Back in Victorian times, wealthier citizens could sometimes be found wandering among London's poorer, informal neighbourhoods, distributing charity to the needy.
"Slumming" - as it was called - was later dismissed as a morally dubious and voyeuristic pastime. Today, it's making a comeback; wealthy Westerners are once more making forays into slums - and this time, they're venturing right across the developing world.
According to estimates by tour operators and researchers, over one million tourists visited a township, favela or slum somewhere in the world in 2014. Most of these visits were made as part of three or four-hour tours in the hotspots of global slum tourism; major cities and towns in Johannesburg, Rio de Janeiro and Mumbai.
There is reason to think that slum tourism is even more common than these numbers suggest.
Consider the thousands of international volunteers, who spend anything from a few days to several months in different slums across the world.
The gap year has become a rite of passage for young adults between school and university and, in the UK, volunteering and travel opportunities are often brokered by commercial tourism operators. In Germany and the US, state sponsored programs exist to funnel young people into volunteering jobs abroad.
International volunteering is no longer restricted to young people at specific points in their lives. Volunteers today are recruited across a wide range of age groups. Other travellers can be considered slum-tourists: from international activists seeking cross-class encounters to advance global justice, to students and researchers of slums and urban development conducting fieldwork in poor neighbourhoods.
Much modern tourism leads richer people to encounter relatively poorer people and places. But in the diverse practices of slum tourism, this is an intentional and explicit goal: poverty becomes the attraction - it is the reason to go.
Many people will instinctively think that this kind of travel is morally problematic, if not downright wrong. But is it really any better to travel to a country such as India and ignore its huge inequalities?
It goes without saying that ours is a world of deep and rigid inequalities. Despite some progress in the battles against absolute poverty, inequality is on the rise globally. Few people will openly disagree that something needs to be done about this - but the question is how? Slum tourism should be read as an attempt to address this question. So, rather than dismissing it outright, we should hold this kind of tourism to account and ask; does it help to reduce global inequality?
My investigation into slum tourism provided some surprising answers to this question. We tend to think of tourism primarily as an economic transaction. But slum tourism actually does very little to directly channel money into slums: this is because the overall numbers of slum tourists and the amount of money they end up spending when visiting slums is insignificant compared with with the resources needed to address global inequality.
But in terms of symbolic value, even small numbers of slum tourists can sometimes significantly alter the dominant perceptions of a place. In Mumbai, 20,000 tourists annually visit the informal neighbourhood of Dharavi, which was featured in Slumdog Millionaire. Visitor numbers there now rival Elephanta Island in Mumbai - a world heritage site.
Likewise, in Johannesburg, most locals consider the inner-city neighbourhood of Hillbrow to be off limits. But tourists rate walking tours of the area so highly that the neighbourhood now features as one of the top attractions of the city on platforms such as Trip Advisor.
Tourists' interest in Rio's favelas has put them on the map; before, they used to be hidden by city authorities and local elites.
Despite the global anti-poverty rhetoric, it is clear that today's widespread poverty does benefit some people. From their perspective, the best way of dealing with poverty is to make it invisible. Invisibility means that residents of poor neighbourhoods find it difficult to make political claims for decent housing, urban infrastructure and welfare. They are available as cheap labour, but deprived of full social and political rights.
Slum tourism has the power to increase the visibility of poor neighbourhoods, which can in turn give residents more social and political recognition. Visibility can't fix everything, of course. It can be highly selective and misleading, dark and voyeuristic or overly positive while glossing over real problems. This isn't just true of slum tourism; it can also be seen in the domain of "virtual slumming" - the consumption of images, films and books about slums.
Yet slum tourism has a key advantage over "virtual slumming": it can actually bring people together. If we want tourism to address global inequality, we should look for where it enables cross-class encounters; where it encourages tourists to support local struggles for recognition and build the connections that can help form global grassroots movements. To live up to this potential, we need to reconsider what is meant by tourism, and rethink what it means to be tourists.