Crime rates almost doubled in China between 1992 and 2004. Over the same period, sex ratios (males to females) in the crime-prone ages of 16-25 years rose sharply, from 1.053 to 1.093. Although scarcity of females is commonly believed to be a source of male antisocial behavior, a causal link has been difficult to establish. Sex-ratio variation is typically either small or related to social conditions liable to also affect crime rates. This paper exploits two unique features of the Chinese experience: the change in the sex ratio was both large and mainly in response to the implementation of the one-child policy. Using annual province-level data covering the years 1988-2004, we find that a 0.01 increase in the sex ratio raised the violent and property crime rates by some 5-6%, suggesting that the increasing maleness of the young adult population may account for as much as a third of the overall rise in crime.