The wealth of data on inbred strains in particular has enabled the use of multivariate approaches to understanding drinking (see Phillips and Belknap 2002). Trait (phenotype) data collected on a dozen or more inbred strains are valuable for exploring common genetic influences among multiple traits by correlational and multivariate analyses. Parallel patterns of strain differences among traits indicate common genetic influences which can implicate common genetically-mediated mechanisms for further investigation. In a recent meta-analysis (Belknap et al. 2008), we analyzed genetic commonalities across four different drugs subject to abuse (ethanol, morphine, pentobarbital, and diazepam) tested on 14–15 standard inbred mouse strains for a series of traits appropriate for all four drugs across multiple doses. A correlation matrix based on strain means was first constructed (bivariate analysis), followed by multivariate analyses based on clusters of correlated traits as the unit of analysis. Multidimensional scaling and cluster analysis showed that for two-bottle choice preference drinking, morphine and ethanol were similar in their pattern of strain differences, but different from diazepam and pentobarbital. This finding was not anticipated based on the pharmacological classes
