It is important to note that the objectives of the twin studies considered here (Rose et al., 2001a, b; Iacono et al., 2003; Rose et al., 2004; Pagan et al., 2006; Hicks et al., 2007; Kendler et al., 2008; van Beek et al., 2012; Baker et al., 2011) and of this study are quite different. The twin studies investigate the presence of a “disease” condition, although exactly which condition varies considerably among studies. (Of the outcomes in the nine twin studies cited above, four had drinking amounts, either as frequency amounts or levels without consideration of abuse symptoms; four had alcohol abuse symptoms, one as a count variable and the others as binary, and one had intoxication levels.) The objective of this study, as a survival analysis, is to analyze the factors contributing to an event, the onset of a condition. Once the condition has come to pass, it is not of further interest in survival analysis. The genetic effects which produce the condition are only significant at the onset of the condition, and their effects persist only if the
