And then there's this: Even if marriage were a miracle drug for poverty, it's not obvious how the government would promote more weddings.

How Promoting Marriage Can Backfire

It's often forgotten, but Clinton-era welfare reform was partly designed to encourage more stable, two-parent families, so that the government wouldn't need to support so many single mothers. After it passed in 1996, states began using federal funds to experiment with marriage promotion programs for low-income couples. The efforts were generally small scale. But they got a major boost in 2002, when the Bush administration funded the Building Strong Families project, a giant experiment in eight cities meant to test the idea that the government could help cultivate healthy relationships. More than 5,000 low-income couples, all unwed parents, were randomly given the chance to take classes focused on topics like childcare and communication, or assigned to a control group that did not take part.

The results? Lackluster. "This final impact analysis finds that BSF had little effect on couples’ relationships," researchers reported in 2012. Three years after the program ended, couples who had the opportunity to take the classes were actually somewhat less likely to stay together. Those fathers were also somewhat less likely to be involved in their children's lives.

Why on earth would taking a course about how to be a better parent and spouse make couples more likely to split up? The researchers speculated that, ironically, the courses helped some parents realize that they were in unhealthy relationships, and may have discouraged some fathers by dealing a blow to their self-esteem. "The need for fathers to 'step up' and be more responsible was one of the strongest messages that couples took from the program," they wrote. "This expectation may have led some fathers in particularly disadvantaged circumstances to instead distance themselves from their partner and children."

In short: Preaching care and responsibility backfired.

The study did have one big problem: class attendance. Of the parents who were selected to take part, only 10 percent actually completed their program. Across all eight cities, only 45 percent came to any classes at all, despite incentives like transportation and on-site childcare. The experiment's results mix all the couples assigned to the program, whether or not they came to class.

This suggests another lesson: Unless marriage courses are mandatory, most low-income couples won't show up.

In Oklahoma City, where participation was highest, the results offered a glimmer of hope. As Brigham Young University Professor Alan Hawkins has noted, children of couples in the experimental group were 20 percent more likely to have lived with both parents since birth.

Not a Panacea: Just 'Another Tool'

Hawkins' own research offers some more reason to be optimistic about marriage promotion. In a paper for Family Relations, he and his co-authors looked at funding for healthy marriage initiatives across the country, and found that states that spent more per-resident generally had fewer single parents, lower rates of child poverty, and more married couples. Unfortunately, removing Washington, D.C. from the analysis rendered the results statistically insignificant.