Adoption studies stand as a cornerstone in understanding the intricate interplay of genetics and environment, often referred to as the nature versus nurture debate, in shaping human traits and behaviors. These pioneering research methodologies offer unique insights by dissecting the influences of biological inheritance and upbringing. But What Do Adoption Studies Compare exactly? At their core, they compare individuals raised apart from their biological families, allowing researchers to disentangle the contributions of genes and environment in a way that traditional family studies cannot.
The essence of adoption studies lies in contrasting outcomes among different groups of people. Primarily, these studies compare adopted individuals to both their biological and adoptive families. This triangulation is crucial for several reasons. By examining traits in adopted individuals and comparing them to their biological parents (with whom they share genes but not environment), researchers can gauge the strength of genetic influence. Conversely, by comparing adopted individuals to their adoptive parents and siblings (with whom they share environment but not genes), they can assess the impact of shared upbringing and environmental factors.
A landmark example of this approach is evident in early research investigating the genetic predisposition to alcoholism. A study conducted in Copenhagen, Denmark, by Goodwin and colleagues in the 1970s, exemplifies how adoption studies illuminate the nature versus nurture dynamic in complex human conditions. This study serves as an ideal illustration to understand what adoption studies compare in practice.
In the Copenhagen study, researchers meticulously utilized Danish official registries to identify biological parents with a history of alcoholism who had relinquished their children for adoption shortly after birth. They also established a control group of biological parents without known alcoholism histories who had also given up children for adoption. The crucial element was to follow up with the adult sons and daughters of both groups, all raised by adoptive parents unrelated to their biological families. The central question was: if genes play a significant role in alcoholism, would the adopted-away offspring of alcoholic biological parents exhibit higher rates of alcoholism compared to those adopted away from non-alcoholic biological parents?
Furthermore, the Copenhagen study incorporated an ingenious design element to directly assess environmental influence. The researchers identified a subset of biological parents who had given up one child for adoption but had raised another child themselves. This unique subgroup allowed for a direct comparison between siblings with the same genetic predisposition but differing environmental exposures. If the environment of an alcoholic parent significantly elevates alcoholism risk, then the non-adopted children reared by alcoholic parents should demonstrate a higher risk compared to their adopted-away siblings raised by non-alcoholic adoptive parents. This comparison directly addresses the question of what adoption studies compare regarding environmental impact.
The findings of the Copenhagen study revealed compelling insights. Notably, 8.9% of biological fathers and 1.6% of biological mothers who placed their children for adoption had a history of hospitalization for alcoholism. This prevalence was significantly higher than the estimated 2% for men and 0.5% for women in the general Copenhagen population of the same age range who had been hospitalized for alcoholism. This elevated baseline risk within the adoptee population itself is a critical consideration when interpreting adoption study results.
Goodwin and colleagues reported a striking result: adopted-away sons of alcoholic biological parents showed a 3.6 times higher risk of alcoholism compared to adopted-away sons of non-alcoholic biological parents (control group). Interestingly, non-adopted sons raised by alcoholic parents exhibited a comparable 3.4 times increased risk. In contrast, the findings for daughters were less pronounced and statistically non-significant in the initial analysis. However, when considering lifetime prevalence estimates for alcoholism in women, the risk ratios for daughters of alcoholics, both adopted and non-adopted, became more notable, suggesting a potential, albeit weaker, genetic influence in women as well.
From these findings, several key conclusions emerged. Firstly, the Copenhagen adoption study provided substantial evidence for a genetic influence on alcoholism risk, particularly in men. The study design, by comparing adopted-away offspring of alcoholic and non-alcoholic parents, effectively isolated genetic factors from the postnatal environment. Secondly, the study offered limited evidence for a significant independent environmental impact of being raised by an alcoholic parent. The risk for non-adopted sons was not significantly greater than for adopted-away sons, suggesting that genetic predisposition was a stronger driver of alcoholism risk in this cohort. However, the researchers acknowledged that with the sample sizes used, moderate environmental influences could still remain undetected. Therefore, what adoption studies compare in this context are primarily the genetic predispositions inherited from biological parents versus the environmental influences of adoptive (or lack thereof in biological families) upbringing.
It is crucial to acknowledge the limitations inherent in adoption studies, as recognized by the original researchers. One potential confounder is selective placement, where adoption agencies might unintentionally place children of biological parents with certain traits (like alcoholism) into adoptive homes with similar characteristics. In the Copenhagen study, the absence of direct interviews with adoptive parents meant that the possibility of selective placement could not be entirely ruled out. If selective placement were prevalent (i.e., children of alcoholic parents were more likely to be adopted by families with alcohol problems), it could inflate the apparent genetic effect. However, the researchers noted that family history reports from the adoptees themselves suggested that alcohol problems were, if anything, less prevalent in the adoptive fathers of sons of alcoholics compared to the control group, making selective placement an unlikely explanation for the observed findings. Understanding these limitations is crucial when interpreting what adoption studies compare and the conclusions that can be drawn.
In conclusion, adoption studies offer a powerful methodological approach to disentangling the complex contributions of nature and nurture to human traits. By strategically comparing adopted individuals to their biological and adoptive families, researchers can isolate and quantify the relative influences of genes and environment. The Copenhagen adoption study on alcoholism serves as a seminal example of how these studies are designed and what critical comparisons they facilitate. They allow us to explore what adoption studies compare: the distinct yet intertwined roles of inherited predispositions and environmental factors in shaping who we are. Through rigorous methodologies and careful interpretation, adoption studies continue to provide invaluable insights into the enduring question of nature versus nurture.
References
- Goodwin, D. W., Schulsinger, F., Hermansen, L., Guze, S. B., & Winokur, G. (1973). Alcohol problems in adoptees raised apart from alcoholic biological parents. Archives of General Psychiatry, 28(2), 238–243.
- Goodwin, D. W., Schulsinger, F., Moller, N., Hermansen, L., Winokur, G., & Guze, S. B. (1974). Drinking problems in adopted and nonadopted sons of alcoholics. Archives of General Psychiatry, 31(2), 164–169.
- Goodwin, D. W., Knop, J., Hermansen, L., & et al. (1977a). Alcoholism and adoption studies in humans. In M. Roff, L. Robins, & M. Pollack (Eds.), Psychopathology: Contributions from the biological, behavioral and social sciences (pp. 127–147). New York: Raven Press.
- Goodwin, D. W., Knop, J., Hermansen, L., & et al. (1977b). Alcoholism and adopted-away daughters of alcoholics. Archives of General Psychiatry, 34(9), 1005–1009.
- Heath, A. C., Bucholz, K. K., Madden, P. A. F., Dinwiddie, S. H., Slutske, W. S., Bierut, L. J., Neuman, R. J., Baller, R., Nurnberger, J. I. Jr., & Reich, T. (in press). Genetic and environmental contributions to alcohol dependence risk in a national twin sample: Heavy drinking and DSM-IV criteria. Journal of Studies on Alcohol.
