In the paper, published in the Proceedings of the National Academy of Sciences (PNAS), Dr Therese Hesketh of the UCL Institute of Child Health and Dr Zhu Wei Xing from the Zhejiang Normal University in China warn that measures to reduce sex selection and change cultural attitudes are urgently needed.

Dr Therese Hesketh, of the UCL Institute of Child Health, 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. Sex-selective abortion and discrimination in care practices for girls have led to higher female mortality.

Although health care 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.

"Over the next 20 years, in parts of China and India there will be a 12 to 15 per cent excess of young men. These men will remain single and will be unable to have families, in societies where marriage is regarded as virtually universal and social status and acceptance depend, in large part, on being married and creating a new family. Many of these men will be rural peasants of low socioeconomic class with limited education.

"When there is a shortage of women in the marriage market, the women can 'marry up', inevitably leaving the least desirable men with no marriage prospects. For example, in China 94 per cent of all unmarried people age 28-49 are male and 97 per cent of them have not completed high school. In these communities, the growing number of young men with a lack of family prospects will have little outlet for sexual energy.

"This trend could lead to increased levels of antisocial behaviour and violence, as gender is a well-established correlate of crime, and especially violent crime. Gender-related violent crime is consistent across cultures. Furthermore, when single young men congregate, the potential for more organised aggression is likely to increase substantially, and this has worrying implications for organised crime and terrorism.

"But as the number of women in a society drops, so their social status should rise and they should benefit from their increased value. This will lead to more balanced sex ratios as more couples choose to have girls. However, measures to reduce sex selection are still desperately needed and should include strict enforcement of existing legislation, equal rights for women, and public awareness campaigns about the dangers of gender imbalance.

"The good news is that the situation can improve if such measures are taken. In South Korea the sex ratio has already declined, and gender preference data from China is also encouraging. In a recent national survey, 37 per cent of Chinese women surveyed claimed to have no gender preference, and 45 per cent said the ideal family consisted of one boy and one girl.

"Almost equal numbers of the women expressed a preference for one girl as for one boy. Fundamental changes in attitudes are starting to happen, which will hopefully see the bias in sex ratio gradually decline over the next two to three decades. However, the damage for a large number of today's young men and boys has already been done."

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