In the USA, the great eugenicists of the first half of the 20th century were the "Progressives". As it says here:
A significant number of Progressives -- including David Starr Jordan, Robert Latham Owen, William Allen Wilson, Harry Emerson Fosdick, Robert Latou Dickinson, Katherine Bement Davis, and Virginia Gildersleeve--were deeply involved with the eugenics movement.
And as we read further here:
The second stage in the development of the eugenics movement extended from 1905 to 1930, when eugenics entered its period of greatest influence. More and more progressive reformers became convinced that a good proportion of the social ills in the United States lay in hereditary factors....
An educator, biologist, and leader of the American peace movement, Jordan's main contribution as a major architect of American eugenics was to bridge the gap between eugenics and other reform groups. Like other progressives, Jordan subscribed to the Populist-Progressive criticism of laissez-faire capitalism.
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And in Great Britain, too, the leftists of the first half of the 20th century were outspokenly in favor of eugenics. As just one instance, that famous philosopher, peacenik and anti-nuclear camapaigner, Bertrand Russell spoke in favor of it. Writing in "Icarus Or the Future of Science" in 1924 he clearly approved of it, though he did voice doubts about its being employed for the wrong purposes. In a letter to his first wife, feminist Alys Pearsall Smith, about socialism and "the woman question," he wrote of eugenics in words that could well have been Hitler's -- even echoing Hitler's bad grammar:
"Thee might observe incidentally that if the state paid for child-bearing it might and ought to require a medical certificate that the parents were such as to give a reasonable result of a healthy child -- this would afford a very good inducement to some sort of care for the race, and gradually as public opinion became educated by the law, it might react on the law and make that more stringent, until one got to some state of things in which there would be a little genuine care for the race, instead of the present haphazard higgledy-piggledy ways."
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And are feminists conservative? Hardly. And feminists are hardly a new phenomenon either. In the person of Margaret Sanger and others, they played an active and prominent role in the USA in the first half of the 20th century, advocating (for instance) abortion. For her energetic championing of eugenics, Margaret Sanger won a public admirer in no less a figure than Hitler himself. Naturally, the American eugenicists were virulently racist, desiring to reduce the black population. They shared Hitler's view that Jews were genetically inferior, opposing moves to allow Jews fleeing from Hitler into the United States. If Hitler's eugenics and racial theories were loathsome, it should be acknowledged that his vigorous supporters in the matter at that time were leftists and feminists, and their opponents were conservatives.
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The few real critics of eugenics in the early 20th century were mainly conservatives and Christians like G.K. Chesterton who saw eugenic planning as just another arm of the wider campaign to impose a "scientific" socialist planning. In fact Chesterton subtitled his anti-eugenics tract "Eugenics and Other Evils" as: "An Argument Against the Scientifically Organized State."