AbstractIn most mammals, including humans, the postnatal acquisition of normal social and nonsocial behavior critically depends on interactions with peers. Here we explore the possibility that mixed-group housing of mice carrying a deletion ofNlgn3, a gene associated with autism spectrum disorders, and their wild-type littermates induces changes in each other’s behavior. We have found that, when raised together, maleNlgn3knockout mice and their wild-type littermates displayed deficits in sociability. Moreover, social submission in adult maleNlgn3knockout mice correlated with an increase in their anxiety. Re-expression ofNlgn3in parvalbumin-expressing cells in transgenic animals rescued their social behavior and alleviated the phenotype of their wild-type littermates, further indicating that the social behavior ofNlgn3knockout mice has a direct and measurable impact on wild-type animals’ behavior. Finally, we showed that, unlike male mice, female mice lackingNlgn3were insensitive to their peers’ behavior but modified the social behavior of their littermates. Altogether, our findings show that the environment is a critical factor in the development of behavioral phenotypes in transgenic and wild-type mice. In addition, these results reveal that the social environment has a sexually dimorphic effect on the behavior of mice lackingNlgn3, being more influential in males than females.