A recent study of Dutch twins (van Beek et al. 2012) is similar to the current report in that it aimed to clarify genetic influences on alcohol problems over time. The sample included individuals assessed from the age of 15 through to 32 years (ages were collapsed into six categories), who were administered the CAGE inventory (Dhalla & Kopec, 2007) to examine alcohol problems. As in the current study, van Beek et al. (2012) found that unique environmental influences were both stable and dynamic; that is, E factors remained relevant over time and were complemented by novel E influences coming ‘online’ at each subsequent age range. They also found that a single C factor, with forward transmission, influenced CAGE scores over time. However, in contrast to our identification of two significant genetic factors, van Beek et al. (2012) found that a single genetic factor, with no innovation over time, accounted for genetic influences on CAGE scores. Previous studies have indicated that alcohol consumption and problems are modestly to strongly genetically correlated in adulthood (Grant et al. 2009; Kendler et al.
