A report by the Brookings Institute  assesses what metropolitan areas the middle class most inhabits, as well as how concentrated middle-class communities are, what forces shape them, and how they’ve changed since 2000.
Defining the middle class as the middle three quintiles of the national income distribution—only adjusted to take account of regional price parities and household size—the study found that the metropolitan areas with the largest concentration of middle-class families are manufacturing centers, military towns, and Mormon communities: what the Brookings Institute refers to as “one of the three M's’’” These areas tend to have a high number of workers not only in manufacturing but also construction and administration. They also are mostly suburban in nature, lacking the subsidized housing and public transit found in older cities with a greater percentage of low-income residents. Demographically, they also tend to be less diverse, with predominately white populations.
While small and mid-sized metro areas have the most homogenous middle-class communities, the majority of middle-class families can nevertheless be found in or around larger cities that tend to support the same labor force in addition to lower-paying and higher-paying jobs.
The study also revealed fluctuations in the middle class since the beginning of the millennium. Overall the middle-class community has shrunk slightly, but this is due to a corresponding increase in higher incomes. The number of middle-class families in the areas described above have also decreased in relation to the number found in larger metro areas.
Since 2000, the concentration of middle-class families in the South has grown substantially but fallen in the Northeast, along with the West Coast, and in a few cities located in the Midwest. Areas, where the middle class has grown, tend to have developed as newer metropolitan areas with distinct suburban characteristics.
Metropolitan areas with the lowest share of middle-class families tend to be tech capitals and college towns. Whereas tech capitals are predominantly populated with high-income workers, college towns are mostly split between high-income faculty members and low-income students. For this reason, areas like the San Francisco Bay and towns like Boston, Boulder, and Huntsville, Alabama tend to have a much lower percentage of middle-class families. Also, older cities tend to have smaller middle classes, such as many cities in the Northeast like Bridgeport, Philadelphia, and New York.