Last week, a 33-year-old woman in California made headlines around the world when she gave birth to eight babies. She had been on fertility treatment and, it emerged, already had six children.

The story sparked an outcry over the ethics of giving fertility treatment to a woman who already has a large family. Now that woman has 14 children to raise.

The California octuplets are sure bucking the trend. Almost all the countries in the world are now experiencing a downturn in fertility, and Japan most of all.

It’s a well-known problem. Japan has a population of some 127 million people, and that is expected to fall to 100 million by 2050 and to 64 million by 2100. Quite simply, the fertility problem will forever change the country.

It’s a national crisis and everyone has an explanation for it: Japanese people are now choosing not to marry, or not to have children, or to have children later in life.

Another reason commonly given to explain the crash in fertility is Japanese parents’ increasing investment in “super children.” In other words, instead of having two or more offspring who have to share resources, it refers to couples deciding to have one child who can be (expensively) educated to an extreme level. Juku cram schools, special tuition and extracurricular activities to give your child the boost it needs to succeed — or at least get to a good university — is how this translates in practice.

This greater investment is also known to researchers of demographics as “embodied capital.”

Not many people appreciate that there is an evolutionary explanation for fertility decline, too. Look a bit wider: It is happening all over the world, in North America, in Europe — and even, now, in Africa. I spoke to Ruth Mace, a biological anthropologist at University College London, and she told me that all African countries are now experiencing a fertility decline.

The reason is urbanization, and that is another area in which Japan leads the world.

It may be an accident of mountainous geography that has led Japan’s population to be concentrated on the coast and in cities. But accident or not, city life in Japan is highly advanced. The greater Tokyo area, for example, is home to more than 30 million people.

In this, as in many fields where Japan leads, the rest of the world follows. City life is where the rest of the world is heading. Already, half of the world’s 6.6 billion people live in cities, and by 2030 — not that far in the future — some 5 billion humans will be city folk.

So how is urbanization impacting on the population decline? And how can science explain what appears to be mainly a societal phenomenon?

In evolutionary biology it’s well known that animals with more resources will generally have more offspring. So it has been problematic explaining why fertility declines as countries such as Japan get richer.

But here’s why.

Generally, the bigger an animal, the more slowly it grows, the slower its metabolism, the longer it lives and the lower its reproductive rate. So a mouse grows faster than an elephant, has a faster metabolism, more offspring, and dies quicker.

This difference is explained by what is called the metabolic theory of ecology. Bigger animals have a bigger network of blood vessels that are used to deliver resources to their cells. So the efficiency of resource delivery is less in big animals. But it’s not just in big animals.

The bigger any network is, the less efficient it is at delivering resources. A city is a networked system, and big cities — they don’t get much bigger than Tokyo — are also far less efficient at using resources.

Humans use about 8,400 kilojoules of energy per day, which is equivalent to about 100 watts per day. That’s the energy we use “naturally,” and about what we would burn if we lived in natural environments. But of course we don’t — we live in cities.

If you take into account the additional energy we use, which mostly comes from burning oil, gas and coal, then in the biggest cities our daily consumption is up to 11,000 watts each.

As Melanie Moses at the department of computer science at the University of New Mexico explained in this week’s Nature, humans in developed countries such as the United States, the United Kingdom and Japan consume energy at a rate sufficient to sustain a 30,000-kg primate.

Such a gigantic beast doesn’t exist, of course, but if it did, the metabolic theory of ecology predicts it would have offspring at an incredibly slow rate — fewer than two per lifetime, which is what we are seeing.

This, says Moses, explains the “demographic transition” that is hitting Japan the hardest, but which is affecting the whole world. Human reproduction declines with the additional “metabolism” of fossil fuels.

So what’s the answer? Moses says our infrastructure must be decentralized. Energy must be delivered from wind, solar and tidal resources, rather than stemming from oil reserves that are located in clusters far away from consumers. Cities have to be built in ways to reduce transport and travel distances so as to boost the efficiency of our energy use.

If we and our cities become more efficient in terms of energy use, the scientists predict, we’ll have more children. That might be the only way that Japan can survive in anything like the form we know and love.

The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human: How New Biology Explains Your Journey Through Life).”

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