 By contrast, constructing plausible theories that account for the moral status of other human beings – not only the degree of their status, but in some cases also whether they have it at all – is more challenging (see section 5). Debates about disability rights and the permissibility of eugenics rest in part on theoretical disagreements about the moral status of cognitively impaired humans. These issues include controversies regarding the treatment of cognitively disabled infants, such as the past U.S. practice of allowing infants with Down syndrome to die. Debates concerning abortion, stem cell research (see the entry on the  ethics of stem cell research),  and the question of what to do with unused frozen embryos from in vitro fertilization also rest on the theoretical question of the moral status of extremely underdeveloped human beings at various stages of development: zygote, embryo, and fetus (see section 5.2). The moral status of both underdeveloped and cognitively impaired human beings is often taken to be at issue when it comes to the use of pre-implantation genetic diagnosis and amniocentesis. In addition, medical advances that prolong life, as well as debates about euthanasia, have led people to question the moral status of humans incapable of consciousness, such as those in a persistent vegetative state and anencephalic babies (born without the higher brain).