Selective abortion of female foetuses and migration to towns has left rural Chinese villages with very few women. The men who are unable to marry because of this imbalance are more likely to suffer from poor mental health, such as depression, feelings of aggression and low self-esteem.
In the villages of Guizhou, southern China, for every woman in her early twenties, there are two available men of the same age. The problem is gets worse as people age – for a woman in her late thirties, there are just over 75 unmarried men.
In a culture where marriage is deemed necessary for a man's social standing and future security, this is having a large impact on men's mental health, according to a study in the Philosophical Transactions of the Royal Society B.
A total of 2,256 men from 48 villages in Guizhou completed questionnaires to assess their mental health. The men were between the ages of 20 and 40, including 1,339 with partners and 917 who had never had a long-term partner.
Those who had never been married were more likely to have depression, low self-esteem and aggressive and suicidal tendencies. The older the unmarried man, the more likely he was to have these mental health problems.
This constitutes "social tragedy", set to last for at least another generation and possibly longer, the study authors say. By 2020, there is estimated to be an excess of 20 million men of marriageable age across China, with 135 million men to 115 million women.
"These unmarriageable men represent the victims of a distortion of the sex ratio driven by a perfect storm of socio-cultural and economic factors," they write in the study.
China's One Child Policy, introduced in 1979, was not likely to have been the cause of these massive sex imbalances, the authors argued. The policy only strictly applied to people in urban areas – in more rural parts of the country, families were permitted two children. But there has been a long-standing prejudice against girls that goes back way beyond 1979.
"The male preference relates to care in older age, being able to help with work, and even 2,500-year-old Confucian traditions that say people should have at least one son," study author Therese Hesketh of University College London told IBTimes UK.
Given the weight of this sentiment, coupled with the widespread availability of ultrasound technologies and the relative ease of access to abortions in China, the number of girls born in rural communities plummeted.
The other factor contributing to the imbalance is migration. Both men and women tend to leave their villages to seek work in larger cities. While men often return eventually to their birthplace, women who marry in the cities tend to stay – which is why the imbalance is greater for older men in the rural areas.
While the men had higher levels of aggression, they were no more likely to commit acts of aggression, Hesketh said.
"We don't think there's more violence in these places. Most violence in most parts of the world happens behind closed doors, much of which is male violence on female partners or children. If you don't have those, you're much less likely to be able to act out violence," Hesketh said.
"The actual crime figures are no higher than places where there is a normal sex ratio."
However, Andrea Den Boer, senior lecturer in international relations at the University of Kent, who was not involved in the research, argued that the wider evidence supports a link between sex imbalances skewed towards men and increased rates of violence.
"Even with uncertainty about the exact imbalance, most researchers are concerned about the possibility of increased social unrest in high sex ratio areas, particularly in areas where unemployment is rising and in areas where the presence of ethnic minorities could add to social tension," she said.
And as well as putting pressure on men's mental health, the imbalance isn't always good for women either, Den Boer pointed out.
"Some have believed that the female deficit would improve women's status, but that isn't the case for many women who end up being trafficked or abducted – there has been a dramatic increase in prostitution as well as sex- and bride-trafficking and abduction of women in the past 20 years," Den Boer said.
"Some research also points to the psychological issues faced by the parents of never-married men – they face discrimination by villages and can be ostracised within their communities."
Den Boer agreed that the sex imbalance is set to get worse before it gets better in China. Exactly how it plays out is likely to depend on how China develops in the coming years, she said.
"If the economy remains strong and the employment rate remains high, then the employment of the male adult population will mitigate some of the effects of the sex imbalance. Real problems will arise if these men find themselves unemployed as well as unmarried."