And the World Bank for Useful Com- Ments. We Gratefully Acknowledge Support from the State of Coahuila, and Thank

Housing, food, and water are considered to be basic requirements for daily living. Unfortunately, inadequate housing with poor water supply, sanitation, and ventilation threatens the lives and health of some 600 million urban dwellers worldwide (see, among others, Cebu Study Team 1991; S. A. Esrey et al. 1991; Galiani, Gertler, and Ernesto Schargrodsky 2005; and World Bank 2005). For this reason, most countries in the world devote substantial resources to upgrading slum areas and improving housing quality for poor groups in the population. For example, the US Government spends more on housing programs than on other better-known welfare programs, such as food stamps and temporary assistance for needy families (Edgar O. Olsen 2003). In the developing world, where urbanization is strongly associated with the rapid spread of slums, and where slum dwellers account for 45 percent of the urban population, policies to improve the welfare of slum dwellers focus on such areas as upgrading slum housing in situ and relocating slum dwellers to better quality, low-cost housing (World Bank 2005). Despite the importance of housing as a