In animal husbandry, the word "cull" means to remove undesirable animals from the herd — the scrawny and the sickly. To hear the word applied to human beings is harsh, but that's just how , professor in the Department of International Affairs at Texas A&M University, means it when talking about the growing worldwide trend of families — largely in Asia, Southeastern Europe, the Middle East and some parts of Africa — to have sons rather than daughters.

"This is a very troubling turn, that in the 21st century the culling of females is once again becoming more prevalent worldwide," she wrote in an email interview.

Nature favors boys at birth, with a consistent worldwide gender ratio of about 105-107 males born for every 100 females. But females eventually catch up. Girls have greater resistance to disease right from the start; they don't take as many risks, from playfully jumping out of trees to drinking and driving; they're less likely to fight in wars or barrooms; and overall women live longer than do men. In developed countries, like the U.S., the U.K., and Canada, women begin outnumbering men at about age 55.

So in the natural course of things, the male advantage in numbers decreases over a lifetime, with women ultimately gaining the advantage in numbers.

But in some countries the balance is tipped unnaturally toward an overabundance of boys, an imbalance that is likely to last through the reproductive years. Several things have combined to lead to what researchers call "missing women." Many countries have a deep-seated cultural preference for sons over daughters. Inexpensive blood tests that can determine the sex of a fetus as early as seven weeks have been developed. And countries around the world have imported ultrasound equipment. "Ultrasound is available even in very poor countries," says Hudson. "The Chinese government actually imported ultrasound machines mounted on carts in the 20th century, so that even the most remote village would have access to this technology."

In 1995, only six countries had such a marked imbalance of boys to girls. Today, 21 countries have a skewed sex ratio favoring boys. The growth of gender imbalance in only two decades points to widespread acceptance of modern technology that can predict the sex of the fetus, according to Hudson.

Technology has enabled even the poorest of countries to bypass the natural gender balance. "It's largely due to the abortion of females," says Hudson. "But it's also due to passive neglect, such as underfeeding, underimmunization, and failing to take girls to the doctor when they're sick." Abortions of females can happen before anyone in the community notices a pregnancy, she says. And when girls are abandoned or neglected so severely that they die, it often doesn't create much of a stir among people who understand the preference for boys. "No one raises it as a public issue within the community, so while it's not secret, it isn't commented upon," says Hudson.

The result of sex-selective abortions, infanticide and neglect of baby girls, according to the United Nations Population Fund, is more than 117 million "missing" females in Asia alone, and many more around the world.

And for every missing woman, there is a surplus man who will never establish a family. "Men are unable to marry," Hudson says, and frustrated, single men are more likely get into trouble. "It leads to instability. In masculinized societies, there are issues such as rising violent crime rates, increasing rates of gang activity and rebel group activity, increasing prostitution and trafficking, and greater constraints on the movement of women."

One country with a tradition of preferring male offspring has successfully corrected the imbalance. "South Korea is the only country I know of that has clawed back its abnormal sex ratios back to the normal range," says Hudson. And it did this not by trying to change culture, tradition, hearts or minds — but by changing laws. In South Korea, sons were responsible for performing ancestral rites and for the care and support of elderly parents. When the government began promoting a two-child norm in the 1970s, Hudson wrote in Foreign Policy, the ratio of boys to girls climbed to a peak of 116.5 to 100 in 1990. That's when the South Korean government began to overhaul laws that favored sons. Women gained full rights in inheritance and in heading families. The government enforced a ban on prenatal sex testing. A pension system was established so that neither sons nor daughters were fully responsible for the care of the elderly. And today, South Korea's ratio of boys to girls reflects nature's average.

But a growing number of countries continue traditions, policies and practices that favor sons over daughters. "These trends do not bode well for the stability and security of nations, regions and even the international system," says Hudson. "There is a real price to be paid for the devaluation of female life."

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