DO SLUMS keep people in poverty or help them get out of it? It is an important question: about one-third of the urban population of developing countries (860m people) live in them. But with little data on slums and their inhabitants, it is a hard one to answer. Nonetheless a recent paper from economists at the Massachusetts Institute of Technology (MIT) suggests that slums are often traps rather than springboards.

Economists have tended to accentuate the good side of slums. By offering a toehold for rural migrants seeking their fortune in cities, they are thought to foster upward mobility. Edward Glaeser, an economist at Harvard University, argues that the buzz of slums encourages entrepreneurship and hard work. One survey from Rio de Janeiro found that a majority of slum-dwellers interviewed in 1969 and found again in 2001 were no longer living in the favelas. That suggests many slum-dwellers had moved on to bigger and better things. Other research shows that about one-third of households in Nairobi’s slums have established their own business.

Yet the MIT paper, which offers simple statistics about 138,000 slum households from around the world, suggests that slums are often an impediment to advancement. Poor hygiene, and the debilitating illnesses it propagates, is one curse. The majority of slum-dwellers in the MIT sample have no private latrine; in one Mumbai slum, taps are shared by more than 100 people. According to the African Population and Health Research Centre hygiene is regularly worse in slums than in rural areas. In the slums of Tongi and Jessore in Bangladesh, 82% of respondents report that a member of their household has been sick in the past month.

Sickness, in turn, can spell economic disaster: days off work and expensive medical bills make it harder to save money and escape the slum. Research from an informal settlement in Dhaka shows that if sickness has prevented slum-dwellers from working for a spell, they are significantly more likely to admit that they are short of money or have recently begged.

Other factors make saving difficult. Many slum-dwellers rent their homes, with relatively few residents squatting. Given the proximity of slums to the city, rents are high. That eats into incomes. In a settlement in Dakar over 20% of people’s income is devoted to rent. That is a hefty charge given the poverty of its inhabitants, especially so when compared with the norms in the countryside. In Kibera, a big slum in Nairobi, the authors of the MIT study show that rents represent one-third of non-food expenditure, whereas in rural areas in Kenya 90% of households pay no rent at all.

The question for economists is whether slums are just an expression of the wider problem of persistent poverty in developing countries or whether they themselves exert an effect on their inhabitants. Tracking the lives of individual slum-dwellers casts some light on that. The longer a family has lived in a Bangladeshi slum, for example, the lower its income. Similar results are found in Kibera, even though Kenya has seen rapid growth in living standards over the past decade.

Such statistics could well be biased: households that have improved their condition may no longer live in the slum, dragging down the average for those that stay behind. Yet the evidence suggests that people find it hard to get out. The average resident of Kibera has lived there for 16 years. Some 40% of Kolkata’s slum-dwellers have lived in slums for more than three decades—an amazing proportion given the regular stream of arrivals.

The evidence is far from conclusive. Yet the paper suggests that economists are often too optimistic about the economic role played by shanty-towns. Even if slum life is better than rural poverty, there may be few grounds for celebration. Slums and poor people may not just co-exist; some people may be stuck in poverty because they live in slums.