<p>Criminological research has identified a robust sex gap in offending and this gap extends to affiliation with peers involved in antisocial behavior. Despite this, there is a dearth of research which has identified mediating mechanisms underlying these sex differences in deviant peer association. With prior research identifying sex variation in cognitive development, differential development pertaining to the dual systems model may have relevance here. This study examined sex differences in deviant peer association and tested for mediation effects of dual systems imbalance. Data from the Adolescent Brain Cognitive Development study were analyzed. Structural equation modeling was used to test for direct and indirect effects of interest. Results indicated that male participants reported greater deviant peer association scores than female participants. This effect was significantly mediated by differences in dual systems imbalance, with this imbalance accounting for about 70% of the direct effect of biological sex on deviant peer association.</p>