E. Kay Trimberger Ph.D.

Adoption Diaries


What Behavioral Genetics Teaches Us About Adoption

How nature and nurture interact.

Posted Aug 02, 2020

Behavioral genetics was founded as an interdisciplinary field in the 1970s to separate itself from any connection with genetic determinism or with eugenics. (Eugenics aspired to improve humanity through selective breeding, which led to racist and anti-Semitic policies.) The founders of behavioral genetics were mainly quantitative psychologists with no training in genetic science. That is changing now. They did not study group differences like race or ethnicity, and rarely gender. Rather, they looked only at differences in individual development and looked at both genetic and environmental forces and how they interact. They considered the environment both inside and outside the family.

What was most important to me was that they used adoptive families for their studies, comparing siblings (both adoptive and biological) within the same family and comparing families formed by adoption to biological families of the same ethnicity and social class background (called a control group). Twins separated through adoption were part of these studies done at universities in Minnesota, Colorado, Texas, Pennsylvania, Oregon, and others.

These were ethical studies, not like the secret studies described in the documentary Three Identical Strangers. All participants in behavioral genetics studies were volunteers and many stayed in the studies for 20 to 30 years. As they became teenagers and adults, the adoptees were given yearly reports on the findings. In Creole Son, I use specific studies that were relevant to the issues my son and I faced, and I integrate their findings with my experience.

For these researchers, adoption was a way to separate the impact of nature and nurture and to understand more about individual development over time. Until recently, they were not interested in adoption theory or practice. The main criticism of these studies focuses on the admitted fact that they find correlations, not causes, and some correlations are not very strong. A second major question raised about these studies is whether you can generalize from adopted families to the whole population. Here, I didn’t care, because I was only interested in what I could learn about adoption, especially my own; I wanted to create a model that those involved in adoption could use for personal understanding.

My favorite study is one that was done at the University of Colorado Adoption Project, which started in the 1970s and continued for more than 20 years. Using 13 tests of cognitive abilities, the researchers picked 245 adoptees given up at (or near) birth. They gave these tests to the birth mothers and adoptive parents, along with a control group of biological families. As the adoptees were growing up, they tested them at various ages and when they were 16, the adoptees, adoptive parents, and control group were given the same tests as their birth mothers had been given at about the same age.

The results were counterintuitive. The adoptees only had similarities with the adoptive parents in the first four years. By age 16, the adoptees had no similarities to the adoptive parents, but had developed moderate resemblance to the birth mothers, comparable to that of children who grew up in a biological family. The researchers concluded that environmental transmission from parents to offspring had little effect on later cognitive ability. A Texas study found a similar pattern in psychological characteristics, but the correlation was not as high.

This study gave me some insight into why the young boy with whom I felt so close and in tune with deviated from me as a teenager and beyond, making choices that were so different from mine. These findings mean that adoption educators should counsel parents to expect these differences. Many biological parents have the same experience, but it is more prevalent in adoptive families. One can still love a child and appreciate some of such differences, many of which may be positive, but adoptive parents cannot assume that the child will make choices or have personal characteristics like others in their family.*

*An earlier version of this post was published as part of an interview with Bella DePaulo on her blog on PsychCentral, Single at Heart.