Mutant mice simulating human CNS disorders are used as models for therapeutic drug development. Drug evaluation requires a coherent correlation between behavioral phenotype and drug status. Variations in behavioral responses could mask such correlations, a problem highlighted by the three-site studies of Crabbe et al. (1999) and Wahlsten et al. (2003a). Factors contributing to variation are considered, focusing on differences between individual animals. Genetic differences due to minisatellite variation suggest that each mouse is genetically distinct. Effects during gestation, including maternal stress, influence later life behavior; while endocrine exchanges between fetus and parent, and between male and female fetuses dependent on intrauterine position, also contribute. Pre and perinatal nutrition and maternal attention also play a role. In adults, endocrine cyclicity in females is a recognized source of behavioral diversity. Notably, there is increasing recognition that groups of wild and laboratory mice have complex social structures, illustrated through consideration of Crowcroft (1966). Dominance status can markedly modify behavior in test paradigms addressing anxiety, locomotion and aggressiveness, to an extent comparable to mutation or drug status. Understanding how such effects amplify the behavioral spectrum displayed by otherwise identical animals will improve testing.