The scientific and social context of the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders -- Methodology used in the VATSPSUD -- Twinning and twin models -- Internalizing disorders -- Externalizing and substance use disorders -- Twin model assumptions -- Childhood experiences and risk for psychopathology -- Adult experiences and risk for psychopathology -- Sex differences -- Genetic and environmental influences on stability and change -- The genetics of what?: comorbidity, general versus specific effects, and risk indicators -- Three extensions of the twin model -- The genetics of the environment -- Mechanisms for genetic control of exposure to the environment -- Is the relation between environmental risk factors and psychiatric disorders causal? -- Genetic control of sensitivity to the environment -- Integrative models.