It has been a week since Mohammed Sani moved to Lagos, Nigeria's commercial capital. A scrawny 22-year old from Kebbi State in the northwest, he came looking for work. He has certainly found it.

At 5 each morning, he fills 10 plastic 25-liter jugs with water from a borehole, paying 20 naira for each one (about 5 cents). He then pulls them around Yaba, his new neighborhood, on a cart, selling each one for 25 naira. By sunset at 7 p.m., he has perhaps 700 naira of profit ($2) in his pocket — not much in Lagos. "If I find a better business, I will try it," he says.

Young people migrate to cities the world over looking for opportunity. Lagos, a sprawling lagoon city of some 21 million people, is no exception. In dense traffic jams, young men weave through the cars selling plastic pouches of drinking water and tissues. On street corners they run generators and will charge your phone or photocopy a document. But most people never get much further than where they start: working extraordinarily hard for very little.

Migrants to African cities are not worse off than they were in the countryside. But urbanization in Africa does not provide nearly as good a ladder out of poverty as it does elsewhere.

Africa is the world's fastest urbanizing continent. In 1950, sub-Saharan Africa had no cities with populations of more than 1 million. Today, it has around 50. By 2030, over half of the continent's population will live in cities, up from around a third now. The fastest-growing metropolises, such as Nairobi, Kenya's capital, are expanding at rates of more than 4 percent per year.

In most parts of the world, crowding people together allows businesses that wouldn't otherwise exist to thrive. In Africa this process seems not to work as well. According to one 2007 study of 90 developing countries, Africa is the only region where urbanization is not correlated with poverty reduction. The World Bank says that African cities "cannot be characterized as economically dense, connected, and livable. Instead, they are crowded, disconnected, and costly."

What ties them together, and sets them apart from cities elsewhere in the world, according to the Brookings Institution, an American think tank, is that urbanization has not been driven by increasing agricultural productivity or by industrialization. Instead, African cities are centers of consumption, where the rents extracted from natural resources are spent by the rich. This means that they have grown while failing to install the infrastructure that makes cities elsewhere work.

In Lagos, the island of Ikoyi, which was once a garden suburb for British colonial officers, is now a wealthy residential area lived in by oil executives and politicians, with a golf course. But if you want to live here, you must "bring your own infrastructure," jokes Giles Omezi, a Nigerian architect. Every private home or apartment block has not only its own security guards and generator, but its own borehole and water treatment system too.

The poor new arrivals, meanwhile, get by with almost nothing. Underneath a bridge that connects the Nigerian mainland to Lagos' islands, the slum of Makoko sprawls out into the lagoon — the houses at the edge are built on stilts in the water on foundations of rubbish. Once a fishing village, it is now home to anywhere between 80,000 and 300,000 people from all over west Africa. Water has to be brought in by cart. Sewage runs in the narrow streets.