The personhood criterion

One popular argument for abortion, infanticide, and euthanasia is the personhood criterion. In principle, this has the added advantage of demarcating those who enjoy human rights (e.g. women) from those who don't (e.g. babies, the developmentally disabled, the senile).

If you're a feminist, that kills two birds with one stone by conferring rights on women, as persons, while dehumanizing their babies, who (allegedly) lack personhood.

However, this argument either proves too much or too little. Physicalism implies a reductionistic view of human nature. If human beings are simply organized matter, it's hard to embed personhood in that framework. If, therefore, you combine the personhood criterion with physicalism, you end up denying women's rights or human rights generally. For instance:

It also seems to be the case that some forms of Scientific Naturalism are committed to the denial of “persons as substantive selves that essentially possess a first-person point of view” (See Dennett 2006, 107). Daniel Dennett, for example, holds that persons will not be part of the ultimately true scientific account of things. Dennett holds that to think of humans as persons is simply to adopt a certain “stance” toward them that he calls the “intentional stance,” but it is clear that the kind of picture of humans we get when we think of them in this way does not correspond with their intrinsic metaphysical properties. It is not clear how systems towards which we adopt an “intentional stance” could be truly autonomous and thus have the kind of value Kant believes human persons have. 

http://plato.stanford.edu/entries/moral-arguments-god/#ArgHumDigWor