Humans are biologically similar, sharing almost all of their genetic material. Many questions relevant to alcoholism,(1) however, concern how people differ. Why do some people abuse alcohol? Why do some heavy drinkers, but not others, become physiologically addicted to alcohol? Twins are a unique resource for identifying the genetic and environmental sources of these differences. The study of the causes of differences (i.e., variation) among people is a major goal of human behavioral genetic research, of which twin studies form a subset.
Twin studies have been used to address important questions about the causes of alcoholism, including the following:
* How important to the development and course of alcoholism are genetic and environmental influences?
* Do genetic influences differ in importance for different groups of people (e.g., males and females or different age groups)?
Based on partial answers from these investigations, new twin studies are seeking answers to even more specific questions, such as the following:
* Do the same sets of genes (or the same environments) produce the same effects in males and females?
* What environmental characteristics are most hazardous for people at high genetic risk for developing alcoholism?
* What environmental characteristics pvotect against alcoholism among people who have a strong family history of the disorder?
* To what degree is the frequently seen overlap between alcoholism and other disorders (such as depression and anxiety) a result of the same sets of genes influencing both disorders?
These questions have important implications for understanding the causes of alcoholism and ultimately providing knowledge to help design treatment and prevention strategies for people at high risk of developing the disorder. This article describes how twin studies are designed, offers examples of research questions for which twin studies are useful, and reviews their limitations.
THE TWIN STUDY DESIGN
The Value of Twins
Because of their unique genetic status, twins play a valuable role in teasing apart genetic and environmental influences on people's development of alcoholism. Genes are segments of DNA that form the blueprints for the development of the human body. Genetic expression that influences behavior is only partly preprogrammed; the biological processes regulated by genes are modified in complex ways by experiences with the environment. Thus, neither genes nor experiences operate independently of one another. In this sense, the often-cited dichotomy of "genes versus environment" is false; both are required for the development and expression of human characteristics. Knowledge of how genetic and environmental factors act both separately and together to influence the development of alcoholism can inform strategies (e.g., biological, individual, or societal) for intervention and prevention.
When researchers study genetic influences on behavioral variation, they focus on the small percentage of genes that differ among people. Fraternal, or dizygotic (DZ), twin pairs, like ordinary siblings, have, on average, 50 percent of these genes in common, whereas identical, or monozygotic (MZ), twin pairs have 100 percent of these genes in common. By comparing MZ with DZ twin pairs, researchers can use this difference in the twins' degrees of genetic resemblance to estimate the relative importance of genetic and environmental influences on the development and course of disorders such as alcoholism.
When searching for genetic and environmental influences on alcoholism, researchers can only observe a characteristic in a subject, such as the presence or absence of the clinical diagnosis of alcoholism. Researchers, however, are actually trying to estimate the contribution of genetic and environmental effects to the subject's liability for alcoholism (the theoretical components contributing to the risk for alcoholism are portrayed in figure 1). …