Bad ideas have a habit of sticking around. More than two hundred years ago, Thomas Malthus predicted that population growth would inevitably outstrip the pace of food production, leading to calamity. Since then, his model has been used in all manner of heinous ways, first to justify slashing aid to the poor, and later by eugenicists in the early twentieth century.

Malthus’s theory is still with us today. Only this time it’s some segments of the environmental movement who propose illiberal methods to prevent a Malthusian catastrophe.

Columnist Dan Savage made waves a few years back after half-jokingly suggesting that “abortion should be mandatory for about thirty years,” and that population control would need to be instituted. Before that, British journalist Alex Renton argued that governments in the developed world should offer incentives and penalties to induce families to have less children, while hinting that, down the line, it may have to happen “the hard way.” Canadian reporter Diane Francis posited that a “planetary law, such as China’s one-child policy” was necessary to prevent global warming, and that “birth restriction is smart policy.”

This idea dates back to at least 1968, when Paul Ehrlich published his neo-Malthusian classic, The Population Bomb, which predicted ecological and social collapse the world over and the starvation of ten million people a year through the 1970s. To halt rapidly approaching catastrophe, Ehrlich offered a range of measures, some benign (change our lifestyles to be less resource intensive), some alarmingly authoritarian (taxes on children and baby supplies, forced sterilization of fathers with three or more children). At one point, Ehrlich suggested the government may eventually have to add a “temporary sterilant to staple food, or to the water supply.”

Ehrlich and his organization, Zero Population Growth (ZPG), quickly amassed tens of thousands of followers convinced of the imminent collapse of civilization. If what they advocated seems extreme, it’s perhaps understandable given their panic — though when one hears advocates say they’d “like to see people have fewer children, and better ones,” one suspects there was more to it than this. Just read Ehrlich describe his come-to-Jesus experience in Delhi:

The streets seemed alive with people. People eating, people washing, people sleeping. People visiting, arguing and screaming. People thrusting their hands through the taxi window, begging. People defecating and urinating. People clinging to buses. People herding animals. People, people, people, people. . . . Would we ever get to our hotel? All three of us were, frankly frightened.

Ehrlich’s ideas caught on among some environmentalists. One was ecologist Garret Hardin, author of The Tragedy of the Commons, who wrote in 1971 that “continuing to support the right to breed is suicidal” and that “if we defend the freedom to breed, we shall ultimately lose all other freedoms.” Another was John Holdren, who coauthored a textbook with Ehrlich in 1977 that outlined — though didn’t advocate for — several possibly coercive population-control measures that could be allowed if “the population crisis became sufficiently severe to endanger the society,” including forced abortions. The passages caused a stir when Holdren was nominated as Obama’s science adviser in 2009.

Still another environmentalist taken with these ideas was David Brower, the first executive director of the Sierra Club and founder of prominent environmental organizations like the League of Conservation Voters and Friends of the Earth. Brower commissioned The Population Bomb after hearing Ehrlich on the radio. In his foreword to the book, he charged that uncontrolled population was “a menace” and registered his hope that organizations like the Sierra Club would “awaken themselves and others” to the importance of population control. Over the decades, Brower would repeatedly state that overpopulation was one of the world’s biggest problems, and that a large part of this was linked to immigration. He remained on the Sierra Club’s board until 2000.

Through the 1970s and ‘80s, the Sierra Club proselytized about the need to curb immigration. It told Congress that immigration policy “is the determinant of future numbers of Americans,” and in 1989, it officially adopted the policy that “immigration to the US should be no greater than that which will permit achievement of population stabilization in the US.” The Wall Street Journal reported in 1992 that the organization helped form the anti-immigration California Coalition to Stabilize Population.

There were other overlaps with immigration restrictionists. One of the Sierra Club’s former presidents joined the board of advisers for the Carrying Capacity Network, an anti-immigration environmental group that today rails against sanctuary cities and warns about supposedly criminal Muslim immigrants. Another former member became the executive director of Californians for Population Stabilization.

None was more prolific than John Tanton, whom the Southern Poverty Law Center calls “the racist architect of the modern anti-immigrant movement.” Tanton, a leading Sierra Club official during the 1970s and president of ZPG, founded a network of thirteen anti-immigration think tanks and advocacy groups, including the Center for Immigration Studies, whose “data” Trump has heavily relied on.

The Sierra Club’s stance on immigration changed in the late ’90s, when its rank and file defeated a push by its anti-immigration wing to explicitly support restrictionism. In 2013, it officially adopted a plank that endorsed a path to citizenship for undocumented immigrants while taking no stance on broader US immigration policies.

Today, calls for restrictionist policies and forced birth control are limited to a fringe of the environmental movement. This in itself is a sign that liberal environmentalists can be pushed to embrace policies that can fight climate change without running roughshod over the rights and freedoms of ordinary people. It was, after all, Sierra Club members who ushered in progressive changes in the organization.

There is perhaps hope that with pressure from both within and without, liberal environmentalists can be won over to the idea that capital itself will need to be tamed and overcome to keep the Earth habitable for future generations. Ehrlich was right — when it comes to securing the future, it really should be about people, people, people, people.