This paper uses survey and experimental data from prison inmates and comparable non-inmates to examine the drivers of rising criminality in China. Consistent with socio-biological research on other species, we find that China's high sex-ratios are associated with greater risk-taking and impatience amongst males. These underlying behavioral impacts explain some part of the increase in criminality. The primary avenue through which the sex-ratio increases crime, however, is the direct pressure on men to appear financially attractive in order to find a partner in the marriage market. These marriage market pressures result in a higher propensity to commit financially rewarding crimes.