Writing in The Guardian, Jimmy Carter and Fernando H Cardoso observe that child marriage has been a major brake on progress towards six of the eight millennium development goals. They argue that is time to recognise that we cannot improve the lives of the poorest and most marginalised women and girls until the impact of child marriage is addressed directly and openly - and we make a commitment to ending it.

At the coming review summit on the millennium development goals, the focus will rightly be on those areas where progress has been most disappointing. High on this list is the failure to improve maternal health in the poorest countries.

There will no doubt be discussion about rich countries' commitments to increase funds and whether governments in the developing world have used resources effectively. Unfortunately little attention will be given to child marriage and its damaging impact on the health of millions of girls and women.

There is, in fact, compelling evidence that child marriage has been a major brake on progress towards no less than six of the eight MDGs. Our hopes of reducing child and maternal mortality, combating HIV/Aids and achieving universal primary education are damaged by the fact that one in seven girls in the developing world – and it is overwhelmingly girls who suffer this fate - are married before they reach 15. So, too, are our ambitions to eliminate extreme poverty and promote gender equality.

The statistics are stark. In poor countries, babies born to mothers under 18 are 60% more likely to die in their first year than those born to older women. Girls under 15 are five times more likely to die during pregnancy and childbirth than women in their 20s. Lack of information, marriage to much older men and the inability to negotiate safe sexual practices also puts child brides at greater risk of HIV infection than their unmarried peers.

Child brides are more likely to drop out of school to concentrate on domestic chores and child rearing. But this bias against educating girls starts even earlier. In societies where girls are normally married off young, there can seem little point in investing in their education.

Poverty is a major driver of child marriage. In many poor countries and communities, marrying off a daughter relieves a family of an extra mouth to feed. A bride price or dowry can also be a much-needed windfall for desperate families.

All this has a damaging inter-generational impact. The children of young and poorly educated girls tend to do less well in school and have lower earnings as adults, perpetuating the cycle of poverty.

Child marriages take place in every continent but they are particularly common in south Asia and parts of sub-Saharan Africa. Child marriage rates reach 65% in Bangladesh and 48% in India – 76% in Niger and 71% in Chad. In the coming decade an estimated 100 million girls will be married before they reach 18.

You might think, given the powerful evidence of the damage caused to individuals and societies, that the practice of child marriage would be high on both national and global agendas. But what is striking is the discrepancy between the scale and seriousness of the problem and the attention it has been given.

We understand, of course, the reluctance to intervene in what is traditionally considered a family matter. We recognise that child marriage is embedded deep in the traditions of many societies and is all too often sanctioned by religious leaders. Change will not be easy.

There is some evidence that, thanks to grassroots campaigns and new economic opportunities for women, child marriage is in decline in some parts of the world. However, at the current rate of progress, it will take hundreds of years to disappear. The challenge is how we can help communities accelerate change.

This is why we and our fellow Elders are committed to drawing attention to the damage that child marriage is causing and to supporting those working towards ending it. This means a new emphasis on engagement, debate and education – especially at the community level.

We actively seek wider engagement with religious leaders on this issue. No religions we know explicitly promote child marriage. The fact that religious leaders condone and sanction it in many societies owes more to custom and tradition than doctrine. But we cannot allow the distortion of faith or long-standing custom to be used as an excuse to ignore the rights of girls and women, and to hold their communities in poverty.

What we have learned over the years is that social change of this kind cannot be imposed from above. Laws have little impact. The overwhelming majority of countries already outlaw child marriage through domestic legislation or are signatories to international treaties that prohibit it. But this has not fed through to change on the ground. In Zambia, for example, the legal minimum age for marriage is 21, yet 42% of girls are married by the age of 18 and nearly one in 12 by the time they reach 15. Similar contradictions are evident in many countries.

We need the authorities to take the law more seriously but change will happen fastest when communities recognise that the economic and social value of educating girls outweighs their bride price. This requires sensitive debate, thoughtful leadership as well as financial assistance to keep girls in school. We must also give greater support at community, national and international level to groups working to end this practice.

Most of all, it is time to recognise that we cannot improve the lives of the poorest and most marginalised women and girls until the impact of child marriage is addressed directly and openly - and we make a commitment to ending it.