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.
