Unlike the rest of the Phoenix metropolitan area, population density in central Phoenix dropped during the 1970s and 1980s. The primary cause was a decrease in the number of housing units. Rising vacancy rates contributed, but the increase in vacancy rates was similar to that of the entire metropolitan area. Between 1990 and 1995, population density rose in central Phoenix. A sharp decline in vacancy rates was a major factor in the turnaround, though the vacancy rate decline only matched that of the entire metro area. Another major factor in the increase in density was the rising number of people residing in prisons, homeless shelters, or on the streets.