State media reported on Monday, citing a study that blamed sex-specific abortions as a major factor, that more than 24 million Chinese men of marrying age could find themselves without spouses in 2020.
The study, by the government-backed Chinese Academy of Social Sciences, named the gender imbalance among newborns as the most serious demographic problem for the country's population of 1.3 billion, the Global Times said.
"Sex-specific abortions remained extremely commonplace, especially in rural areas," where the cultural preference for boys over girls is strongest, the study said, while noting the reasons for the gender imbalance were "complex."
Researcher Wang Guangzhou said the skewed birth ratio could lead to difficulties for men with lower incomes in finding spouses, as well as a widening age gap between partners, according to the Global Times.
Another researcher quoted by the newspaper, Wang Yuesheng, said men in poorer parts of China would be forced to accept marriages late in life or remain single for life, which could "cause a break in family lines."
"The chance of getting married will be rare if a man is more than 40 years old in the countryside. They will be more dependent on social security as they age and have fewer household resources to rely on," Wang said.
The study said the key contributing factors to the phenomenon included the nation's family-planning policy, which restricts the number of children citizens may have, as well as an insufficient social security system.
The situation influenced people to seek male offspring, who are preferred for their greater earning potential as adults and thus their ability to care for their elderly parents.
The Global Times said abductions and trafficking of women were "rampant" in areas with excess numbers of men, citing the National Population and Family Planning Commission.
Illegal marriages and forced prostitution were also problems in those areas, it said.
Authorities put the normal male-female ratio at between 103-107 males for every 100 females. But in 2005, the last year for which data were made available, there were 119 boys for every 100 girls, the newspaper said.
However, the study said that in some areas the male-female ratio was as high as 130 males for every 100 females, a report by the Mirror Evening newspaper said.
The report said the study urged the government to relax the so-called "one-child" policy and study the possibility of encouraging "cross-country marriages."
China first implemented its population control policy in 1979, generally limiting families to one child, with some exceptions for rural farmers, ethnic minorities and other groups.
It has said the policy has averted 400 million births.
Researchers said the gender imbalance problem cropped up in the late 1980s when the use of ultrasound technology became more prevalent.
This allowed women to easily determine the sex of their foetuses, leading to an increased number of sex-selective abortions.