twin samples are drawn and the outcomes which are modeled. The samples in the twin studies are drawn from the general population, not from the densely affected families which form the bulk of the sample used here. Thus genetic effects will be more difficult to find in the twin studies, particularly for the rarer, more genetically affected conditions. In a number of studies outcome definitions are broad, and are not subject to as strong genetic effect as more restricted outcomes such as alcohol dependence or externalizing disorders. The most dramatic example of this is the difference between the cross-sectional results from the Minnesota twin studies (Iacono et al., 2003; Hicks et al., 2007) in which the outcomes are narrowly defined and the cross-sectional results from a Dutch twin study (van Beek et al., 2012) with the very broad outcome of having one or more alcohol abuse symptoms. The Minnesota twin studies have A > 0.6 for ages 11 and 17, while the Dutch twin study has A < 0.3 for ages 15–17 and 18–20, where A is the additive genetic effect.
