Running Head: GENETICS OF IN-GROUP FAVORITISM

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this Abstract Objective: Research has shown that in-group favoritism is associated with concerns over the maintenance of social norms. Here we present two studies examining whether genetic factors underpin this association. Methods: A classical twin design was used to decompose phenotypic variance into genetic and environmental components in two studies. Study 1 used 812 pairs of adult US twins from the nationally representative MIDUS II sample. Study 2 used 707 pairs of middle-age twins from the Minnesota Twin Registry. In-group favoritism was measured with scales tapping preferences for in-group (vs. out-group) individuals; norm concerns were measured with the MPQ-traditionalism (Study 1) and right-wing authoritarianism (RWA: Study 2) scales. Results: In Study 1, heritable effects underlying traditionalism were moderately (c. 35%) overlapping with the genetic variance underpinning in-group favoritism. In Study 2, heritable influences on RWA were entirely shared with the heritable effects on in-group favoritism. Moreover, we observed that Big Five openness shared common genetic links to both RWA and in-group favoritism. Conclusion: These results suggest that, at the genetic level, in-group favoritism is linked with a system concerned with concern over normative social practices, which is, in turn, partially associated with trait openness.