TOKYO -- Asia's recent demographic upheaval is well documented: Rapid economic development and urbanization over recent decades have pushed up life expectancy, leading to aging populations and the array of socioeconomic challenges that quickly follow suit.

But another demographic shift is now rearing its head, and it brings with it a new set of pressures. Several of Asia's largest countries are experiencing male surpluses -- where men outnumber women in the population -- among young adults. This is the result of a "boy preference" among parents in the 1980s, when today's 20-somethings were born.

By 2035, Asia will have 65 million more men than women between the ages of 20 and 40 -- the years between which couples tend to marry and have children. Among this age group the imbalance will amount to 5% of the region's population, meaning there will be 11 men for every 10 women. So how did we get to this point?

The causes

Research suggests most of the male surplus can be traced to the strong patrilineal nature of agriculture in many parts of Asia 30 years ago, and the size of the sector at the time. The degree to which these characteristics are present also explains variations in the magnitude of the male surplus in different countries.

Chinese culture still bears the marks of Confucianism, which emphasizes male authority over women. Meanwhile, the impact of religion and tradition more broadly in India means it remains one of the most patriarchal societies in the world.

In such male-dominated societies, the preference for boys has often been a matter of simple economics. From easily quantifiable matters such as dowry payments from the bride's family to the groom's, to the custom of a woman caring for her husband's parents in their old age, many view a son as an investment and a daughter as a cost.

This socioeconomic mechanism is still prevalent in India, where sex imbalances remain stubbornly high. State-level data from the second India Human Development Survey (2011-12) show an association between the percentage of people who expect sons to support them in old age and the area's child sex ratio -- the number of males per 100 females ages 0-6.

The relatively agrarian and rural nature of Chinese and Indian society in the 1980s also played a role in the male skew. Agriculture still accounted for more than 30% of each country's gross domestic product in the early '80s, and more than 75% of their populations lived in rural areas. This reliance on physically taxing work, coupled with the clearly defined gender roles in patrilineal societies, meant that having a son was seen almost as a necessity.

Using this logic alone, we would expect each country's male skew to be highest in rural areas. But while India's overall child sex ratio is more male-heavy than China's, it is most skewed in urban areas. Most current research puts this down to access to prenatal sex screening and abortion facilities, which is much greater and more affordable in urban areas. This appears to outweigh the agricultural factor.

Regional differences in the enforcement of China's one-child policy have exacerbated its measured preference for sons, with parents in some parts of the country permitted a second child if the first was a girl, and the regulations enforced more loosely in rural areas.

China's late registrations However, new research has found evidence that this same lax enforcement of the one-child policy may mean that China's rural sex imbalance is not as pronounced as first thought. University of Kansas professor John James Kennedy found that rather than pointing solely to lost females -- either as a result of abortion or infanticide -- many of the missing girls may simply not have been officially registered at birth.

This is not to dismiss as insignificant the infanticide and lethal neglect that results in the deaths of children: China and India are stark outliers globally in having higher infant and child mortality rates among girls than boys.

According to a 2015 UNICEF analysis, India is the only sizeable country* where girls die before their fifth birthday at a higher rate than boys. UNICEF also calculated the discrepancy between a country's childhood mortality sex ratio and the ratio that would be expected given mortality rates in the population as a whole. Not only did China and India both score as having higher female childhood mortality than would be expected, India's score actually deteriorated between 1990 and 2015.

Kennedy analyzed Chinese census data and tracked the sex ratio of the cohort born in 1990, following it through to 2010. He found that hundreds of thousands of additional people -- mainly female -- showed up in the cohort each year, despite negligible immigration. In other words, people who had been left off official records at birth were being registered some years later.

Kennedy suspects this was because the son preference, in combination with the one-child policy -- relaxed in 2015 -- meant many parents wanted their first officially registered child to be a son, and would thus hold off registering daughters until after a son was born and entered into the census. Furthermore, daughters would often only be registered when it became a necessity, such as for attendance at a municipal school or for marriage.

"What we see in China seems to be about the weak birth registration system, and incentives not to register additional children, so we might expect to see these changes in registration over time. Why? Schools and marriage, both of which require registration," said Kennedy.

A Nikkei Asian Review analysis of U.N. population data corroborated Kennedy's results and also identified a similar -- albeit less pronounced -- pattern of late birth registration in India.

The registration surges can be seen in the marked declines (toward balance) in the sex ratio for children around age 10 in China and India. This is the age where children often move from local schools, where registration is often not required, to larger schools where official status is needed. Another jump is visible around age 20 in China, when many women in rural areas marry.

But regardless of late registrations, China and India both still exhibit highly male-skewed child sex ratios today, with only muted signs of a rebalancing.

The solution

A wealth of research has gone into how best to address boy preference, but the bottom line appears to be deep social and cultural change. At one point there was an emerging belief that improving female education could be the silver bullet: Geographical patterns in India's sex ratio at birth hinted at an association between female literacy rates and a more balanced gender split among newborns. But subsequent analyses including additional possible explanatory factors were unable to reproduce the result.

Furthermore, the aforementioned UNICEF study found that India's boy preference has become more acute over the last 25 years, despite progress in female education over the same period.

Rather, it appears that the less quantifiable underlying attitudes around the two sexes are associated both with better female literacy and more balanced sex ratios.

The new consensus is that changing deeply rooted attitudes toward men and women's relative standings in society must be at the heart of any long-term rebalancing of Asia's gender-skewed populations.

As well as the direct mechanism of reducing sex-selective abortion, infanticide and lethal neglect of females, a more progressive attitude toward girls and women should set in motion economic processes whose longer-term indirect impact could become self-reinforcing.

South Asia's sex ratio for labor force participation rates (31% female to 81% male) is among the most uneven in the world, and India's is even worse than the regional average, at 27% female to 80% male. If the stereotypes that sustain such disparities can be eroded, so too will the excuse for parents -- and whole societies -- to view girls as burdens rather than sources of enormous potential.

*Defined by the author in this context as countries with 50,000 births or more per year (159 out of 195 countries in the UNICEF database).

John Burn-Murdoch is on assignment from the Financial Times to the Nikkei Asian Review.