Violent crimes (murders, rapes, and assaults) are higher in countries with a relative scarcity of men according to research using both United Nations and INTERPOL data and controlling for economic development, income inequality, urbanization, population density, police presence, and drug trafficking (Barber, 2009a). This is an apparent contradiction given that males are more criminally violent and the author argues that the most plausible explanation is that there is more direct mating effort, and hence more violent crime, in countries having a relative scarcity of men (or a low sex ratio). Alternative explanations that are discussed and found wanting include cultural determinism. Causal links connecting the marriage market and violent crime include possible sociological, physiological, and developmental mechanisms that offer exciting prospects for future researchers. (Published Abstract)