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.

So there must be something more going on to create smaller families in richer countries. And, here, biology can offer some clues. Natural selection is not just about having a lot of kids. After all, a parent isn't an infinite source of food and protection. The more offspring an animal has, the less energy it can give each one. If a hawk can't supply its chicks with enough food, they may not live long enough to have chicks of their own.

It turns out that animals have evolved a balance between offspring and effort. Some can even adjust how many offspring they produce, depending on whether they are under stress or live comfortably. Ruth Mace, an expert on family size at University College London, argues this week in the journal Science that humans are governed by the same kinds of rules.When the standard of living goes up, the cost of living goes up too. It takes a family in Addis Ababa (the urban capitol of Ethiopia) a lot more money to raise an additional child than a family out in the Ethiopian countryside. That may be one reason why the population is exploding in rural Ethiopia, while in Addis Ababa it is actually shrinking.

Humans, of course, are not like other animals. Our cultures are far more complex and powerful than those of other species. And Mace suggests that human culture may drive the cost of raising children up with no end in sight. In an affluent country, raising children becomes much more than providing enough food to eat. Parents can also send their kids to college to prepare for well-paying jobs, for example. The benefits they enjoy can be great, but the investment is as well, as any parent who has signed a tuition check will attest. But the more affluent a country gets, the more things parents come to see as essential for raising children.

If Mace is right, then as long as the world keeps creeping out of poverty, families will continue to shrink. How small they can go is an open question. But perhaps we should stop thinking of families with only children as some odd fluke of neurotic New York life. It's just human biology played out to a logical extreme.

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Carl Zimmer won the 2007 National Academies Communications Award for his writing in The New York Times and elsewhere. His next book, Microcosm: E. coli and the New Science of Life, will be published in May.