It's changing the world.
And if people think the United States will be immune from the political and economic repercussions, says Hvistendahl, the Asia correspondent for Science magazine, they had better think again.
"We've never seen an imbalance at this level," says the Swarthmore College graduate and author of the new book Unnatural Selection: Choosing Boys Over Girls and the Consequences of a World Full of Men.
Cultural preferences for boys, falling birthrates, and economic needs - in China, sons care for aging parents - have met the introduction of cheap ultrasound technology that allows women to learn the sex of their fetuses. The result is an epidemic of gender-selective abortions.
Asia alone has seen the elimination of 160 million future women - more than the entire female population of the United States, Hvistendahl documents. That's creating millions of "surplus men" who will never be able to marry because there won't be enough women to go around.
The potential ramifications? Multiple. And worrisome: Growth in sex trafficking, prostitution and crime, in sales of child brides and in kidnappings of girls or women.
Statistically, men are more violent than women, and unmarried men more violent than married men. So governments fear unrest. There's also evidence that having more men dampens economies - they don't need to buy consumer goods if they're not having families.
"The gender imbalance is a local problem in the way a superpower's financial crisis is a local problem, in the way a neighboring country's war is a local problem," writes Hvistendahl. "Sooner or later, it affects you."
In this country, studies show, some Asian immigrants select for boys even as they earn more money and become U.S. citizens. Wealthier, native-born Americans also are selecting for gender - through new technologies in artificial insemination and in vitro fertilization that can control the sex of the embryo.
Clinics such as the Center for Human Reproduction in New York and the Fertility Institutes, with offices in New York, Los Angeles, and Mexico, tout their abilities in online ads and on websites.
"We can virtually guarantee your next child will be the sex of your choice," the Fertility Institutes boasts.
The United States, Hvistendahl contends, bears some responsibility for the situation in Asia.
In India, population-control advocates from the Ford Foundation and other agencies in the 1960s and 1970s envisioned sex selection as a way to lower birthrates. In 2007, she says, General Electric introduced to China a low-end, compact ultrasound machine that could be hooked up to a personal computer.
Now a confluence of data, media reports, and international scrutiny is drawing new attention to the issue.
In March 2010, the Economist published a cover story with a bold pink headline: What Happened to 100 Million Baby Girls? It was the first time many people heard gendercide. Three months later, a Christian group called All Girls Allowed formed in Boston, seeking to end China's one-child policy, which generally limits parents to a single offspring in a land where boys are prized.
Last fall, the United Nations summit on millennium goals concluded with new commitments to gender equality.
"We're seeing a snowballing effect," said Brian Lee, executive director of All Girls Allowed. People are starting to see the gender gap "not just as a human-rights issue, but as an economic issue, as a stability issue."
China and India get most of the attention because of their huge populations and rising economies, but also because demographers expected sex-selective abortion would decrease as wealth increased. Instead, the opposite has occurred.
Hvistendahl, who has spent the last five years in Asia, found that in India, sex-selective abortion started in the upper classes, among women who were lawyers and doctors, then trickled down. Wealthier, more educated people tend to have fewer children - so they face more pressure to have sons. They have earlier access to new technologies, because they tend to live in cities and because they have money to pay for obstetric care and ultrasound.
By demanding lower birthrates, governments in China and India created a situation where women are pressured not to reproduce - and to produce a son.
The results show in the sex ratio, expressed in the number of males per 100 females. The natural ratio at birth is 105 boys to 100 girls; anything over 106 is off balance.
In India, the figure is 112. In China, according to its 2010 census, 118. The government says that by 2020, Chinese society will include 24 million bachelors who have little chance of finding a mate.
For instance, in Jiangsu province's Suining county, the ratio was 152 boys per 100 girls. Parents told Hvistendahl that sonograms followed by abortions were the cause - and that the punishing economics of the one-child policy played a role. To bribe an ultrasound technician cost $150, no small sum, but the fine for having an extra child was 10 times more.
In China, where sons serve as a kind of social security system, blame has centered on the 1979 birth law. Though now more flexible, the policy has encouraged horrors from infanticide to forced abortion.
"A shift away from the one-child policy wouldn't necessarily fix the problem," said historian Jeffrey Wasserstrom, the author of China in the 21st Century: What Everyone Needs to Know.
What could help, he said, is enforcing the law: Sex selection is illegal in China (and India). The introduction of a social-care system for the elderly could be huge.
"That would reduce the pressure to have sons," he said. "A stronger social-welfare net would be a good a thing for the people, and for the security of the country."
But the Chinese experience doesn't fully explain why gender-selective abortion occurs in countries such as Albania, which has social security but no one-child policy.
What Albania also has is a preference for sons and a dramatic drop in birthrate, from an average 3.2 children per woman in 1990 to 1.5 in 2010. And cheap and available ultrasound.
Contact staff writer Jeff Gammage at 215-0854-2415 or email@example.com.