Adolescent Drinking Behavior and Genes : The FinnTwin Studies

by National Institute of Health

(Page 2 of 5)

Longitudinal studies using genetically informative samples enable researchers to assess the magnitude of genetic and environmental influences and their interplay across development for a variety of behaviors. Therefore, these research designs also are particularly useful in understanding the development of drinking behaviors. The most widely used design is a longitudinal/ developmental study of child or adolescent twins and their parents. Several research groups in the United States and other countries are engaged in developmental behavior-genetic research of this kind, and many of these groups explore risk factors for alcoholism as part of their research aims. This article focuses on two longitudinal studies of young Finnish twins and their parents known as the FinnTwin studies.

Finland is particularly suitable for conducting longitudinal twin studies because of the availability of comprehensive birth records, the ease of locating twins as they grow up, and the high level of compliance of organizations and individuals with research requests.

To acquire a large sample of twins that is representative of the entire population, it is almost imperative that investigators have access to a comprehensive birth registry for the region or country under investigation. However, many countries, including the United States, do not have such registries. Some researchers in the United States have attempted to obtain comprehensive data sets by identifying all twin students enrolled in a large public school system or by systematically accessing birth records within a State and then searching for the current addresses of the twins so identified.

In the Nordic countries, in contrast, the entire population is enrolled at birth into a centralized population registry. In Finland, every newborn citizen is given a unique personal identification number that includes the person's date of birth and a link to the biological mother. Accordingly, researchers who have approval to access the registry can identify all people born on the same day to the same mother, thereby identifying the nation's entire population of twins and other multiples. Moreover, Finnish law requires all citizens to report all residential moves in a timely manner, which means that families of twins can be readily located and, in principle, followed up without loss. Finally, for cultural and historic reasons, compliance with requests for research participation is unusually high in the Nordic countries. This high compliance rate and the requirement to report residential moves make it possible to conduct followup assessments with large numbers of subjects, which otherwise presents a particular challenge when researching drinking behavior and its consequences.

Design of the FinnTwin Studies

The two FinnTwin studies differ in the age groups they study and in their followup intervals. One study, FinnTwin, includes all twins born in five consecutive birth cohorts, 1975 through 1979, who were enrolled in the study when they reached age 16. Systematic followup surveys of these twins were completed at ages 17 and 18.5 and at one point when the twins were between ages 22 and 25. The second study, FinnTwin, identified all twins in the five consecutive birth cohorts 1983 through 1987 who were enrolled at age 11 or 12, with followup assessments at ages 14 and 17.

Each FinnTwin study includes about 2,500 pairs of twins, with equal proportions of brother - sister, brother - brother, and sister - sister twin pairs. Among the same-sex pairs, about half are identical, or monozygotic twins and half are fraternal, or dizygotic twins. MZ twins develop when a single egg is fertilized by a single sperm, and the resulting embryo divides shortly after fertilization to produce two individuals who share all their genes. DZ twins, in contrast, develop when two egg cells are independently fertilized by separate sperm at about the same time, resulting in the development of two individuals who, like ordinary siblings, share, on average, half of their genes.

The fundamental approach in twin study research is to compare the extent to which MZ twins resemble each other in relation to a specific trait or behavior with the resemblance between DZ twins for the same trait or behavior. In MZ twins, the genetic influences on that trait or behavior are identical because the co-twins share all their genes. In DZ twins, however, the genetic influences can differ because the co-twins share only about 50 percent of their genes. With respect to environmental influences, there should be no substantial differences between MZ and DZ twins because, in childhood and adolescence, all or nearly all twin pairs live with one another and with the same biological parent or parents. Therefore, both MZ and DZ twins will share equally important aspects of their household experience, peers, neighborhood, school, and other environmental factors that may enhance or reduce the risk for early alcohol use and abuse.

Based on these premises regarding genetic and environmental influences, the following expectations can be articulated: if genetic factors are primarily important in the development of a measured trait or behavior, MZ co-twins will resemble one another about twice as much as do DZ co-twins. If genetic factors are of only modest importance and shared environmental influences are predominant, the two kinds of twins will show comparable resemblance for the trait or behavior studied, because both share the environmental influences relevant to that outcome.

The following sections summarize some of the findings of the FinnTwin studies regarding the contributions of various genetic and environmental factors to a variety of drinking-related behaviors. In general, the results from the FinnTwin studies have been consistent with ongoing research with adolescent twins in other European countries as well as in the United States.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.