Unlike the US Supreme Court and typical opinion on most rich countries, John Stuart Mill does not recognize bearing a child as within the sphere of personal liberty. He argues in paragraph 15 of On Liberty “Chapter V: Applications” that many other people are involved: the child itself, as well as everyone whose wage might be affected by having more people in the country: 

It is not in the matter of education only, that misplaced notions of liberty prevent moral obligations on the part of parents from being recognised, and legal obligations from being imposed, where there are the strongest grounds for the former always, and in many cases for the latter also. The fact itself, of causing the existence of a human being, is one of the most responsible actions in the range of human life. To undertake this responsibility—to bestow a life which may be either a curse or a blessing—unless the being on whom it is to be bestowed will have at least the ordinary chances of a desirable existence, is a crime against that being. And in a country either over-peopled or threatened with being so, to produce children, beyond a very small number, with the effect of reducing the reward of labour by their competition, is a serious offence against all who live by the remuneration of their labour. The laws which, in many countries on the Continent, forbid marriage unless the parties can show that they have the means of supporting a family, do not exceed the legitimate powers of the State: and whether such laws be expedient or not (a question mainly dependent on local circumstances and feelings), they are not objectionable as violations of liberty. Such laws are interferences of the State to prohibit a mischievous act—an act injurious to others, which ought to be a subject of reprobation, and social stigma, even when it is not deemed expedient to superadd legal punishment. Yet the current ideas of liberty, which bend so easily to real infringements of the freedom of the individual in things which concern only himself, would repel the attempt to put any restraint upon his inclinations when the consequence of their indulgence is a life or lives of wretchedness and depravity to the offspring, with manifold evils to those sufficiently within reach to be in any way affected by their actions. When we compare the strange respect of mankind for liberty, with their strange want of respect for it, we might imagine that a man had an indispensable right to do harm to others, and no right at all to please himself without giving pain to any one.

Taken at his word, John Stuart Mill seems to give moral support to the cruel one-child policy that China has just abandoned in favor of a looser, but still heavy-handed two-child policy. 

I think John Stuart Mill is quite wrong for reasons that Steven Landsburg explains best. In his book More Sex is Safer Sex: The Unconventional Wisdom of Economics, Steven writes:

The reason you are wealthier than your grandparents, and the reason your grandchildren will be wealthier than you, is that each generation free-rides on the inventiveness of its ancestors. A generation ago, your parents were free to choose among three television channels, probably broadcasting in black and white, showing programs that could not be taped for later viewing. They used electric typewriters, of which the latest models featured a marvelous innovation: a “delete” key that enabled you automatically to erase the last character you had typed. If you wanted to erase the character before that one you were out of luck.

For many of the comforts we enjoy today, we can be grateful to the inventors of cable television, video recorders, and the personal computer–and to the stroke of good fortune that prevented their parents from joining Zero Population Growth. 

The engine of prosperity is technological progress, and the engine of technological progress is people. The more people, the more ideas. The more ideas, the more we prosper. [pp. 25,26]

There are some big benefits of population growth. Most importantly, they’re spillover benefits: when I decided to have a child, you were a winner. To decide whether the earth is over- or under-populated, we’ll want to weigh those spillover benefits against any spillover costs we can think of. 

But first let’s acknowledge the benefits and costs that don’t spill over. The day my daughter was born, my family’s per capita income fell by one-third (because it was now shared among three people instead of two). Without offsetting benefits, that would have been one of the worst days of my life. Instead, it was the best. (Indeed, the economist Peter Bauer once observed that if per capita income were the only measure of human happiness, then the birth of a farm animal would be a blessing and the birth of a child would be a curse.)

Large as they are, these private (nonspillover) costs and benefits are quite irrelevant to the population issue, because people already have every incentive to account for them when they calibrate their family sizes. And they do. Family sizes are quite sensitive to changes in economic conditions. [p. 33]

Steven Landsburg goes on to show that (short of theft) resource usage is not a spillover cost, since those resources come from one’s own family or one’s own paid contributions to the world. Extra crowding is by and large not a spillover cost, since many places are not crowded, and people choose to live in cities because of the benefits of high density. Affecting prices (including any effects on wages) has countervailing spillovers–helping those for whom one direction of price change is beneficial and hurting those on the other side of those transactions–including those affected indirectly–say as customers of the firms that pay the workers.   

Indeed, in a rare lapse, when talking about children, John Stuart Mill forgets the point he made just a few paragraphs earlier in, in paragraph 3 of On Liberty “Chapter V: Applications”–a paragraph I discussed in “John Stuart Mill on Legitimate Ways to Hurt Other People”:

Whoever succeeds in an overcrowded profession, or in a competitive examination; whoever is preferred to another in any contest for an object which both desire, reaps benefit from the loss of others, from their wasted exertion and their disappointment. But it is, by common admission, better for the general interest of mankind, that persons should pursue their objects undeterred by this sort of consequences. In other words, society admits no right, either legal or moral, in the disappointed competitors, to immunity from this kind of suffering; and feels called on to interfere, only when means of success have been employed which it is contrary to the general interest to permit—namely, fraud or treachery, and force.

So overall, I think Steven Landsburg easily got the better of John Stuart Mill in this tussle.