Downvoted- this is misleading. It might technically be true that we cannot sustain the population of Earth at US living standards currently, but the main reason is that a large portion of the population is nowhere near US levels of productivity. Even given this, we produce enough food for everyone on the planet with agriculture in Asia and Africa at productivity levels far below what could be achieved with common US technology. So if those people can generate enough prosperity to make houses and iPhones, which we did without the benefit of borrowing tech from more advanced societies, then the world can easily sustain many more people at US living standards.

There are certainly environmental problems remaining, but a child born today in almost any region of the world has the highest expected standard of living so far in history. As Caplan points out in the book, new people have large positive externalities, and their contributions more than make up for any drain on resources. And of course environmental problems tend to become less severe as societies develop.

A person enjoying life is an extreme good, overpopulation is really not an issue, so have more kids!