There's been some heady philosophical discussion over at Free Exchange over ... existence.
Yes, people generally prefer existing. But the possible people implicit in couples' germ cells are not actual people, and therefore do not have preferences. Conception and birth are preconditions for having preferences. I call this the "lucky souls fallacy". Imagine pre-actual persons gathered outside the gate of existence. Each soul holds a number in its tiny incorporeal hands, badly hoping to be called. An ethereal presence stands at the gate shouting numbers. Lucky souls get to go to the front of the line, through the gate, and straight into a real pulsing zygote.
Only thus does the "decision to have kids" create a "massive benefit" to the kid. Lucky soul! But Mr Mankiw is right. What childbirth does is create a life -- a new nexus of benefits and harms, a new container of utility (to be reductively economistic about it). But by itself reproduction confers no benefit on the child produced, since there was no prior hollow soul longing to be filled by the breath of being.
And thank goodness this is true: if it weren't, we'd face the "repugnant conclusion."
“For any possible population of at least ten billion people, all with a very high quality of life, there must be some much larger imaginable population whose existence, if other things are equal, would be better even though its members have lives that are barely worth living.”
That said, I do take the position Glaeser does not, that under certain circumstances bringing a child into the world does represent an external benefit to society as a whole. This, of course, is by no means self-evidently clear: I can also see a plausible case for voluntary human extinction, particularly after having recently spent several hours in the company of some very boring and self-regarding people.
The website for Alan Weisman's excellent The World Without Us begins with a flash animation sequence. As it fired up, I saw the silhouette of a man who, I assume, had just finished the book. I then imagined that said silhouette, now full of despair over a world gone terribly wrong, would then stand on top of a chair and hang himself. That, alas, didn't happen.