WASHINGTON, JUNE 11, 1999-What's the best thing parents can do to ensure their children's future prosperity? Stay married.

According to a new paper from The Heritage Foundation, children from divorced homes are far more likely than children from married homes to live in poverty-and less likely to earn a decent income themselves when they become adults.

In fact, the marital status of a child's parents is the most important determinant of whether a child lives above or below the poverty line in the United States, writes Patrick F. Fagan, Heritage's William H.G. FitzGerald senior fellow in family and cultural issues. His conclusions are based on a thorough review of the available social science research-conducted with the assistance of Heritage's Center for Data Analysis-on the effects of family status on income.

"Poverty is the result of many factors, but most have to do with marriage, sex before marriage, living together outside of marriage, and divorce after marriage," according to Fagan, a former family counselor. "We can understand why some people choose divorce, but the economic and social science data show it causes more harm than good." Among his findings:

Divorce reduces the income of families with children by an average of 42 percent, and almost 50 percent of these families live in poverty.

Nearly 75 percent of families in the bottom quintile of income have only one parent, while 95 percent of families in the highest quintile are headed by married couples.

Three-quarters of all women applying for welfare do so either because of a divorce or because a live-in relationship has ended, and the ones who leave the welfare system when they get married are the least likely to return.

Married couples in their mid-50s amass four times the wealth of divorced individuals (an average of $132,000 versus $33,600).

The harmful effects of divorce go beyond the immediate problem of lost income, Fagan says. The children of divorced parents are more likely to have out-of-wedlock children and twice as likely to cohabit than are the children of married parents. They are also twice as likely to drop out of high school and 25 percent to 50 percent more likely to experience behavioral problems such as anxiety, depression and hyperactivity.

Cohabitation also adversely affects income because of its "significant relation to divorce," Fagan says. Cohabiting couples who marry are twice as likely to divorce. The divorce rate quadruples for those who have cohabited with someone other than a future spouse.

The number of divorced couples rose precipitously in the post-World War II years, from about 300,000 in 1950 to a peak of 1.2 million in 1980, Fagan notes. The divorce rate leveled off somewhat in recent years, but still stands at about 1.1 million.

Federal efforts to reduce poverty so far have promoted only welfare dependency and single-parent homes, Fagan says. One of the surest ways for Congress to help poor children is by promoting policies that support marriage, such as eliminating the provision of the Earned Income Tax Credit that reduces benefits to women who wed the fathers of their children-which in effect dissuades poor women from getting married.

For their part, state governments should enforce full compliance with child-support laws and make divorces harder to obtain when children are involved, Fagan says. And church leaders and non-profit organizations should prepare couples for marriage by fostering local "community marriage covenants" and by encouraging regular religious worship by families (which research shows reduces the risk of divorce).

"The effects of marital breakdown on national prosperity and the well-being of individual children are like the action of termites on the beams in a home's foundations," he writes. "They are weakening, quietly but seriously, the structural underpinnings of society."