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Sprawling out. Small households have a larger impact on the environment.

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In rich and poor countries alike, people are living in smaller households, which makes their impact on the environment more severe, a new report argues. The trend is steepest for countries with high biodiversity, and experts predict that it will be difficult to reverse.

Human population growth will bring widespread environmental devastation, according to the doomsday prophecy of The Population Bomb, a book written in 1968 by evolutionary biologist Paul Ehrlich. Some scientists think that the book's predictions were overly bleak. But while studying human impacts on panda populations in China during the 1990s, ecologist Jack Liu of Michigan State University in East Lansing made a disquieting discovery. He observed that the number of humans on the panda reserve has increased by about 70% since 1975, but the number of households has ballooned by about 110%. That's because more people are living alone or in smaller groups.

To see whether household numbers are burgeoning around the globe, Liu, Ehrlich, and colleagues at Stanford University in California gathered reams of data from the United Nations and censuses in 142 countries. From 1985 to 2000, household size declined at a rate that outpaced population growth, resulting in 155 million more households than could be explained by the census alone, the team reports in the 13 January online issue of Nature. Even worse, household shrinkage was much more severe in "biodiversity hotspot countries" than in nonhotspot countries, as designated by the nonprofit Conservation International, headquartered in Washington, D.C.

"There is no question that the number of households has an impact on the environment," says population policy researcher John Bongaarts of The Population Council in New York City. But it will be a challenge to temper that impact, because it is rooted in rising incomes, rising divorce rates, and the breakup of extended family households, he says. "All of these trends are hard to reverse, and I don't see [any] government doing anything about it."

Related sites
Ehrlich's site
The Population Council
Conservation International