In 20 years parts of China and India will have up to 15 percent more men than women, who could find themselves marginalized in society, a study says.

“The ratio of men to women in most populations is remarkably constant if left untouched. The tradition of son preference, however, has distorted these natural sex ratios in large parts of Asia and North Africa,” said Dr. Therese Hesketh of the University College London Institute of Child Health.

“Sex-selective abortion and discrimination in care practices for girls have led to higher female mortality.”

Although healthcare for women is generally improving, these advances have been offset by a huge increase in the use of sex-selective abortion, and there are now an estimated 80 million missing females in India and China alone, says Hesketh.

When there is a shortage of women, the women can “marry up” and inevitably the least desirable men have no marriage prospects, according to the study published in the published in the Proceedings of the National Academy of Sciences.

“This trend could lead to increased levels of antisocial behavior and violence, as gender is a well-established correlate of crime, and especially violent crime,” says Hesketh.