Genomic imprinting, the epigenetic process by which transcription occurs from a single parental allele, is believed to influence social behaviors in mammals. An important social behavior is group living, which is enriched in Eutherian mammals relative to monotremes and marsupials. Group living facilitates resource acquisition, defense of territory and co-care of young, but requires a stable social group with complex inter-individual relationships. Co-occurring with increased group living in Eutherians is an increase in the number of imprinted loci, including that spanning the maternally expressedCdkn1c. Using a 'loss-of-imprinting' model ofCdkn1c(Cdkn1cBACx1), we demonstrated that twofold over expression ofCdkn1cresults in abnormal social behaviors. Although, our previous work indicated that maleCdkn1cBACx1mice were more dominant as measured by tube test encounters with unfamiliar wild-type (WT) males. Building upon this work, using more ecologically relevant assessments of social dominance, indicated that within their normal social group,Cdkn1cBACx1mice did not occupy higher ranking positions. Nevertheless, we find that presence ofCdkn1cBACx1animals within a group leads to instability of the normal social hierarchy, as indicated by greater variability in social rank within the group over time and an increase in territorial behavior in WT cage-mates. Consequently, these abnormal behaviors led to an increased incidence of fighting and wounding within the group. Taken together these data indicate that normal expression ofCdkn1cis required for maintaining stability of the social group and suggests that the acquisition of monoallelic expression ofCdkn1cmay have enhanced social behavior in Eutherian mammals to facilitate group living.