Epidemiological studies strongly suggest that genetic factors operate at all steps of addiction, including vulnerability to initiation, continued use, and propensity to become dependent (Box 1). Twin studies have been popular to investigate the relative contributions of genetic and environmental factors. In addition to estimating genetic liability, twin studies provide information about environmental contributions, including the availability of and exposure to a substance, and shared and unique environments. The genetic influence on addiction has proved to be substantial, and HERITABILITIES for most addictive disorders are moderate to high.
