I said I wouldn't post any more about this, but I'm going to cheat a little by posting links to other people, starting with Yuval Levin:
... surely the most essential problem with the eugenics movement was not coercion or collectivism. It wasn’t even the revolting notion of some duty to improve the race. The deepest and most significant contention of the progressive eugenicists was that science had shown the principle of human equality to be unfounded. These eugenicists badly misread Darwin. The eugenicists of today, in contrast, employ actual scientific principles to support their beliefs; nevertheless, their abuse of science is no less misguided. It is, again, being used to demonstrate distinctions among human beings that—the new eugenicists claim—are so fundamental as to make some lives not worth living, and therefore not worth protecting.
The challenge of eugenics was, and is again, a challenge to our egalitarianism. That is what lies at the heart of the abortion debate, and of the larger debate about emerging biotechnologies. These arguments are not about when a new human life begins—an empirical matter not in real dispute—but about whether every human life is equal. That question is a perfectly serious one, and there are defensible positions on both sides. But too many American progressives have answered in the negative without thinking through the consequences. And increasingly the reasons they give are not liberal reasons—reasons of liberty and personal choice—but scientific reasons, be it the great promise of some very particular avenue of medical research, or the instrument readings that demonstrate Down’s or another genetic condition.
See also Cheryl Miller, Matt's remarks and Reihan's response, and Cheryl again.