Democrat presidential candidate and avowed socialist Sen. Bernie Sanders of Vermont has aligned himself with those who think that babies are nasty little carbon emitters and need to be eliminated in large numbers to save the planet. He has endorsed taxpayer funding of abortions around the world as part of a plan to combat overpopulation and fight climate change.

His startling proposal came in response to a questioner at CNN’s seven-hour climate town hall, who suggested that “the need to curb population growth”–that is, population control–was a way “to address climate catastrophe.”

He jumped up out of his seat and practically shouted, “Yes!” He went on to claim that poor women around the world were having lots of babies they didn’t want.

Actually, women in poor countries, like women everywhere, pretty much have the number of children they want. Unless of course they live in a country which practices population control. Take China, for example, which for decades has dictated to women how many children they can have–one–and when they can have it.

A new documentary called One Child Nation explains how this worked in practice in China. Bernie should watch it before proposing population control as a “solution” to global warming–or anything else, for that matter.

The documentary details the 35 years of suffering that Chinese women and girls endured under the policy, which ended in 2015.

It’s a story that I am intimately familiar with, as I was in China in 1980 when the policy began. I saw firsthand the brutal way it was enforced by Communist Party officials. The sight of sobbing women being subjected to forced abortions and sterilizations is a vision of hell that I am unlikely to ever forget, not least because I was in the operating room with them—standing only a few feet away from the operating table in shocked disbelief.

Over the years, I have given countless talks, written dozens of articles for publications such as the Wall Street Journal, the Epoch Times, and the Washington Post, written best-selling books, and have appeared on national television—all to highlight the horrors of China’s misguided population-control policy. Many other voices have emerged over time to speak out on behalf of Chinese women and their babies as well. Together, we formed quite a chorus.

For anyone who was paying attention, there was never any doubt about what was happening in China. The policy allowed one child for urban residents, and two children for country folk, with an escalating series of threats and punishment used to enforce these strict limits.

Officials began their campaigns to pressure women and their husbands into abortions by firing them from their jobs, destroying their homes, seizing their livestock and other property, and imposing impossibly steep fines. But those Party officials carrying out this cruel and inhumane policy didn’t stop there.

Pregnant moms who refused to abort were summarily arrested by local Family Planning officials and taken by force to abortuaries, where their unborn children were killed by lethal injection.

To handle the large number of women who were already seven, eight, or even nine months pregnant, Party doctors invented a new crime against humanity: the “Cesarean section abortion.” As the term itself suggests, this entailed opening up women’s bodies like so many tin cans so that their now dead or dying babies could be more easily removed and thrown away. Cases of infanticide—where “illegal” babies were killed by government doctors at birth—were common.

Many of these horrors—although not all—are recounted in One Child Nation. But while nothing in the documentary surprised me, it has apparently stunned many progressives. In fact, to judge from the buzz about the film, many on the left seem to be genuinely astonished by the sheer evil of China’s policy, and talk about it as if they were becoming aware of it for the first time.

Nick Schager, for example, writing for The Daily Beast, is so appalled by what he learned from watching the documentary that he can hardly contain himself. “China’s horrifying child-killing policy,” he writes breathlessly, was a “reign of terror” that resulted “in countless abductions, forced abortions, and child deaths.”

The viewers at the Sundance Film Festival, were the documentary won the Grand Jury Prize, were equally taken aback. Pregnant women abducted from their homes by the millions and forcibly aborted? Children killed by the millions at birth, or sold to orphanages to be sold in turn to foreigners? How could this have happened? Such a brave and groundbreaking documentary, they apparently believed, deserved to be widely recognized. Hence the award.

The progressives’ belated awakening to the horrors of the one-child policy is, I suppose, a vindication of sorts for those of us who, at great cost, have sought to expose the policy over the past few decades. As Monica Showalter writes in American Thinker:

Remember Stanford scholar Steven Mosher? Way back in the 1980s, the man was reviled in scholarly circles for exposing just these brutal realities about China. Instead of being praised for adding to the scholarly body of knowledge, he was abused, slandered, accused of process crimes, and eventually kicked out of his Ph.D. program because he reported the truth about what was happening. This was at the urging of the Chinese government, which wanted all news of its cruelty kept hidden—the lies-violence cycle that Alexander Solzhenitsyn described as so necessary to all totalitarian tyrannies. I recall that controversy back when I was a student studying Chinese history, and my professor (I won’t name him, because, well, I liked him) called Mosher “a rat” because his revelations about China’s forced abortions and human rights violations angered the Chinese communist government and caused it to limit opportunities for scholarly research on China. Yet the only thing Mosher was really guilty of was the highest responsibility of a scholar, which was to tell the truth.

Yet the left—including most of my Stanford faculty colleagues—not only turned a blind eye to the truth, some of them even defended China. How could they ignore the brutality that was China’s one-child policy for 35 long years? How could they defend the indefensible?

Not everyone did, obviously. American pro-life organizations heard the cries of the women of China for help, and sought to aid them in various ways. Pro-life U.S. administrations, beginning with Reagan and ending with Trump, have cut funding to the U.N. Population Fund and other organizations because of their involvement in Beijing’s policy. But the left generally averted its eyes. Why?

Part of the reason is that, when it comes to criticizing the human rights abuses of socialist or communist regimes, leftists are always late to the party. Long after the mass graves begin filling up, they are still inclined to give their fellow socialists a pass. They instinctively understand that “fundamentally transforming” a country into a communist paradise is dirty work, and that “class enemies” who stand in the way of progress and stubbornly refuse to be “transformed” must be gotten out of the way—eliminated—somehow.

In fact, leftists are generally only willing to acknowledge the human rights atrocities of any particular communist regime only after it has collapsed, when the evil that it has committed is laid bare for the whole world to see. Bernie Sanders was still celebrating the Soviet Union as late as 1988, long after its crimes were legend and when it was clearly on its last legs.

The People’s Republic of China still exists, of course, but the one-child policy itself has been quietly abandoned by the Chinese Communist Party. The Party line has shifted, which perhaps explains why leftists have finally decided to break their long silence.

My biggest complaint about One Child Nation is that it fails to adequately address—much less debunk—the one-child policy’s bogus economic rationale.

Party propaganda at the time went into overdrive, warning the Chinese people that too many babies were an existential threat to China’s future. Without a drastic reduction in the birth rate, China would suffer an economic, environmental, and societal collapse so catastrophic that people might even resort to eating each other. One still-credulous old woman assured the filmmakers that, without the one-child policy, “China would have faced famine and potential cannibalism.”

Under the Party’s new policy, however, all would be well. The elimination of large numbers of children would lead to wealth and happiness for those who survived the cut. The god of prosperity would smile upon China, everyone’s standard of living would double, and China itself would recover its national glory.

So it was that, on the orders of the Party, China embarked upon “cannibalism” of a different sort: It began to devour its own children. Babies were the new “class enemies” who stood in the way of socialist progress. Like all such “enemies of the people,” they had to be ruthlessly eliminated, even if this meant forcibly aborting their mothers or killing them at birth.

Fundamentally transforming a country into a communist paradise is, as I said earlier, dirty work.

Over the ensuing three-and-a-half decades, an estimated 400 million children, both born and unborn, were sacrificed to the lie that their deaths were necessary for China’s economy to thrive. It is a lie because it was access to Western markets, capital, and technology, combined with the native entrepreneurial ability and work ethic of the Chinese people, along with their sheer numbers, that has allowed China’s economy to grow over the past few decades.

The truth is that people—especially young people—are an indispensable aid to the economic development of any country over the long term. And the leaders of the Chinese Communist Party, in eliminating 400 million of the most hardworking, intelligent, and business-minded people on the planet, were literally destroying China’s future. It was the wanton destruction of human capital on a scale never before seen in human history.

All this is to say that the one-child policy has not only broken the hearts and spirits of hundreds of millions of Chinese women, it has made China poorer as well.

And as Chinese birth rates continue to plummet, and its work force continues to shrink, an even darker future may be in the offing. China may be entering a decades-long demographic recession, brought on by the devastation of China’s most valuable resource: its people.

The real-world effect of eliminating roughly half of two generations of Chinese young people is the aging and dying population of today. In fact, China is aging more rapidly than any other country in the world. Unlike its Asian competitors—Japan, South Korea, and Taiwan—it will grow old before it grows rich.

This is obviously not the first time that the Chinese Communist Party has compromised China’s future. Each and every one of the massive social engineering programs carried out by the Party—from the People’s Communes of the 1950s, to the Cultural Revolution of the 1960s, to the crackdown that followed the Tiananmen Massacre, to the current vicious persecution of religious believers—have set back China’s real economic and social progress.

Why should anyone with even a passing familiarity with the brutal history of the Party expect the one-child policy to have turned out any differently?

One would think that China’s brutal one-child policy would have discredited population control programs for all time, but socialist Bernie Sanders is still pushing them.

He believes that women in the developing world are “having more children than they want” and would choose to abort them if given a “choice.”

The truth is that nearly all women around the world are happy with their family size. Population control programs of the kind that Bernie supports do not expand women’s options, they narrow them. They force women to contracept, sterilize, and abort wanted children out of existence.

Just like they did in China.

Steven W. Mosher is the President of the Population Research Institute and the author of Bully of Asia: Why China’s Dream is the New Threat to World Order.