After decades of concern about population explosion, U.S. scientists are warning of a new, long-term, and hidden global menace -- a household explosion.

A previous norm of one large household sheltering many people is giving way across the world to households comprised of fewer people -- sometimes young singles, sometimes empty nesters, and sometimes just folks more enamored with privacy, the researchers said.

Even though population growth has been curbed, the trend to smaller households is ratcheting up the impact on natural resources and the environment worldwide, researchers at Michigan State University report in the journal Population and Environment.

"Long-term dynamics in human population size as well as their causes and impacts have been well documented," said Jianguo "Jack" Liu, leader of the study that yielded the first long-term historical look at global shifts in how people live. "But little attention has been paid to long-term trends in the numbers of households, even though households are basic consumption units."

More households require more lumber and other building materials, the researchers said, and smaller households are generally less efficient, with fewer people using proportionally more energy, land and water.

Household size has been declining in some countries for centuries, the researchers said, adding a largely unrecognized factor to humans' impact on the environment.

Average household size in developed countries declined rapidly from approximately 5 members in 1893 to 2.5 now, while a rapid decline in average household size in developing nations began around 1987, they said.

"We've documented that the changes we're seeing in household size across the globe essentially doubles the number of homes needed per-capita," researcher Nils Peterson said. "This will put enormous strain on the environmental life support system we rely on, even if we achieve a state of zero population growth."