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The sheer number of households on the planet is now rising much faster than the global population.

Demographers are not as worried today as they were several decades ago about the prospect of a "population bomb," a scenario where so many people come to populate the planet that we exhaust its resources. Population growth has slowed in many parts of the world. And in much of North America, Europe, China, and Brazil, fertility rates are so low that local populations are on pace to decline.

These trends, however, don't cover the whole story of human impact on the environment. The growth in the number of humans on earth may be slowing. But something very different is happening in the growth of human households.

Researchers Mason Bradbury, M. Nils Peterson, and Jianguo Liu identify some hidden but seismic shifts on this front in a new paper in the journal Population and Environment. For years – in some countries, centuries – the average household has been shrinking in size. As a result, the total number of global households is growing much faster than the growth of the world population itself.

Why does this matter? In the U.S. and Europe, the average household included about five people in the late 1800s. Now it has more like 2.5. That means the same number of people today live in twice as many homes, requiring twice as many resources to build and furnish them, to heat and cool them, to pave roads to their front doors. This "household explosion" has long been underway in developed countries. But it's rapidly accelerating throughout the rest of the world.

Now, if developing societies follows suit – mimicking a pattern where household size plummets in tandem with urbanization and industrialization – then "billions of households could be formed,” Bradbury and his co-authors warn, “despite declines in population growth."

This graph from their paper reflects data the researchers collected on 213 nations, territories, and colonies, relying on historic data going as far back as 1600 in documents like censuses and books. Across all of these decades and countries, the definition of "household" has varied. But, acknowledging that limitation, the authors have tried to highlight some underlying patterns. In the graph below, each point represents the average household size of a particular nation during the census corresponding to that year:

"Long­term dynamics of household size and their environmental implications," by M. Bradbury et al. in Population and Environment.

According to that picture, developed nations reached a tipping point in 1893 when household size began to rapidly drop. Over the course of the next century, households fell in size by half, from 5.0 to 2.5. In 1987, developing countries appear to have reached a similar threshold, with an even steeper downturn since.

With some fluctuation, this long-term pattern holds in nearly every country in the study:

Across all of these places and time periods, and with only a few exceptions, the number of households grew faster than the number of people.

A lot of forces have been driving us in this direction. The population is aging. Women are having smaller families, and children are leaving home sooner to start their own households. Singletons are on the rise, alongside the social acceptance of unmarried women living alone. Divorce rates are rising, while inter-generational households are growing less common. And as once-poor parts of the world are becoming wealthier, families that previously crammed under one roof together can now afford to spread out a little more.

In the U.S., demographers and economists have been waiting for household formation to increase, as a hopeful sign that Millenials' job prospects are improving enough to abandon their parents' basements. On a global scale, the trend may similarly have positive implications for local economies, in the construction of new housing and the demand for more household goods.

But this is also what worries scientists concerned about global sustainability. Smaller households are on average less efficient (the opposite is true of smaller houses). They demand, per person, more land, more energy, more water. As Bradbury and his co-authors frame it:

From a more simplistic perspective, declining household sizes, from over 5 to approximately 2.5, will mean approximately twice as many houses will be needed per capita in any areas of the world yet to undergo the shift in household size. If the average household size had been 2.5 people globally in 2010, then the number of households would have been 41 % higher, resulting in 800 million additional households...

That's also 800 million more refrigerators and ovens and climate-control systems, 800 million more homes that need roads and sewage hookups and access to a power grid. If every one of those homes were the size of the average American home circa 2002, the researchers calculate that would mean constructing about 72,000 square miles of new housing on the planet.

That’s an intentionally dramatic illustration; American homes are a colossal outlier. But, in countries like China, the average home is already rapidly scaling up in size.

Top image: Mikhail Kolesnikov/Shutterstock.com

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