is unlikely because when we examined all forms of DA in both twins and all full siblings, we estimated the “special twin environment” – which would cause excess resemblance DZ pairs compared to all siblings – to account for only 3% of the variance in liability (Kendler et al., 2013a). Prior evidence for analyses of DA in Swedish siblings suggests that this effect would be still smaller in siblings quite close to one another in age (Kendler et al., 2013a, 2013b).
