Timely in light of Lauren's recent post, here's an article worth a look about why wealthy nations produce fewer per capita offspring, although it might be an instance of the comments being more informed than than the article that inspired them. I disagree with the author's premise that "...exactly why families are shrinking is a mystery" - I think that modern families are shrinking because people have decided to allocate the resources they have (mental, financial, and time) to the care and raising of fewer children that they can pay more attention to. People act in their own self-interest, even (or especially) at a deeply subconscious level, and if you need more children to run the farm or support you when you are old, you have them. If you don't, you have fewer, and one of the better points the author makes is that in environments where it is expensive to raise a child, people naturally only have as many as they can support. This is not rocket science.
That said, big props to the author for using "biological imperative," which is one of my favorite phrases and is not used often enough in public discourse.