After an unforgivably long time, the revolutionary idea that girls should get the same education as boys seems to be gaining acceptance. What's surprising, then, is that one of the great impediments to keeping girls in school - particularly girls in the developing world - remains largely unremarked upon and unaddressed. By this we mean the stubborn persistence of child marriage.

Child brides remain alarmingly common. An estimated 10 million marriages occur worldwide each year in which at least one spouse, usually the female, is younger than 18. In 16 countries, half of girls marry by that age. West Africa has the highest incidence of child marriage, South Asia the largest numbers. In many places, brides are preposterously young. In Niger, 36 percent of girls are wed before turning 15.

These unions produce a host of social ills. Younger brides become pregnant at younger ages, often before their bodies fully mature, putting mother and child at risk. In the developing world, a baby born to a mother under 20 is 50 percent more likely to be stillborn or die in the first month than one born to a mother 20 to 29. Pregnant girls younger than 15 are five times more likely to die giving birth than women over 20 are.

A particular danger for teen mothers is obstetric fistula, a condition that can develop during untended, obstructed labor and lead to constant leakage of urine or feces; such girls are often cast out of society.

Girls who marry young also typically leave school. Their children are more likely than those of older brides to be undereducated and malnourished.

Generally, the younger the bride, the greater the age gap between her and her husband, which translates to a power imbalance that often leads to spousal abuse.

In recent decades, several countries have increased the legal marital age to 18 to comply with international conventions defining marriage as an agreement between consenting adults. Yet legal change alone has made little difference. The prevalence of child marriage is decreasing - but very slowly, and in many places not at all. Some members of Egypt's Parliament actually want to rescind the country's law that sets the marriage age at 18.

In societies where this tradition is entrenched, particularly among the poorest of the poor, parents often regard early marriage as a way to eliminate an economic burden. They may see educating a daughter as a waste of resources and seek to hasten the day when she moves to her husband's home.

Programs that target this economic dynamic have reduced child marriages. In Ethiopia's rural Amhara region, for example, parents of girls 10 to 14 were given $6 to pay for their daughter's school supplies plus a goat worth $25 if she remained enrolled for two years. After that period, girls in the program were one-tenth as likely to be married and three times as likely to be in school as their peers. Similar programs in Malawi and Bangladesh have also reduced child-marriage rates.

In addition to helping pay the costs of keeping a daughter at home, these programs encourage a view of girls as children rather than brides. Parents can imagine a return on investment in educated daughters equipped for jobs. And schooling gives girls the skills, social networks and confidence to better negotiate their futures.

The challenge is to expand these interventions. Governments will need to make them part of national education policy, and donors will need to help with funding. The Ethiopian program was a partnership of local authorities and the Population Council, a nongovernmental organization focused on health and development issues, with funding from the Nike Foundation, the United Nations and the U.S. Agency for International Development.

High child-marriage rates are associated with weak national indicators not only in maternal health and education but also poverty, food security and HIV incidence, since badly informed and weakly connected mothers make poor decisions for themselves and their families. If girls are educated, they won't be the only ones who benefit.