If you start researching the intellectual movements of the first 30 years of the twentieth century, you will find that eugenics was extraordinarily popular on the left. I’ve been looking at Spanish feminists and co-operativists of the 1920s and 1930s, and they were all eugenicists. And so were their colleagues and friends in the rest of Europe and the US. Margaret Sanger was not an outlier. She was extremely typical.
The reason for eugenicism’s popularity among the leftist crowd of the early twentieth century is that these were the people who were absolutely convinced that their ideas were going to save the world. They were possessed by a missionary zeal. But what stood in their way was that the masses were completely uninterested in their ideas. There are only two conclusions you can draw when that happens: the ideas are no good or the masses are defective. And nobody likes abandoning their pet theories.
The rhetoric of the urgent need to eliminate the “inferior” masses that stood in the way of “superior” human beings changing things for the better had to be abandoned by the left once Hitler adopted it. But the sensibility that informed these beliefs never disappeared. A belief in dumb, unwashed masses that stand in the way of everything good because something is physically wrong with them is still an organizing principle on the left.