By Deborah Zabarenko, Environment Correspondent

WASHINGTON (Reuters) - Irked spouses looking for a reason to stay married were offered a novel rationale by U.S. researchers on Monday: divorce is bad for the environment.

The global trend toward higher divorce rates has created more households with fewer people, scientists at Michigan State University reported in the Proceedings of the National Academy of Sciences.

More households means more energy expended to build, fuel, and provide water for them, the researchers wrote.

"Globally, the number of households is increasing much faster than the number of people," said co-author Jianguo "Jack" Liu in a telephone interview.

"Even in regions with declining population, we see substantial increase in the number of households. Divorce is the main reason for reducing the number of people in a household," he said.

The average divorced person's household is about 40 to 50 percent smaller than the average married person's household, Liu said. But whether there are three or six people in a house, the amount of fuel needed to heat them is about the same.

Divorce tosses out any economies of scale, the researchers found.

In the United States, divorced households used 73 billion kilowatt-hours of electricity and 2.850 trillion litres of water in 2005 that could have been saved if households had stayed the same size as when they were married. Thirty-eight million extra rooms were needed, with associated costs for heating and lighting.

In the United States and 11 other countries such as Brazil, Costa Rica, Ecuador, Greece, Mexico and South Africa between 1998 and 2002, if divorced households had combined to have the same average household size as married households, there could have been 7.4 million fewer households.

The number of divorced households in those countries ranged from 40,000 in Costa Rica to almost 16 million in the United States around 2000. The number of rooms per person in divorced households was 33 percent to 95 percent greater than in married households.

Liu acknowledged that it wasn't necessarily that marriage that was good for the environment. Rather, it was the size of the household that counted. And this could mean simply living together outside of wedlock.

"If you really want to get divorced, you know that two people cannot stay together, you don't want to stay together forever, then maybe you remarry with somebody else, or live together with somebody else you like," Liu said.