I inserted myself in the middle of another fruitless debate, arguing against both sides. This one was primarily between Wesley J. Smith, a conservative bioethicist at the Discovery Institute (!), John Derbyshire, and Josh Rosenau over "human exceptionalism". My comments are here and here. A lot of hear and not much light, as usual. But it introduced to me a useful term, personhood theory. This is used by some bioethicists to describe the process of deciding who is or isn't a "person" under law and ethics. It makes sense to me that there should be such theories, although they may vary widely. For instance, I (and most sensible people) don't consider a 16-cell blastocyte to be a person, but many of the religious do.
But to Wesley Smith the very idea of "personhood theory" is anathema. He doesn't merely offer a competing definition of person, he regards the entire topic as an occasion for bluster and obfuscation. I can't quite understand why generally anti-science conservatives are wedded to the idea that it's 46 chromosomes that define a person.
The two main theories in play seem to be either genetic (the right-to-life conservative view) or based on some kind of cognitive quality such as self-awareness or language use). Smith's entire output seems devoted to warning that the latter criterion is perilous, leading us down a variety of slippery slopes to euthanasia, infanticide, and all manner of horrors. He's got a point, this stuff is very problematic. But throwing up hands and refusing to think about it does not strike me as a useful or interesting approach.
Marvin Minsky once proposed (jokingly, I think) that you aren't fully human until you can speak in sentences with subordinate clauses, which would allow infanticide up to age 3 or so.
It is obvious (to me at least) that "person" is a social construct. Some societies permit infanticide, others don't. The default for tribal societies seems to be to consider everyone outside of the tribe as somewhat less of a person than those within. The desire of conservatives for some kind of moral absolutism based on biology is doomed to failure, as is obvious from the glaring inconsistencies in their position. Given that, personhood theory should be a subject of intense interest.