The impact of socioeconomic status (SES) and genetic polymorphisms on individual differences for externalized behaviors have often been investigated separately in studies of children and adults. In a general population sample of 607 Italian preadolescents, we examined the independent and joint effects of SES and the dopamine receptor D4 (DRD4) and serotonin transporter linked promoter region (5-HTTLPR) polymorphisms upon rule-breaking and aggressive behaviors measured with the Child Behavior CheckList/6–18. We found evidence, which was based on both one locus and two-loci genotype analyses, that low SES and DRD4 long and 5-HTTLPR long alleles, both alone and in interaction, are associated with higher aggressive behavior scores. The effects were similar but more modest and limited to one locus genotype analyses for rule-breaking behavior. Consistent with studies that showed the effects of societal moderators on the heritability of externalized behaviors across different segments of the population, we suggest that diminished social constraints associated with low parental SES may act as enhancers of the genetic influence of specific DRD4 and 5-HTTLPR alleles over aggressive behaviors in preadolescence.