The question is rather one of causality—marriage may not lift people out of poverty, but financial well-being sure does seem to make marriages stronger. As Kristi Williams of the Ohio State University told Annie Lowrey in The New York Times last year, “It isn’t that having a lasting and successful marriage is a cure for living in poverty. Living in poverty is a barrier to having a lasting and successful marriage.”

So if not an absence of a marriage license, what is keeping families in poverty? No surprises here: Though the economy may be growing, very few people are seeing that money show up in their paychecks, and that means people are struggling. According to Elise Gould at the Economic Policy Institute, rising inequality accounts for a far greater percentage of the country’s poverty problem than does the rising rate of single parenthood. If today’s families looked as they did in 1979 (meaning fewer single parents), the poverty rate would be 1.6 percentage points lower, she finds. If the distribution of income today looked as it did in 1979, the poverty rate would be 7.1 percentage points lower.

However, there is one thing about being married that very much does tend to lift families out of poverty: having two incomes. There’s no demographic in America doing better income-wise than dual-earner households, who on average earn more than $100,000 annually. It’s simple math: Most of the time, two incomes combined is going to result in a sum greater than $40,000. In fact, according to Fremstad, “in 2014, 18 percent of families (with kids) with incomes below 150 percent of official poverty had two or more working family members. And only 3.2 percent had two or more full-time, year-round workers.”

And that’s a big part of the reason that, overall, married couples are doing better than single-parent households—just 7 percent of married couples with two incomes are making less than $40,000, according to data from the Census Bureau. By contrast, of married families in which only one person is employed, 31 percent are in that bracket. (Unsurprisingly, married families with no earners tend to be in the poorest category, but this group is very small by comparison, with fewer than one million people nationwide.)

So why don’t more married couples in poverty send both adults out into the workforce? Of course, the normal reasons for unemployment are likely suspects: a paucity of jobs in the region where a family lives; a lack of valuable skills on the part of the potential employee; health troubles, both mental and physical; and so on. But one factor may have particular salience for this group of low-earning families with kids: the difficulty of finding—and affording—childcare.

If one parent works, the other parent can stay home and take care of the kids. But once both parents are in the workforce, unless there is another relative around to pitch in, parents will have to pony up for daycare, nursery school, or after-school programs. For many families at the bottom of the income distribution, after childcare costs that second income may not be much of a net boost, and can even be a net loss. As a result, getting a second job may not be worth it, even if their incomes together would rise above that $40,000 bar (and thus put them beyond the count of most poverty measures).