I’ve often wondered why women in wealthy societies have fewer children. Melanie Moses (who teaches Computer Science at the University of New Mexico) offers a solution in an 
article entitled, “Being Human: Engineering: Worldwide Ebb,” appearing in the 2/5/09 edition of Nature (available online only to subscribers). This phenomenon is counter-intuitive because evolution by natural selection would seemingly predict that human animals with more resources would have more babies. Moses employs the Metabolic Theory of Ecology (MTE), an approach for understanding the dynamics of flow through networks. It was developed
to explain why so many characteristics of plants and animals systematically depend on their mass in a very peculiar way. . . According to the theory, the larger the animal, the longer its cardiovascular system (its network of arteries and capillaries) takes to deliver resources to its cells. That delivery time, which in turn dictates the animal’s metabolic rate, is proportional to the animal’s mass raised to the power of ¼. Thus, because its circulatory system works less efficiently, an elephant grows systematically more slowly than a mouse, with a slower heart rate, a lower reproductive rate and a longer lifespan.
Moses argues that this idea that networks become predictably less efficient as they grow has “profound” consequences. With regard to fertility, she starts with facts regarding our energy consumption.
The average human uses up only about 100 watts from eating food, consistent with predictions based on body size. But in North America, each person uses an additional 10,000 watts from oil, gas, coal and a smattering of renewable sources, all of which are delivered through expansive, expensive infrastructure networks.
How do energy networks interact with the reproductive choices of humans?
The decline in human birth rates with increased energy consumption is quantitatively identical to the decline in fertility rate with increased metabolism in other mammals. Put another way, North Americans consume energy at a rate sufficient to sustain a 30,000-kilogram primate, and have offspring at the very slow rate predicted for a beast of this size . . . As infrastructure grows we get more out of it, but must invest more into it, reducing the energy and capital left to invest in the next generation.
Moses disagrees with alternative explanations, such as availability of birth control or decisions to marry later, because these don’t explain decisions to have fewer children in the first place. She also dismisses the idea that “as societies become wealthier, greater educational investments are made in each child to make them competitive in labour markets” because investments in eduction correlate inversely with fertility rates.
[photo: permission of www.dreamstime.com]
Wow… viewing a modern technological human as equivalent to a 30 tonne ape seems quite profound! It gives a very visceral idea of the power we wield, and the environmental impact of such. As an explanation for decreasing fertility with increasing wealth it's interesting; I too have wondered about the reason for that lucky bit of human nature. I'm curious about the exact mechanisms proposed though… surely it can't be biological as it is in other mammals, so how does it work?
The question never seemed counterintuitive to me. But the fact that people find it so seems indicative of a misunderstanding of the source of causal effect. While it is correct that the Darwinian model should show that higher resourced humans would have more progeny, that is only when the actions and effects of what we might term unmodified natural selection are considered.
But humans at that level have become capable of competing as sources of effect with "nature." We set the conditions now, or at least enough of them that a straightforward Darwinian model is disrupted.
In simpler terms, we reach a point at which what "we" want is a match for what "nature" wants. Then the model breaks down.
I think it's telling that Moses used North America as her model. North America uses way more energy per person than Europe does, yet has significantly higher birthrates. I call reporting bias.
I also don't understand how Moses can possibly think the advent of birth control (and women's rights in general) is disconnected from the decision to have fewer children. Back in the day, when wife rape was legal and socially acceptable – or when a couple without birth control might decide to just go for it and hope for the best – children happened. There were even economic incentives to have children, in the form of using your children as unpaid labor on farms or paid-by-someone-else labor with their money going into your pockets. Children were often necessary, even downright profitable, especially for farmers. And with the child mortality rate where it was, it was probably a good idea to have a few spares just in case something happened to your eldest. Now that they're unnecessary to most people's livelihoods, and in fact an economic sink, it absolutely makes sense that fewer people would choose to have children.
AnonMiss, I think you have the economics of large families in a somewhat distorted view. A major shift has occurred in the the function and definition of family.
Agrarian cultures, often had large families, and it is true that the children were a workforce. But the family also included aunts, uncles and the grandparents (kinda like the Waltons), and the children learned to accept hard honest work with a sense of accomplishment. The eldery were taken care of by the adult children and most developed an idea of social conscience in the process. Eventually the children would become the adults and would take over the farmin as their parents retired.
In industrial society, large families are much less practical and the model family is the "Nuclear" family. Industrialization has brought with it the factory mythology where the children are processed in knowledge factories called schools, the parents work in a factory or office to pay for an urban life style.
The information age seems to be ushering in another shift from the nuclear families to single parent families with a single income earning adult and a dependence on paid child care.
The trend is trivializing the importance of a nurturing family to the social growth of our children.