One of the key insights of pioneer works on housing in cities in the Global South was that there was a range of housing types in each city through which low-income groups built,  purchased, rented or occupied accommodation. These needed to be understood if policies for  improving conditions were to be effective. Some of the pioneer works also discussed how much  these ‘housing sub-markets’ and their locations were shaped by labour markets and by politics.  Although there were points of comparison between cities – for instance in squatter settlements  and houses built on illegal subdivisions – their relative importance and the range and relative  importance of other relatively inexpensive housing submarkets differed. During the 1970s, there  were also detailed case studies of housing sub-markets in particular cities and some detailed studies  of particular ‘informal settlements’ and these highlighted how much these were shaped by local  contexts. These works influenced many governments and international agencies to move to more  support for upgrading and away from slum clearance.

But this attention to city-specific understandings and to the priorities of low-income groups got lost  in the 1980s and then in the 1990s swamped by the way that international agencies sought to  incorporate some attention to housing issues in their policies. Most of the large and diverse range  of housing sub-markets through which those with low-incomes got accommodation got termed  ‘slums’ and as international goals and targets on ‘slums’ began to be set, so ‘slums’ had to be  defined. But they were defined very loosely (and often with considerable inaccuracies). National  governments are now meant to measure and monitor their ‘slum’ population.