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.
Creating things, be they X Y or Z, usually has diminishing marginal returns. You can only make so many Xs before it becomes a worse idea than Ys, and only so many Ys and Xs before Zs become better ideas.
Unfortunately, if your factory workers have aesthetic fondness for Xs, or Ys are accidentally produced by the coffeemakers in the break room, or if Bob of the Church of the SubGenius commands Zs be made rather than As, you may wind up with a suboptimal productions. In such a situation, someone may come to you and suggest that you make Zs, but you say no. But if this is a bad bargain, Zs always are!
Of course, the right answer is, 'conditions right now mean that Zs are not the greatest marginal return for investment, but Zs are still pretty nifty, and if Zs fell down to some level N such as 0, then maybe Zs would return to being the best investment'.
In this analogy, Zs are babies. Arguably they are not a very good investment in the First World or anywhere. Populations don't need to grow. It'd be fine if populations gradually shrunk. But if birth rates were zero then that's different, and would cause in not terribly long a catastrophic decline, and if it continued for more than a few decades, it would literally cause the extinction of the human race. It's hard to argue that babies didn't become the best possible investment at some point along the way to extinction. But that doesn't mean they are the best investment now.
My wording in the grandparent was misleading. This:
Was meant to refer only to having a baby in current conditions regardless of whether or not it would be sacrificed to save an adult. I meant that if 80 years of production minus 18 years of investment was a net negative or a net positive, that wouldn't change whether you saved the adult or the baby.