, the director of one of Denver's agencies was enthusiastic about participating in the study and the University of Colorado provided some initial financial support.  During the ensuing 25 years the Colorado Adoption Project (CAP) has been supported by multiple grants from the William T. Grant Foundation , the Spencer Foundation , the National Science Foundation, and the National Institutes of Mental Health , Child Health and Human Development , and Drug Abuse . 

In 1976, CAP staff began recruiting birth mothers (and fathers if possible) through Denver Catholic and Lutheran Social Services, the two largest adoption agencies in the Rocky Mountain region.  These parents completed a three-hour battery of psychological measures, many of which were based on an earlier large-scale study of parents and children, the Hawaii Family Study of Cognition.  In order to avoid influencing the adoption process, adoptive parents were approached by the agency social workers after the adoption was complete.  The willing adoptive parents then completed the same measures as the birth parents.  A control group was recruited through area hospitals and matched on the basis of family demographics.  The first assessments of the children were begun in 1977 when the oldest CAP probands were one-year olds. 

Annual home visits were scheduled for all CAP probands and their younger siblings through age four.  Follow-up phone calls to the parents kept the investigators in touch with the families at ages five and six.  Then in 1983, we began testing children in the laboratory after they had completed first grade.  During the next stage of the CAP, a novel assessment was developed to conduct tests over the phone with both parents and the children at ages 9, 10, and 11.  Although experimental at the time, this method has been very successful, enabling the investigators to collect useful data in a way that is both cost-effective and less burdensome for participating families.  The children were then tested in the laboratory at 12 years and again by phone at ages 13, 14, and 15.  Perhaps most importantly, the three-hour test battery previously administered to the biological, adoptive, and control parents was administered to the probands at age 16 as well as to all of their siblings, both younger and older.  Thus, the CAP had come full circle, with children completing in late adolescence the same test battery their parents had completed 16 years earlier. 

To provide only one example of the many results that have been obtained from the CAP, in a paper ("Nature, nurture, and cognitive development from 1-16 years:  A parent-offspring adoption study") published in 1997 in Psychological Science, Plomin, Fulker, Corley and DeFries reported that children increasingly resemble their parents in cognitive abilities from infancy through adolescence.  Adopted children resemble their adoptive parents slightly in early childhood but not at all in middle childhood or adolescence.  In contrast, during childhood and adolescence, adopted children become more like their biological parents and to the same degree as children and parents in nonadoptive families.  These findings indicate that genes that affect cognitive abilities in adulthood do not all come into play until after adolescence and that environmental factors that contribute to differences in cognitive development are not correlated with parents' cognitive ability. 

The project would undoubtedly be regarded as a landmark study in child development if it had ended at this point.  However, data collection in new domains and at older ages is ongoing.  Via telephone we interview the CAP adolescents on substance experimentation and use, and conduct detailed in-person interviews regarding psychological development at ages 17 and 21.  Recently, Avshalom Caspi , a renowned social demographer at the Institute of Psychiatry in London, has become a collaborator and leads a new component of the study in which we conduct telephone interview regarding adult life-choices at ages 18-30, again with both probands and all of their siblings. 

The founding investigators were ably assisted in the early stages by Steven Vandenberg who had previously initiated the Louisville Twin Study. In 1983, David Fulker joined the faculty at IBG and the CAP team.  John Hewitt became a co-investigator in 1992 and now directs some components of the study, as does Robin Corley who has also been the CAP data manager since 1987.  A diverse group of researchers continues to analyze this rich data set, both by exploring data collected many years ago in new ways and by contemporary analysis of current data.  Today, 25 years after its beginning, the CAP is a remarkably productive project with over 150 publications, including three volumes on the CAP, chapters in edited books, and articles in peer-reviewed journals on topics ranging from the first "Assortative mating by unwed biological parents of adopted children," (Plomin, DeFries, & Roberts) published in Science in 1977 to the recent "Genetics of cognitive abilities and disabilities," (Plomin & Defries) published in Scientific American in 1998.  

Sally Ann Rhea
2001
Colorado Adoption Project Coordinator 
1984 to present

return to top