Advocates for ending child marriage are trying a new tactic: Show governments just how much the practice is hurting their own bottom line.

That's the goal of a new report by the World Bank and the International Center for Research on Women (ICRW), a global research and advocacy group. The report analyzes the impact of child marriage on the national budgets and economic growth of 25 countries where at least one in three women marry before age 18. Its conclusion: By 2030 child marriage will cost developing countries billions of dollars in health-care and education costs as well as lost earnings potential.

Suzanne Petroni, one the report's co-authors and a senior director at ICRW charged with expanding the evidence base on child marriage, is hoping the data will be a wake-up call for governments. Until now, Petroni says, research on child marriage has mainly focused on determining its prevalence, causes and costs to women at the individual or family level.

This prior research has helped bring global attention to the issue and even galvanized some political will to address it. But only up to a point: "The investments that are needed to end the practice have not been forthcoming," says Petroni. "So if we could document and show finance ministers, policy makers, donors and communities that child marriage has a cost to their bottom line budgets, we may finally be able to spur investments in solutions."

The study finds one of the biggest bites to a country's budget results from the link between child marriage and population growth. The report estimates that a girl marrying at 13 will have an average of 26 percent more children over her lifetime than if she were married at 18 or older. And across the countries studied, ending child marriage would reduce total fertility rates by an average of 11 percent. Take Niger, where more than three-fourths of women are married before age 18 — the highest proportion in the world. The study finds that ending child marriage would slow population growth by 5 percent by 2030.

The additional children that are being born as a result of child marriage drive up government spending. For instance, in 18 countries analyzed, the study found that curbing this growth would save as much as $17 billion a year from the education budget.

Of course, in theory rising births can be a boon to a country if the short term cost are more than offset by the benefits of a gaining a larger workforce down the line. But Petroni says that only works if the country provides its children with enough education to grow into economically productive adults. And that's not generally the case in countries with high rates of child marriage.

"Child marriage for the most part takes place in situations of poverty," she says. "And the offspring of child brides tend to be not just more children, but also children who less healthy, less educated and less able to contribute economically to their countries."

The study finds that child marriage can also damage a country's overall economy because of the devastating impact that under-age marriage has on a girl's educational prospects, which, in turn determine her earning power. There's a bit of chicken-and-egg conundrum when it comes to quantifying the connection between child marriage and high school dropout rates, notes Petroni. That's because in places where child marriage is common, it's also often the case that girls don't have easy access to schools. So it can be hard to determine whether a girl drops out because she's been married off, or whether she's married off because she's no longer attending school.

Either way, however, studies indicate that child marriage and lack of education are closely entwined. And this new report estimates that as a result, child marriage reduces the earnings in adulthood for women by an average of 9 percent. In Nigeria alone that amounts to $7.6 billion annually in lost earnings and productivity.

On the upside, the link between underage marriage and education also suggests a powerful tool for curbing the practice: Get more girls into high school. In those 15 countries analyzed in greatest depth, the study estimates that every year of secondary education reduces a girl's likelihood of marrying as a child from four to 10 percentage points depending on the country.

"One of the biggest messages from this study is just how important education is," says Petroni. "It would be great if finance ministers would come away from this committed to invest in quality education for girls."

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