It was not absolutely clear whether it was a moment to cheer or cry. In December, shortly after the participants in a UN conference on family planning had broken up in Kampala, Africa's billionth baby was born. In a continent apparently wracked with all the ills of over-population – hunger, poverty and shocking maternal and neo-natal mortality – it might appear a harbinger of disaster.



For within 40 years, that number could almost double. How then could Africa hope to feed itself, let alone find work and livelihoods for so many? "Sexual and reproductive health" – aka population control – has been added to the Millennium Development Goals mainly because it is now understood that without it there is no hope that the other targets will be met.



Africa's billionth baby, the doom mongers predicted would, if he survived to adulthood, only perish in one of the coming resource wars fought over land or water or oil or minerals, or simply fall victim to the unvarying instability that trails in the wake of over-population.



But there is a counter argument: each new baby is another consumer – and modern economic growth is driven by demand. The billionth baby is the engine of future prosperity.



As Mo Ibrahim, a Sudanese-born British businessman and philanthropist, points out, each baby is a potential worker: "Africa is underpopulated. We have 20% of the world's landmass and 13% of its population. We have a bulge of young people and that brings to the marketplace a huge workforce, whereas Europe's population is ageing. We need to focus on education and training." It is big, concentrated populations that have contributed to explosive economic growth rates in China and India. Get the policies right, he suggested (and his focus is on improving governance) and the billionth baby could yet enjoy a secure old age.



The argument made by the Mo Ibrahim Foundation is that rising mineral prices will lead to the exploitation of Africa's vast natural resources. Put in the right structures of governance and the old post-colonial basket cases like Angola, Democratic Republic of the Congo and southern Sudan will become growth centres. In the short-term, rising populations may pose economic and social difficulties, but at nearly 5% a year, Africa's average economic growth already outstrips the global north and incomes are starting to reflect that.



Analysis of the Asian tiger economies shows a whole series of overlapping change and development that created the ideal circumstances. The age profile of the population is one of them. This is the "demographic dividend", the moment when the economically active in a population outnumber economically inactive dependents, whether young or old.



For that to happen, however, the rate of population growth must slow. From China's one-child policy and Mrs Ghandi's forced sterilisations of the mid-70s, powerful forces imposed smaller families. Smaller families both allow and incentivise parents to earn money rather than to eke out a living from the soil. Population growth in India has fallen from 4% in the 1960s to about 1.7% now. Policy makers dream of a country with the world's second largest population at 1.1 billion people by 2050. But the average number of children in a family is still too high, some fear. A "replacement" policy, a "two per family norm" is the policy makers' objective.



China is at the peak of its population dividend. The average income for people between 20 and 30 has risen by a third. There are about 300 million under 30 years old and it is their consumption patterns that stimulate economic development. China is about to hit the wall of an aging population that by 2030 might begin to slow its growth.



In India, half the population is younger than 25. Between now and the moment perhaps in about 20 – 30 years' time, when rising prosperity starts to raise life expectancy, there is a window where a corner can be turned, where producers become consumers and economic growth takes off. At the end of the period, if development is to be sustainable, the population needs to be balanced and stable – neither too broad a base at the bottom of the graph, nor at the top.



Slowing Africa's population growth is the challenge. "Population growth is a bigger problem than HIV/Aids," one expert told the UN Kampala conference. Large families are not just a cultural tradition; they are evidence of status and masculinity, part of an individual's identity. Some speakers ridiculed the idea of "behaving like Europeans" and restricting themselves to one or two children.



There are practical as well as cultural reasons for large families. For the 80% in sub-Saharan Africa who are subsistence farmers with no state to support the ill or elderly in their family, to have many children is simply a sensible insurance policy, a source of labour first and an old age pension later.



Uganda is one of a handful of African countries where the population, now 33 million, is predicted at the least to treble in the next 30 years. Yet like Mo Ibrahim, president Yoweri Museveni calls his country's population growth "a great resource". And when his wife, Janet, in a piece of soaring irony, opened the Kampala conference she spoke hardly at all of the need for contraception beyond ruing the way it was misunderstood in her country. Mrs Museveni promotes a form of natural birth control called Moon Beads, a system by which women avoid sex at moments of peak fertility. Many Ugandans feel it is a singularly inappropriate method in a country where women too often have no sexual autonomy and where half of all conceptions are unwanted. More babies are born in Uganda (where abortion is still illegal) than almost any other country, trailing only Somalia, Afghanistan and East Timor.



Educating women is one important strategy for slowing population growth. There is a direct correlation between the level of education and the number of children a woman has. Improving maternal health, lowering infant mortality and extending the interval between pregnancies all contribute to smaller families along with raising confidence that babies will survive into adulthood. Uganda has now introduced universal secondary education and is beginning to improve provision of condoms.



The second element is ensuring that economic growth is equitable. Inequitable growth can be as destabilising as no growth as excluded groups exacerbate regional and political tensions. In Uganda, the north and east already feels victim of discrimination. If southern Sudan (where regional inequalities are also a major source of unrest) reverts to civil war, there are some who see conflict spreading across east Africa.



Mo Ibrahim may be right that a growing population could contribute to future prosperity. But without a sharp fall in the rate of growth, it looks as if getting to it will be a painful journey.

• This article was amended on 1 April 2010 to amplify a quote on population and education from Mo Ibrahim. The standfirst and final paragraph were amended on 12 April 2010 to reflect the earlier change.

