Large parts of China and India will have an excess of young men by up to 20 per cent because of sex selection, say researchers who call on policy makers to address the causes.
A preference for sons in China, India and South Korea, combined with easy access to sex-selective abortions, has led to a significant imbalance between the number of males and females born in these countries, researchers say.
In 2005 in China, "it was estimated that 1.1 million excess males were born across the country and that the number of males under the age of 20 years exceeded the number of females by around 32 million," wrote Prof. Therese Hesketh of the UCL Centre for International Health and Development in London and her co-authors.
The outlook for a generation of males entering their reproductive years in the next 20 years is "grim," the team concluded in urging China to relax its one-child policy, especially in rural areas.
It's also important to change the underlying problem of how girls are valued, the team argued.
Historically, the sex ratio at birth or number of boys born to every 100 girls is about 105 boys for every 100 females.
But after ultrasounds were introduced and sex selection became available in the mid-1980s, the gender ratio at birth in some cities in South Korea climbed to 125 by 1992, and was more than 130 in several Chinese provinces in 2005, researchers report in Monday's issue of the Canadian Medical Association Journal.
Similarly in India, the gender imbalance is as high as 125 in Punjab, Delhi and Gujarat in the north but is within normal levels in the southern and eastern states of Kerala and Andhra Pradesh, the team said.
A common pattern driving the change in China, India, and South Korea is that if the first- or second-born are girls then couples will often ensure the second or third child is a boy.
The authors described potential consequences of high sex ratios, such as men who are unable to marry in societies where starting a family often drives social status and acceptance. They also assume that lack of opportunity to marry and have children will result in low self-esteem that could make men psychologically vulnerable.
The researchers noted their ongoing work has found no evidence men are prone to aggression or violence, but their findings suggest most of these men do have low self-esteem and are inclined to depression.
"This may be because there is not yet a large enough critical mass of unmated men to have an impact, or because the assumptions about male aggression do not apply in this context," they said.
The team also pointed to potential pluses of sex selection, such as an increase in the proportion of wanted births, and placing a higher value on women as their numbers in society fall.