Definitions of "middle class" vary widely and often differ by regions of the country and even within states. However, sociologists agree that social class is always defined by lifestyle, rather than by income or wealth alone.
Nearly 100 years ago, Lloyd Warner did the definitive study of social class and eventually settled on six broad descriptions: two upper classes, two middle classes, and two lower classes. He divided the middle of the spectrum into the upper-middle class and the lower-middle class.
The lower-middle class was composed of small business owners, white-collar workers, lower-level management, skilled craftsmen, and semi-professionals. These people were at least high school graduates; some had some college (e.g., bookkeepers, some nurses); some had completed college (e.g., school teachers, registered nurses, middle-managers).
The upper-middle class is the professional class. To the average "man-on-the-street," this is considered to be the "upper class." The upper-middle class is populated by physicians, lawyers, some college professors, engineers, and others for whom college and often graduate degrees are required.
A short-cut version of all of the variable that make up a lifestyle is SES. That stands for socio-economic status, and it's a pretty good indicator of social class. SES is a combination of occupation, education, and income. Generally, these are highly correlated, but they don't NECESSARILY indicate the social class of any one person or family. For example, it's impossible to lead a champagne lifestyle on a beer budget. However, one can have a champagne income but lead a beer lifestyle.
Social class description is so complicated that even professional sociologists who did not major in this sub-speciality have a hard time understanding the subtle differences.
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Source(s):
Stewart and Glynn, Introduction to Sociology
Harold Hodges, Social Class in the San Francisco Bay Area
Strauss and Zweizecki, Social Stratification in the United States