A slum is a heavily populated urban informal settlement characterized by substandard housing and squalor. While slums differ in size and other characteristics from country to country, most lack reliable sanitation services, supply of clean water, reliable electricity, timely law enforcement and other basic services. Slum residences vary from shanty houses to professionally-built dwellings that because of poor-quality design or construction have deteriorated into slums.Slums were common in the 19th and early 20th centuries in the United States and Europe. More recently slums have been predominantly found in urban regions of developing and undeveloped parts of the world, but are also found in developed economies.According to UN-HABITAT, around 33% of the urban population in the developing world in 2012, or about 863 million people, lived in slums. The proportion of urban population living in slums was highest in Sub-Saharan Africa (61.7%), followed by South Asia (35%), Southeast Asia (31%), East Asia (28.2%), West Asia (24.6%), Oceania (24.1%), Latin America and the Caribbean (23.5%), and North Africa (13.3%). Among individual countries, the proportion of urban residents living in slum areas in 2009 was highest in the Central African Republic (95.9%). Between 1990 and 2010 the percentage of people living in slums dropped, even as the total urban population increased. The world's largest slum city is in Mexico City.Slums form and grow in many different parts of the world for many different reasons. Some causes include rapid rural-to-urban migration, economic stagnation and depression, high unemployment, poverty, informal economy, poor planning, politics, natural disasters and social conflicts. Strategies tried to reduce and transform slums in different countries, with varying degrees of success, include a combination of slum removal, slum relocation, slum upgrading, urban planning with city wide infrastructure development, and public housing projects.