A few years ago Bill McKibben (better known for his pessimistic book The End of Nature and his anti-genetic-engineering book Enough) wrote a book called Maybe One, explaining his reasons for getting a vasectomy and deciding that one child in today's world is enough. [...]. And then he lays out his proposition in a single sentence, and asks us to think and talk carefully and frequently about it:

No decision any of us makes will have more effect on the world (and on our lives) than whether to bear another child.

That's it.
[How to Save the World]