A multivariate Cholesky model was used to estimate genetic and environmental influences across the measures of consumption/problem drinking (Neale and Cardon, 1992). Analyses were conducted separately using the measures available in each sample. The Cholesky model allows us to evaluate (1) the magnitude of genetic and environmental influences on each phenotype and (2) the extent to which these influences contribute to the covariation between the phenotypes. Phenotypic variance was decomposed into 3 components: variance because of additive genetic factors (a2); variance because of shared environmental factors (c2); and variance because of nonshared environmental, or individual-specific, factors (e2). Calculation of variance accounted for by each of these factors is performed by comparing MZ twin correlations to DZ twin correlations. Genetic influences correlate 1.0 between MZ twins, who share all of their genetic variation identical-by-descent, and 0.5 between DZ twins, who share, on average, 50% of their segregating genes, as do ordinary siblings. Common/shared environmental effects, as defined in biometrical twin modeling, refer to all environmental influences that make siblings more similar to one another. By definition, these influences correlate 1.0 between
