For as long as humans have congregated in urban centers, people have been forced to move for various reasons. Scholars, though, have struggled to define this displacement. Grier and Grier developed a definition in the 1970s that encapsulates people's general understanding. They defined displacement as occurring when a household is compelled to leave its residence because of conditions that affect the dwelling or its immediate surroundings and (1) are beyond the household's control, (2) occur despite the household's adherence to previously imposed conditions of occupancy, and (3) cause continued occupancy to be impossible, hazardous, or unaffordable.

Natural disasters and human conflict are perhaps responsible for the largest amount of displacement. Hurricane Katrina in 2005 is an example of widespread displacement caused by a natural disaster. Some ...