You’d notice if 160 million women were missing from the U.S. population.

You couldn’t help but notice, actually. There aren’t that many females in the whole country.

Yet that’s how many girls have been lost in Asia to the practice of sex-selective abortion. The crisis is the subject of Mara Hvistendahl’s provocative new book, “Unnatural Selection: Choosing Boys Over Girls and the Consequences of a World Full of Men.”

“Gendercide,” as it’s been called, should worry women. Even in America.

Feminists told us abortion would empower women. Instead, in some places around the globe, abortion disproportionately is used against women. A tsunami of social problems is swelling as a result.

The natural ratio of males to females at birth is 105 to 100. Stats that deviate mean something unnatural is at work—and what’s happening in places like China and India today is anything but natural.

In China, abortion is often a means of compliance with the government’s policy of one child per family. Combined with a Chinese cultural preference for boys to carry on a family line, sex-selective abortion has become particularly prevalent.

The result is a ratio of boys to girls that tops 120 to 100, with some provinces of China reporting an unprecedented ratio of 130 to 100.

In India, 112 boys are now born for every 100 girls.

In recent decades, demographers estimate, the continent of Asia has lost approximately 163 million women to sex-selective abortion. By comparison, the HIV/AIDS epidemic has claimed 25 million lives.

Globally, the ratio by sex already has increased to 107 males for every 100 females because of the sheer scale of the abortion of female babies. Even if the birth ratio normalized immediately, the natural balance would not be restored until 2050.

In addition to the harm for women, gender imbalance prevents men from entering the socially stabilizing institution of marriage. An article in The Economist magazine notes that a rising demand for brides in female-starved communities increasingly is met through coercion and trafficking.

Sex-selective abortion is at work in the U.S. as well. Until recent negative publicity, “it was not unusual to find abortionists advertising the availability of sex-selective abortions in newspapers such as the New York Times,” Steven W. Mosher, president of the Population Research Institute, testified at a congressional hearing in December.

In a 2008 study, Columbia University researchers found that among children born in America to Chinese, Korean and Indian parents, a first-born girl tends to skew the sex ratio of the second or third birth. For second births the male-female ratio was 117 to 100, the report said, and for third births 151 to 100 if the couple already had two girls.

As technology plays a larger role in reproduction, parents can choose whether to carry a child to term based on sex or other genetic characteristics. That leads to Hvistendahl’s grim conclusion: “In China and California alike, mothers have become their own eugenicists.”

Hvistendahl tells how Asia’s gender imbalance was fueled in part by Western pressure for government population control, which allowed an existing cultural preference for boys to evolve into deadly discrimination.

But feminist groups such as the National Organization for Women and NARAL Pro-Choice America have yet to join the call to outlaw sex-selective abortion. Rather than show solidarity with their daughters and sisters in life, they swear allegiance to unrestricted access to abortion—even when it decimates females.

The irony is extremely bitter: “In a world in which women are unnaturally scarce,” the pro-choice Hvistendahl writes, “the right to abort will be the least of our worries.”

Despite the author’s candor, most pro-choice advocates ignore abortion’s role in decreasing the number of women and instead blame gender bias.

“Son preference is a symptom of deeply rooted social biases and stereotypes about gender,” a representative of the National Asian Pacific American Women’s Forum said in congressional testimony. “Gender inequity cannot be solved by banning abortion.”

Jonathan V. Last, who writes about cultural and political issues, begs to differ. The choice is clear, he argued last summer in the Wall Street Journal.

“Restrict abortion,” Last wrote, “or accept the slaughter of millions of baby girls and the calamities that are likely to come with it.”

If there is equality between women and men, it’s rooted in our nature and purpose as human beings. Denying that fundamental dignity inherent in all human life destroys the very basis of equality.

After that, on what basis can feminists advance gender equity? They can’t have it both ways.

Jennifer A. Marshall is director of the DeVos Center for Religion and Civil Society at The Heritage Foundation, where Sarah Torre is a research assistant.