Fig. 1 depicts several possible outcomes in the co-twin control model. Overall, we expect individuals reporting maltreatment to have a higher risk for AAD than those unexposed to maltreatment. If childhood maltreatment directly leads to later AAD, individuals who experienced childhood maltreatment are expected to have higher prevalence of AAD compared with controls, in the discordant pairs as well as in the sample overall (model I, Fig. 1). If the childhood maltreatment–AAD association is non-causal, but arises because both are due to family environmental factors, the twin not exposed to maltreatment is expected to have equally elevated rates of AAD as his maltreated co-twin (i.e. OR 1.0) among MZ and DZ pairs. This is because the twin unexposed to maltreatment is still exposed to the aspects of the familial environment that predispose to the development of AAD (model II, Fig. 1). If the childhood maltreatment–AAD association arises because both are entirely due to genetic factors, rates of AAD in exposed individuals relative to unexposed individuals are expected to be equal among discordant MZ pairs (OR 1.0) but somewhat higher among
