Interestingly, the sex differences we observed were not just in the mean trait values of the two sexes within and across strains. Several sex differences were found in the heritability of traits, indicative of increased within strain variation relative to between strain variations. This increase in variance was not limited to females, for whom the estrous cycle is a source of variation in brain and behavior, including drug abuse (Becker & Hu 2008; Becker & Ramirez 1981; Becker et al. 1982). The estrous cycle is sometimes but not always a source of sex differences in behavior (Mogil et al. 2000; Sternberg et al. 2001). For some neurobehavioral phenotypes, males have a higher variability than females. This is often attributed to the social stress involved in the formation of dominance hierarchies. For example, social stressors related to fighting, such as the resident intruder paradigm have been shown to affect neurogenesis (Mitra et al. 2006) and cocaine-induced conditioned place preference (Mclaughlin et al. 2006). The sexes have also been shown to be differentially susceptible to effects of housing density and social isolation
