David Rusk claims in Cities Without Suburbs that elastic American cities are less segregated than other American cities. I demonstrate through statistical analyses that there is a strong correlation between Rusk's elasticity (an index comprised of a central city's annexation history since 1950 and its population density) and his income segregation index. The statistical correlation between these two variables is stronger than between Rusk's segregation index and any other variable I test, including city age, size, regional location, and black population percentage. I then consider several hypotheses that may explain these correlations and propose that the continuous annexation of peripheral, developing land by a central city prevents the incorporation of affluent suburbs. Suburban boundaries, especially those of affluent suburbs, function as population sorting mechanisms, which segregate migrant households by socioeconomic status and life-style. I ascertain that only rapidly growing, unbounded central cities prevent or ameliorate segregation by being elastic. / Department of Urban Planning