"Is There an Obligation to Have Children?" as does Bryan Caplan in Selfish Reasons to Have Children (he doesn't think all the good reasons to have children are selfish reasons).
Both the Green reason to have no children and the Grey reason to have (let's say) two seem like compelling reasons, but how do they combine? Does the Grey reason for cancel out the Green reason against, leaving the decision-maker at "neutral"? Or is the Green reason too huge to be canceled out by the Grey reason?
It seems obvious that if the population shrinks, older people will pretty quickly start being deprived of social, financial, medical (etc) benefits. It's very hard to see how you can get around the fact that 10 people can provide better support for 10 people than can 5 or 2. What technologies or innovations could possibly come along and change something so basic?
It seems equally obvious that after another 200 years, if the population doesn't shrink, earth's resources will have been more and more depleted, and horrific amounts of despoiling "junk" will have been generated ... but all that's techology-dependent, and not so obvious and basic. Imagine someone in 1800 predicting the terrible congestion of horses and carriages there would be by 2000, because the population was growing. More people, so more horses! Only no, because "more people, more horses" isn't a basic law, and isn't true. New technology made it possible for there to be more people and fewer horses.
So--my sense is that the Grey reason to have two children trumps the Green reason not to have two children. Pray tell, if you think I've gone wrong, how?