Inbred strains of mice, such as C57Bl/6, have become preferred animal models for neurobehavioral studies. A main goal in creating inbred lines is to reduce the effects of individual genetic variation on observed phenotypes. Most studies use only males, and there is increasing evidence that agonistic interactions within the home cage may produce systematic variability in behavior and brain function. Previous studies have demonstrated that the outcomes of aggressive interactions have powerful effects on the brain and behavior, but less is known about whether aggressive interactions within the home cage have similar effects. We assessed group-housed laboratory mice C57Bl/6 for competitive ability and then tested the extent high competitive ability (CA) or low CA was related to gene and protein expression within related pathways. We focused on a broad social behavior network, including the nucleus accumbens (NAc) and bed nucleus of the stria terminalis (BNST). High CA mice had significantly more corticotropin releasing hormone receptor 2 (CRHR2) and estrogen receptor alpha (ESR1) mRNA in the BNST. Our data suggest a simple test of CA could yield valuable information that could be used to reduce error variance and increase power in neurobiological studies using mice.