WASHINGTON — “Save water, shower together,” young people proclaimed a few years ago. It turns out they were right.

Americans spend an extra $3.6 billion annually on water as a result of the extra households created when people divorce, an ecologist at Michigan State University, Jianguo Liu, estimated.

In countries around the world divorce rates have been rising, and each time a family dissolves, the result usually is two households, Mr. Liu said. His analysis of the environmental impact of divorce appears in this week’s online edition of Proceedings of the National Academy of Sciences.

“A married household actually uses resources more efficiently than a divorced household,” Mr. Liu said.

Households with fewer people are simply not as efficient as those with more people sharing, he added. A household uses the same amount of heat or air conditioning whether there are two or four people living there. One person or several use the same refrigerator. Two people living apart run two dishwashers instead of just one.

Mr. Liu, who researches the relationship of ecology with social sciences, said people seem surprised by his findings at first, and then consider it simple.

“A lot of things become simple after the research is done,” he said.

Some extra energy or water use may not sound like a big deal, but it adds up.

America, for example, had 16.5 million households headed by a divorced person in 2005 and just over 60 million households headed by a married person.

By the person, divorced households spent more a person a month for electricity, compared to a married household, as multiple people can be watching the same television, listening to the same radio, cooking on the same stove, and or eating under the same lights.

That means some $6.9 billion in extra utility costs a year, Mr. Liu calculated, in addition to the extra $3.6 billion for water, plus the other costs such as land use.

And it isn’t just America.

Mr. Liu looked at 11 other countries such as Brazil, Costa Rica, Ecuador, Greece, Mexico, and South Africa between 1998 and 2002.

In the 11, if divorced households had combined to have the same average household size as married households, a million fewer households could have been using energy and water in these countries.

“People have been talking about how to protect the environment and combat climate change, but divorce is an overlooked factor that needs to be considered,” Mr. Liu said.

Mr. Liu stressed that he isn’t condemning divorce: “Some people really need to get divorces.”

But, he added, “one way to be more environmentally friendly is to live with other people and that will reduce the impact.”

Don’t get smug, though, married folks — the savings also apply to people living together and hippie communes or Shaker communities would have been even more efficient.