TOKYO – Women hold up half the sky, as Mao Zedong once declared. Decades later, they don’t hold up much of anything in the halls of Chinese power.

In 2012, Liu Yang proved the inverse of Mao’s point, at least as far as China is concerned. The 34-year-old became the first Chinese woman to orbit the Earth. Her milestone highlighted a less heavenly reality: It’s easier for a Chinese woman to circle our planet in outer space than to reach the highest rungs of male-dominated Beijing politics.

Expectations that a woman would be included in the Politburo Standing Committee, China’s most powerful body, came to naught last year. While one did make it to the broader 25-member Politburo, the outcome had China watchers lamenting how the Communist Party remains an old-boys club.

As women cool their heels for another 10 years, China has an even bigger gender problem on its hands: a girl shortage.

By 2020, as many as 40 million more men than women will reach adulthood and enter the world’s most competitive mating market. That estimate, which may actually prove to be conservative, would become a stark economic reality on Xi Jinping’s watch. China’s next president has barely broached this population mismatch in speeches, never mind suggested solutions.

China’s cultural preference for sons is partly about economics. Parents can hope to live with their sons in old age, whereas daughters tend to enter other family systems. The resulting testosterone glut is the most unintended and dangerous side effect of the one-child policy.

Tens of millions of young, ambitious men unable to find girlfriends or wives can’t be good for any economy, never mind one poised to surpass the United States. Will the economics of sexual frustration lead to families auctioning off their daughters? Anyone who has watched the popular Chinese game show “One Out of 100,” where attractive young women pick from a herd of lonely guys, has to wonder. Or state-sanctioned polygamy, whereby women marry multiple men?

Geopolitics is another concern. Officials in Vietnam, Mongolia and Myanmar already are decrying the flow of women to Asia’s biggest economy. In a 2012 study, “The Competition for Brides in East Asia,” Soohyung Lee envisions a future that might shock science-fiction scribes from Stephen King to Margaret Atwood. Who knows, North Korea’s main export may soon be marriage-age women.

Many pundits are hopeful that Xi will be the great reformer that the world has been craving, including Nicholas Kristof, co-author of the 2009 book on Asia’s women, “Half the Sky.”

There are promising signs that Xi will address the towering problems that President Hu Jintao ignored. There’s talk of shutting the labor camps that enrage human-rights activists. Xi’s move to replicate Deng Xiaoping’s 1992 trek through the industrial southeast suggests plans to modernize the economy.

That’s all important. China must stop being the sweatshop of choice for Western manufacturers and start building a more sophisticated demand-driven economy. Just as critical is altering the demographic trajectory.

“It’s critical for China to do everything in its power to redress the deteriorating sex ratio among China’s birth population, even if this means moving toward a two-child policy,” says Valerie Hudson, a co-author of “Sex and World Peace.” “Internal, regional, and even international security is compromised by the fact that approximately 15 percent of its young adult males will not be able to form conventional households. China need only look to its own imperial history to see the destabilizing consequences of devaluing daughters.”

Of course, allowing families to have more kids would create other issues. Environmentalists worry that our planet can’t handle the strain of 7 billion people all polluting at developed-country rates. That calculus gets even uglier if China’s population, now at 1.3 billion, heads toward 2 billion in short order. It’s up to Xi’s team to find a balance between narrowing the gender gap and sustainable growth.

China is hardly alone. India’s girl deficit also requires urgent attention. Today’s protests are over the brutal gang rape and murder of a 23-year-old New Delhi woman. The demonstrators are also spurring more scrutiny of the shameful record on women’s rights in the world’s biggest democracy, including the odds of girls even being born. American feminist writer Mary Anne Warren called the exploding number of sex-selection abortions “gendercide” way back in 1985.

Some historians are concerned that Asia’s bachelor generation could even fuel wars. Harvard University’s Niall Ferguson, for example, wonders if it will incite Arab Spring- like uprisings, Brazilian-style crime or worse. Europe’s continent-wide wars and colonial conquests are sometimes seen as manifestations of population stresses.

It isn’t that hard to imagine how the unmet needs of young men might mix with perceptions that China’s economy is rigged for the party elites. Try as he might to intensify Internet censorship, Xi will find it hard to keep China’s struggling masses from learning how rich Communist Party members are becoming. Tens of millions of young, underpaid and unloved men angry at their leaders is in no one’s interest. Over the next decade, Xi will have to keep China’s demographic sky from falling.

William Pesek is a Bloomberg View columnist.