It's one thing to wish for a boy or a girl when pregnant; but it's something else entirely to take steps to guarantee your wish comes true. Enter China and India, where the ratio of boys to girls is so lopsided that economists project there may be as many as 30 to 40 million more men than women of marriageable age in both countries by 2020.

The question is: Why? It's more than just the historic birth ratio of 105 boys for every 100 girls. Both abortion and infanticide, largely triggered by a long-time limit of one child per family in China, each played a role. The skewed populations have prompted Chinese men, left with a limited pool of potential brides at home, to seek wives in other regions of their own countries as well as those abroad. But a dearth of mates isn't the only concern for population giants China and India, which together account for 2.4 billion of the 6.7 billion people on Earth.

There are 119 boys born for every 100 girls in China today, compared with 108.5 boys per 100 girls during the 1980s. Recent national data is less comprehensive for India, but census records show 115 boys born for every 100 girls in 2003. That represents a major leap from 104 boys per 100 girls in 1981. By comparison, the U.S. is closer to average: 105 boys for every 100 girls this year.

The growing imbalance slows in older age because women tend to outlive men, with the ratio in both countries falling to about 106 men per 100 women after age 60. But such figures are cold comfort for younger men who lack marriage prospects in their age groups.

China's lopsided population woes began in the early 1980s when its government began enforcing a one child per couple rule. The cap was first adopted in 1979 as part of a series of ongoing measures to curb population growth to help the government manage the country's still-limited resources. The move correlated with an attempt by Chinese authorities to improve healthcare that included taking portable ultrasound machines to the most isolated rural villages, which gave women advanced knowledge of the sex of her fetus.

The Chinese have traditionally preferred sons because of their potential to financially support their parents, carry on the family name, and lead ancestor worship, population experts say, and this holds particularly true for rural areas where sons provide much-needed labor. This cultural preference has led many women under the one-child rule to seek abortions, which are legal in China, if they discovered a fetus was a budding girl. The advent of abortion technology has largely replaced the practice of abandoning baby girls, which was more widespread when the one-child rule was first adopted.

Local officials now have flexibility to enforce the policy as they see fit.  Rural Chinese are typically allowed to have two children instead of just one; in fact, only roughly 36 percent of the population, primarily in cities, is subject to the rule, according to the National Population and Family Planning Commission. In recent years, these urban Chinese also flout the rules and have more than one child, typically losing societal benefits and paying a fine based on how much the couple earns.

The existence of families with more than one child has allowed researchers to track the practice of sex selection before birth, particularly since hard data on abortion and infanticide is scarce.

Health policy expert Avraham Ebenstein of Harvard University examined China's 2000 census data and found that the sex ratio of first births for couples was close to the natural sex ratio, but it became increasingly skewed following the birth of one or more daughters. That suggests parents value firstborns regardless of sex, but practice sexual selection for later children if they do not yet have a boy. "The steep rise in sex selection rate between first and second births is responsible for 70 percent of missing girls," Ebenstein says.

There is not a one-child policy in India, but parents there apparently make similar decisions driven by cultural views of daughters as financial burdens—largely because of the dowries required before marriage. The sex ratio for second and third Indian births became increasingly slanted if the firstborn was a girl, but was roughly 50–50 if the first birth was a boy, according to a 2006 Lancet article. The situation led Indian Prime Minister Manmohan Singh to denounce the half-million annual abortions of Indian female fetuses as "a national shame" earlier this year. Killing or abandoning infants has historically existed in India and may also play a role.

Chinese, Korean, and Indian parents in the U.S. with children born in this country show a similar cultural bias according to a recent study in Proceedings of the National Academies of Science. This was particularly apparent in the 2000 U.S. census of the third of three children: boys outnumbered girls by 50 percent if there was no previous son.

Modernization typically leads to a drop-off in the number of children per family, but the preference for sons does not fall as quickly, Ebenstein says. That was evident in modernizing Asian countries such as South Korea and Taiwan, which both saw skewing in the ratio of girl and boy births during the 1980s.

Those countries have recently seen a shift back toward a balanced sex ratio, which spells hope for China and India further down the road. For instance, South Korea had a birth sex ratio of just 107.4 boys for every 100 girls in 2006, compared with 116.5 boys for every 100 girls in 1990. The reverse trend draws power from the strengthening social and economic status of women, as well as the parental desire to have a nuclear family consisting of one boy and one girl.

Baby boy bias is not as widespread in countries outside Asia—at least not enough to prompt parents to attempt to control the sex of their newborns. Studies show the birth sex ratio of males to females fell in North America and Europe during the latter half of the 20th century, although it was not significantly skewed to begin with. South American countries do not have widespread prenatal sex selection because of Catholic beliefs, according to political scientist Valerie Hudson of Brigham Young University, and Africans cherish the earning capacity of daughters. Only some other Central and East Asian countries such as Vietnam now see birth sex ratios near that of China or India.

The growing number of "bare branches"—as the Chinese call young men without the opportunity to marry—was deemed "a hidden danger" that will "affect social stability," according to a 2007 statement by the Central Committee of the Communist Party of China and the State Council. Hudson has also suggested that social instability such as rising crime and even rebellion historically follow any large number of "bare branches," although other social scientists such as Ebenstein remain reluctant to extend such parallels to modern China or India.

A more indisputable result has been Chinese bachelors joining South Koreans and others in searching for foreign wives, particularly from neighboring Asian countries such as Vietnam and even North Korea.  That solution, however, may prove fleeting as Vietnam struggles with its own growing imbalance in birth sex ratio. All countries involved can only hope that their "bare branches" cross borders to make love, not war.