Many experts say one of the best ways to build wealth is to get married and stay married.

In fact, those who marry ” ’til death do us part” end up, on average, four times as rich as those who never marry, according to Barbara Dafoe Whitehead and David Popenoe, who run the National Marriage Project at Rutgers University.

This is partly because marriage provides economies of scale – two people sharing expenses. If both spouses are working, you also have two incomes sustaining the household.

It’s when an elderly spouse dies that the financial equilibrium can be thrown off. And if the husband dies, it’s all that much more devastating to the wife.

Despite general declines in old-age poverty rates, nearly 30 percent of older nonmarried women are either poor or near poor, according to a recent report by the Center for Retirement Research at Boston College.

“Of all the factors associated with poverty in old age, the most critical is to be a woman without a husband,” the center said.

Take a look at these sobering statistics compiled by Nadia Karamcheva, a graduate student in economics at Boston College, and Alicia H. Munnell, director of the retirement research center:

¢ 17.4 percent of single women older than 65 fell below the poverty line in 2004.

¢ An additional 10.8 percent were classified as “near poor,” which means that their income was less than 125 percent of the poverty threshold.

¢ As a whole, 28.2 percent of single older women are either poor or near poor – a clearly vulnerable group.

Not only do single women have high poverty rates, they also are a significant portion of the elderly population, a share that steadily increases with age. Among those 80 or older, nonmarried women account for 56 percent of the population.

The study didn’t look at women who have never married.

“Women who enter retirement non-married tend to end up poor, because the U.S. retirement income system bases benefits on earnings, and women have lower lifetime earnings than men,” Karamcheva and Munnell wrote. “They earn lower wages, are more likely to work part time, and spend fewer years in the labor force.”