Two alternative hypotheses have been proposed to explain how social and genetic mating systems are interrelated in birds. According to the first (male trade-off) hypothesis, social polygyny should increase extrapair fertilizations because when males concentrate on attracting additional social mates, they cannot effectively protect females with whom the); have already paired from being sexually assaulted. According to the second (female choice) hypothesis, social polygyny should decrease extrapair fertilizations because a substantial proportion of females can pair with the male of their choice, and males can effectively) guard each mate during her fertile period. To discriminate these alternatives, we comprehensively reviewed information on social mating systems and extrapair fertilizations in temperate zone passerine birds. We found significant inverse relationships between proportions of socially polygynous males and frequencies of extrapair young, whether each species was considered as an independent data point (using parametric statistics) or phylogenetically related species were treated as nonindependent (using contrasts analyses). When social mating systems were dichotomized, extrapair chicks were twice as frequent in monogamous as in polygynous species (0.23 vs. 0.11). We hypothesize that in socially polygynous species, (1) there is less incentive for females and males to pursue extrapair matings and (2) females incur higher costs for sexual infidelity (e.g., due to physical retaliation or reduction of paternal efforts) than in socially monogamous species.