This article examines the relationship between birth masculinity and socio-economic levels in China. Both 2000 and 2005 data suggest the presence of a non-linear relationship between the sex ratio at birth and socio-economic status, with a lower sex ratio at birth observed among both the poorest and the richest households. This inverted-U pattern is significantly different from what is observed in India and what has been assumed previously for China. Multivariate analyses indicate that this pattern persists after the introduction of several other covariates of birth masculinity such as ethnicity, fertility, migration status, age or parity. These results suggest that further economic advances and socio-economic mobility could contribute to the return to normalcy of the sex ratio at birth.