From Wired.com - Carl Zimmer

Modern life means small families. Starting about two centuries ago, families in Western Europe began to shrink, and then — country by country, continent by continent — the rest of the world followed suit. The trend is so big that it may rein in the world population’s exponential growth, perhaps even causing it to stop growing altogether over the next century.

But exactly why families are shrinking is a mystery. Rising living standards seem to have something to do with it. It’s certainly true that as living standards rose in England — as children died less from diseases, as the country overall became richer — the size of the English family shrank. When other countries became wealthier, their families shrank, too. These days, affluent countries tend as a rule to have smaller families than poor ones.

But why should that happen? After all, the biological imperative to have kids is strong, and if people have more resources, you might expect them to have more kids. As a result, some demographers have decided that the link between more wealth and fewer children has nothing to do with biology — rather, that small families are more like fads that sweep through countries when they get richer.

Yet we shouldn’t abandon biology just yet. The idea that wealthy nations have fewer children than poorer ones is something of an illusion. If you look closer within the groups of people who make up those countries, it turns out wealthier people actually do tend to have more children. In one of the most extreme examples, scientists looked at Harvard graduates worth over a million dollars. Even among these highly successful people, the richest of them tended to have bigger families.