Twin Study Design

Abstract

By studying human pairs of twins, researchers can learn the relative contributions of genetic and environmental factors to the development of alcoholism. Identical (i.e., monozygotic, or MZ) twins share 100 percent of their genes, whereas fraternal (i.e., dizygotic, or DZ) twins generally share only 50 percent of their genes. Using certain techniques and theoretical models, researchers can compare the two types of twin pairs for how often alcoholism occurs in both members of a twin pair. If alcoholism occurs more often in both members of MZ twins, genetic factors are implicated in the origin of the disorder. Twin research also has been applied to studies of differences between men and women in their genetic contribution to alcoholism.

Keywords: AOD dependence; applied research; environmental factors; gender differences; hereditary factors; twin study.

Model demonstrating how three factors determine twins’ resemblance for liability for alcoholism. The genetic makeup (A) of identical (monozygotic [MZ]) twin pairs (upper section) is correlated perfectly (represented as r_A = 1.0), whereas the genetic makeup of fraternal (dizygotic [DZ]) pairs (lower section) is correlated to only one-half the degree of MZ pairs (r_A = 0.5) for their genetic components. Because each set of twins was raised in the same environment, both types are correlated perfectly (r_C = 1.0) for common environmental components (C) and uncorrelated for individual, specific environmental components (E). Thus, resemblance for liability between DZ twins results less from genetic factors than does resemblance between MZ twins.

Figure 1

Components influencing a twin’s risk (i.e., liability) for developing alcoholism. Three theoretical components—additive genetic sources (A); common environmental sources (C), which twins in a pair share; and individual, specific environmental sources (E)—combine to create a person’s liability for developing alcoholism. In this example, liability is theoretical and can only be inferred from the observed clinical diagnosis.