Two divergent perspectives have been articulated in the literature regarding the effect that an unbalanced sex ratio is speculated to have on male-on-female intimate partner violence. Evolutionary psychology proffers that a high sex ratio (i.e., more men than women in the population) propagates competition among males for female mates. This competition for female mates is thought to engender sexual jealousy among men, which in turn results in male-on-female intimate partner violence. In contrast, the Guttentag and Secord thesis argues that a high sex ratio acts to attenuate rather than amplify male-on-female intimate partner violence because the relatively small number of women in the population makes them more highly valued and respected by men. Using data culled from the Federal Bureau of Investigation's National Incident-Based Reporting System (NIBRS) and the Census, we investigate the relationship between the sex ratio and male-on-female intimate partner violence. We define male-on-female intimate partner violence as violence occurring within a marriage or boyfriend/girlfriend type relationship. Multivariate regression results furnish evidence supporting evolutionary psychology by demonstrating that a high sex ratio increases male-onfemale intimate partner violence. Results also show that male-on-female intimate partner violence is higher in cities where more women work. Such a finding further buttresses the logic associated with evolutionary psychology because participation in the workforce is theorized to afford a woman a greater opportunity to meet and interact with men other than her husband or boyfriend.