The world's two most populous countries, India and China, are set for massive internal instability and violence if they cannot lower the ratio of male-to-female babies, a new book argues.
Around two million Chinese men a year will be unable to find a wife
The rapidly increasing number of males being born in comparison to females is leaving a "surplus" of men who will be unable to marry when they grow up.
Official figures from the Chinese government currently put this surplus at two million men a year.
The book, Bare Branches: The Security Implications Of Asia's Surplus Male Population, warns that such gender imbalance will lead to tensions that could spill over into widespread violence.
"There are numerous examples," its co-author Andrea den Boer, from the University of Kent at Canterbury, told BBC World Service's Everywoman programme.
"In India's past - in the areas where female infanticide was practiced to such an extent that there were very few females present at all - there was a lot of inter-tribal, inter-caste, inter-ethnic rivalries that resulted," she said.
She also highlighted a case from the 1850s in China, in the region of Hwi Pai, which once had 129 men for every 100 women. The men, many of whom had received military training and were unable to find jobs, began to engage in banditry.
"Eventually, this led to a large-scale rebellion - they teamed up with other sorts of revolutionaries - and eventually controlled a substantial part of China," Ms den Boer said.
In both India and China, parents are going to great lengths to ensure that they have male children, including female infanticide and sex-selective abortion.
Both countries have a long tradition of favouring males, with a saying in Hindi urging women to bath in milk for baby boys, while a Chinese proverb warns that baby girls will bring depleted fortune.
Ms den Boer stressed that the presence of a large number of males would not necessarily lead to instability and conflict on its own.
But it taps into a large amount of social and biological research - some of it controversial - that suggests young males are responsible for many of the violent attacks occurring in society at any one time.
"There seems to be overwhelming evidence that, for example, a large portion of violence committed within any society is committed by unmarried males," she said.
The BBC's Beijing correspondent, Louisa Lim, told Everywoman that the Chinese government has accepted it has a problem regarding the effects of the country's current gender imbalance - which official figures put at 117 boys born for every 100 girls.
The worst case is in Hinan province, in the south of China, where official figures are 135 boys to every 100 girls.
Officials have warned of many negative consequences for society, including increased prostitution, women trafficking and kidnapping, and bigamy.
"Whether or not it will lead to violence is another question," our correspondent added.
She pointed out that the current male surplus was being "mopped up" by migration to China's big cities, which was in turn fuelling the country's economy.
"Spare male labour [is going] into the cities, where they're beginning to work in construction projects or in factories," she said.
"They're really behind China's economic miracle. So the government has really channelled them down that route."
Meanwhile the BBC's former Delhi correspondent, Jill McGivering, said that while the situation in India varies from one state to another, the situation can be as extreme there.
She said that she had met one family in Haryana, in the north-west of the country, which had resorted to sending out to Bangladesh to find a young woman from a poor family, and brought her back to marry their son in exchange for money.
"It seemed as though she was very displaced - both in terms of language, because the dialect she'd grown up speaking was very different from the one spoken in the village - and also because she was now thousands of miles away from her family in a totally different culture," she added.
There will be comparatively fewer Indian women in the future
And the family had got the idea from their neighbours, suggesting "imported women" may be on the rise as families seek to marry their sons.
However, she added that concerns over violence were not overly high.
"People are saying, 'it's not good for our sons that they're not having the women available to them that they should have'," she said.
"They are saying it is not good for young men not to be able to get married - they'll get up to no good, they'll cause problems.
"But I don't think anyone was saying there will be any major problems, mainly because they're so poor and have to spend so much time working simply to subsist."