As a Humean, who bases his moral philosophy on rational self-interest,
Two points.
1:
Why is this any less arbitrary?
2:
So if I handed you a baby and offered you $10 to kill it, assuming no one else would ever find out, would you do it?
If the answer is some variation on "no, because I would feel bad about it", I can throw in a pill that keeps you from feeling bad about it.
The scenario in 2 is too implausable to be useful, in my opinion.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.