D. Twin data document that most of this heritable influence is not substance-specific, but provides “higher order” pharmacogenomics We are fortunate to have data from studies of identical vs fraternal twin pairs that evaluate the degree to which one twin’s dependence on a substance enhances the chance that his or her co-twin will become dependent on a substance of a different class. Results of these analyses document that most of the genetic influences on addiction vulnerability are common to dependence on multiple different substances, though others do appear to be substance-specific [46, 53, 54].
