Genetic and environmental correlations between measures of brain structure and use were computed to estimate the degree to which genetic and environmental effects are shared between the two measures. Additionally, standardized genetic covariances (covA) were also estimated to indicate the degree to which genetic and environmental effects contribute to the phenotypic correlation (rP). The sum of the standardized genetic and environmental covariances is equal to the phenotypic correlation. The standardized genetic covariance between two measures is defined as rx,y=Ax,y(Vx×Vy) where Axy is the genetic covariance between a measure of brain structure (x) and a measure of cigarette pack-years (y) and Vx and Vy represent the total covariances of x and y . The standardized environmental covariance (covE) is similarly defined using measures of unique environmental covariance.
