CHILD marriage is common in the developing world. One in three girls there is believed to marry before the age of 18, which is how child marriage is generally defined. If the practice is not reduced, another 1.2bn women will have married as children by 2050. Almost all of the countries in the top 20 spots in a ranking of states with the highest rates of child marriage are African. Far less well-known is the prevalence of the practice in America—and almost always among girls. The country’s diplomats are active in international efforts to ban it abroad, but American children are still permitted to marry (albeit, usually, with parental consent and the approval of a judge or a clerk). No American state has passed a law that categorically forbids the practice.

Child marriage is most common in conservative religious communities and poor, rural areas. But it can be found in all socio-economic strata and in secular, as well as pious, families. More than 207,000 American minors were married between 2000 and 2015, according to an investigation by Frontline, a television programme. Over two-thirds were 17 years old, but 985 were 14, and ten were just 12. Twenty-seven states have no minimum age for marriage. Encouragingly, the practice has become less common in recent years. This reflects changing social norms, higher rates of school attendance for girls and a decline in marriage generally. Whereas 23,500 minors got married in 2000, that figure had dropped to a little over 9,000 by 2010. Yet even as recently as 2014 more than 57,000 minors aged 15 to 17 were married. They entered perhaps the most important legal contract of their lives while, in most cases, not being considered legal adults. This means they cannot file for divorce, sign rental leases or seek protection in a shelter if they are abused.

Opponents of a ban on child marriage can be found across the political spectrum. Social conservatives argue that early marriages can reduce births out-of-wedlock as well as the number of single mothers on welfare. They also want to see religious traditions and customs protected. Libertarians say that marriage should be a choice made apart from the state. On the left, the American Civil Liberties Union and Planned Parenthood, a national group that offers reproductive-health services, have defended the practice because banning it would intrude on the right to marry. Supporters of a ban hold that if children are seriously committed to each other, they can wait until they are 18 to marry. And religious customs that hurt children should not be protected.

Parents may think they have their child’s best interest in mind by allowing an early marriage, especially if their daughter is pregnant. But in the vast majority of cases they actually harm her, sometimes irreparably. Between 70-80% of child marriages end in divorce. Married children are twice as likely to live in poverty and three times more likely to be beaten by spouses than are married adults. Around 50% more of them drop out of high school, and they are four times less likely to finish college. They are at considerably higher risk of diabetes, cancer, stroke and other physical illnesses. And they are much more likely to suffer from mental-health problems. That is why activists are so intransigent in pushing for a complete ban. They are gaining ground. Virginia, Texas and New York have introduced laws that restrict marriage to legal adults. (In some states, people under 18 can become legal adults, with the associated rights, in order to marry.) Connecticut has banned marriage for the under-16s. In 11 other states legislation restricting child marriage is in the pipeline; Arizona, Florida, Maryland, Massachusetts, New Jersey and Pennsylvania are considering blanket bans on marriage for those under 18.