I'm actually pretty sure some people who have had 12 kids are heroes or at least very altruistic when objectively analysed.
Many many people that made great contributions have come from large families of overachievers. Genetics and upbringing matter a lot. And productivity gains made by lets say 6 of the kids can easily overshadow anything that one individual could have done (even when adjusted for the fact that the kids start contributing later).
However overall if we look at the world today, the vast majority of people having 12 kids aren't heroes.
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.