Jason Sorens critiques natalism over at Pileus.  Here’s my point-by-point reply.  Jason’s in blockquotes, I’m not.

The main advantage of more people is a deepening of the market and the
division of labor. More people means more ideas and more specialization.
But the law of diminishing marginal productivity suggests that each
additional unit of labor and of human capital is of less value.
Furthermore, in a world of 7 billion people we are going to get roughly
as many outlier geniuses as we do in a world of 9, 10, or 15 billion.

I’m frankly puzzled.  Why wouldn’t outlier geniuses be exactly proportional to total population?  Furthermore, a lot of innovation – probably most – comes not from geniuses, but from tons of smaller contributions of lots of lesser minds.

In fact, there’s a good reason to think that innovation will rise at more than a proportional rate: Population increases not only the supply of innovation (more people to create ideas) but the demand for innovation (more consumers around to pay for ideas).  As I’ve said before:

If, like the Professor, you have only seven potential customers counting yourself, most innovations won’t pay. Suppose the Professor could spend a year of his life working on an idea worth $1 per person. As long as he’s stuck on the island, he’ll be working for $7 a year. He’d be better off picking coconuts. If the Professor could escape the island and bring his idea to a world market of 7 billion customers, though, it would amply repay a lifetime of research.

Sorens continues:

Along with diminishing marginal benefits of people, there are rising
marginal costs. The human footprint on the natural environment increases
with population, and intrudes ever more into ever scarcer (and more
socially valuable) undisturbed habitats.

One way to limit environmental externalities is to (voluntarily or involuntarily) limit population.  But environmental economics teaches us far cheaper and more humane alternatives: taxes and tradeable permits.  Questioning a person’s existence because he drives a car is severe overkill.

People don’t like being crowded. Part of the reason why people move to
suburbs and exurbs is not just high crime and costs in central cities,
but distance from other people. Where do people go to “get away”?
Generally rural and wilderness areas.

Some people don’t like being crowded.  But most seem to love it.  Real estate prices are much higher in densely-populated areas, and the reason is simple: People prefer (all the benefits of population + all the drawbacks of population) to splendid isolation.  If Jason were right, real estate would actually be cheaper in big cities.  It’s not.

More people in a country mean more agency problems
with the government. The people find it more difficult to constrain
their rulers when their rulers don’t pay attention to individual voices,
or even small clusters of people. As a country of over 300 million, the
U.S. would face severe agency problems were it not for the federal
system — and even so, agency problems are significant.

There may be something to this, but it’s not a big deal.  If it were, real estate prices would be noticeably lower in high-population countries and political subdivisions.  And in a world of largely closed borders, there’s an obvious benefit of large polities: They create big free-trade and free-migration zones, with all the attendant wonders.

More people will mean more infectious disease.
It is a basic principle of ecology that a higher population of a
species encourages greater parasitism on that species. As human
populations have increased, so have human diseases.

This is probably true all else equal.  But mankind has gotten much healthier during the population explosion of the last two centuries, and there’s a simple explanation: Larger populations lead to more innovation, including more innovation in medicine and sanitation.

Bottom line: Jason should stop doubting the case for natalism.  Yes, population has some drawbacks.  It’s conceivable that one day these drawbacks will exceed the benefits.  But we have a very long way to go before we reach that point.  Until then, the more the merrier.