Founded in family systems theory, the spillover hypothesis suggests that marital relationships are linked to parenting. The current study used meta-analysis to extend this literature and examine links between marital positivity and marital negativity with absolute levels of parental differential treatment (PDT) of siblings. Multilevel data included 2575 effect sizes nested within 45 sources (articles/raw datasets), nested within 12 unique samples. Lower marital positivity and greater marital negativity were linked to greater differences in treatment. Those main effects, however, were moderated by several source and effect size characteristics. For example, links were stronger when effect sizes were based on differential positive interactions. Caution is warranted in most cases; effect sizes were generally small. Findings, however, suggest that parents should be aware of how they may treat their children differently in connection with poorer marital relationships. Future research on PDT should include a greater focus on the domains of differential treatment.K E Y W O R D S differential treatment, family systems spillover, favoritism, marital quality and relationships, parenting, siblings According to family systems theory, interactions, experiences, and relationships within one unit of the family influence other parts of the family system (Cox & Paley, 1997). Based on this theoretical notion, the overarching goal of the current study was to examine the link between