ASHINGTONThere once was a time when conservatives believed in small government. Now that they control Congress and the White House, however, they have increasingly learned to live with big government as long as it serves their ideological ends. The latest example can be found lurking in the welfare reauthorization bill passed by the House and awaiting action by the Senate: a proposal to spend nearly $2 billion over the next six years to encourage people to marry. States receiving their shares of this money would have to establish services to "encourage the formation and maintenance of healthy two-parent married families." Single people on welfare who marry might even get cash bonuses. Now, marriage is a good thing. A substantial body of research shows that people who marry tend to benefit economically and in other ways, and that high marriage rates are good for society. But before embarking on a giant federal program to encourage marriage, we should consider whether the program will really help the people it is intended to reach: poor single mothers on welfare. First, there is the question of whom these women are supposed to wed. The Harvard sociologist William Julius Wilson and others have shown that in high-poverty areas there are relatively few marriageable men. A study of census data by researchers at Princeton and Columbia found that more than a third of fathers of children born out of wedlock lacked a high school degree; 28 percent were unemployed; and 20 percent had incomes of less than $6,000 per year. Roughly 38 percent had criminal records. The truth is, many single mothers are single because they find their unemployed and undereducated potential partners to be unattractive marriage material. Do we really want to encourage them to marry unsuitable partners? Second, the beneficial effects of marriage on low-income single mothers may be significantly smaller than for American women on the whole. Census data indicate that about half of such unwed mothers already live with their child's father. An additional third are romantically involved with the father, but live separately. In either case, the father can be presumed to be providing at least some support already, so any economic gains through marriage would probably be marginal. In addition, promoting marriage among young women may have unintended negative consequences. Teenage mothers who marry are more likely that those who stay single to have a second child while still young. This makes them more likely to drop out of school and less likely to return for their education later. In addition, young people have far less stable marriages than those who put it off until they are older — they are more likely to divorce and have a higher incidence of domestic violence. Finally, defining the issue as one of marriage misses the real problem: out-of-wedlock births. It is meeting the needs of children, not being single, that helps keep these women in poverty. It would make more sense to focus on helping women delay childbearing until they have completed their educations, established themselves in the workplace and, presumably, married on their own. It seems unlikely the bill's $50 million a year for abstinence education will be of much help. Perhaps conservatives have grown so accustomed to lamenting an America in decline that they fail to realize that most people in our society, including 90 percent of women by the time they reach age 45, still choose to marry. There is no marriage crisis. If liberals had introduced this costly big-government social experiment, conservatives would be outraged, and they would be right. Michael Tanner, director of health and welfare studies at the Cato Institute, is author of ''The Poverty of Welfare: Helping Others in Civil Society.''